WhitakerAudio€¦ · 1 Circuit Description 8 2 Parts List 12 3 PWB Design 17 4 Construction...

WhitakerAudio

Stereo Audio Preamplifier

User and Assembly Manual

Copyright 2013 WhitakerAudio LLC, Morgan Hill, California, USA.

No part of this document may be reproduced without the express written consent of

WhitakerAudio.

Specifications subject to change without notice.

Any trademarks used in the manual are the property of their respective owners.

Note:

This document is intended to assist readers in building an audio product for personal use. No

warranties are expressed or implied. Always use good engineering practices and appropriate

safety precautions.

Table of Contents

1 Circuit Description 82 Parts List 123 PWB Design 174 Construction Techniques 215 Step-by-Step Instructions 23

5.1Assembly of Right Channel PWB 245.2Assembly of the Left Channel PWB 365.3Chassis and Final Assembly 45

5.3.1Front Panel Components 455.3.2Back Panel Components 495.3.3Cable Clamps and Related Hardware 515.3.4Install the PWBs 515.3.5Back Panel Audio Connections 525.3.6Front Panel Connections 605.3.7Install Interconnecting Wiring 705.3.8Final Assembly Check 74

6 Initial Checkout 766.1Functional Tests 806.2Setting Operating Levels 816.3Important Operational Note 83

7 Performance Measurement 847.1RIAA Equalization 877.2Final Touches 88

8 Troubleshooting Guidelines 919 Safety Considerations 9210 Notes and Tube Data 93

10.1Tube Characteristics 9310.1.1 6EU7 9310.1.2 5879 9410.1.3 7025 (12AX7) 97


The Stereo Preamplifier is a high-quality device intended for discriminating listening. The

product features quality components throughout. The preamp is available either as a kit or

assembled as a part of the signature series of WhitakerAudio products. This product shares the

craftsmanship and impressive industrial design of the other offerings in the WhitakerAudio line.

This unit, like the other products, is limited in number

The WhitakerAudio high-quality classic vacuum tube based preamplifier provides five

available inputs:

• Phonograph

• Microphone (high impedance)

• Tuner (100 kΩ input impedance)

• Auxiliary (100 kΩ input impedance)

• Front panel auxiliary (100 kΩ input impedance)

An input switch selects the desired source, which is fed to a tone-control stage and finally to

an output buffer amplifier. An isolated “Tape Out” connector is provided for side-chain

applications, such as recording on audio tape or on a computer. The Tape Out port is connected

prior to the tone control stage.

WhitakerAudio

The “Front Aux” input position connects to a front-panel jack for connection of a personal

music device to the preamp.

No power supply is included in the design, as the assumption is that this unit will be paired

with a separate power amplifier, such as the 20 W stereo amplifier or 40 W stereo power

amplifier. A multi-pin connector ties the power supply of the amplifier to the preamp chassis.

This has the benefit of reducing cost and at the same time reducing noise from the supply that

could be picked up by the low-level preamp circuits.

The stereo preamplifier is built around two printed wiring boards (PWBs), engineered for top

performance. A ground plane covers the component side of the PWB. Component placement and

board traces have been engineered to minimize hum and noise. The boards feature a unique

mounting technique for the five vacuum tubes used in each channel that keeps heat away from the

PWB and minimizes hum in the amplifier. The PWBs are engineered to reduce off-board

connections, thereby simplifying layout and providing for controlled characteristics from one unit

to the next.

6


The phonograph circuit uses a 7025 (12AX7) tube as the active stage. The microphone circuit

uses a 5879 tube. The tone control stage is built around a 6EU7. The output buffer amplifiers are

7025 tubes.

This product is offered in both kit form and completely wired and ready to go. This document

provides a technical description of the product and complete assembly instructions. The user

manual is the same for both kit and assembled products.

Physical dimensions: 19-in wide by 12-in deep by 6-in high. Note that the depth specification

does not include back panel cables. Weight approximately 16 lbs.

This product has been designed and built for discriminating consumers, using classic

technologies and new components. Beyond the great specs, the preamplifier sounds very nice.

Clean. Warm. Interesting. Like a tube amp should sound.

The preamplifier generates little heat and (for a tube amplifier) consumes little power. Turn it

on and let it run. Put on some music and enjoy audio as it should be heard.

The stereo preamplifier shown with the companion 20 W stereo audio amplifier.

7

WhitakerAudio

1 Circuit Description

A schematic diagram of one channel of the preamplifier is shown in Figure 1.1. Two such circuits

are used for the stereo preamplifier.

The phono preamp is a two-stage high-gain circuit intended for use with a magnetic

phonograph pickup device. A nominal load of 47 kΩ is assumed1. The 7025 twin triode features

low hum and noise, and is designed specifically for use in high-fidelity circuits that operate at low

signal levels. Typical voltage gain of this stage is in excess of 130.

The audio signal from the phono pickup is applied to the grid of V1a via the rear panel RCA

connector. Interstage coupling between the two phono amplifier sections includes an RIAA

equalization network consisting of C3/R6, C6/R10, C7, and C8. The output of the preamp is

coupled from the plate of the second stage by coupling capacitor C9 to the input of the tone

control amplifier through output level control R16 and input switch SW1. Capacitor CP is used to

optimize the high frequency response of the circuit if needed. This is a “select on test”

component. A typical value would be 20 pF, if used.

The microphone preamp stage (V2) is intended for use with high-impedance dynamic

microphones. Optionally, an external impedance transformer may be used to interface a low-

impedance microphone. The circuit uses a 5879 low-noise sharp-cutoff pentode in a conventional

amplifier circuit with high voltage gain (approximately 60). The output of V2 is coupled through

C10 to output level control R17 and to input switch SW1.

The Tuner and Auxiliary inputs are routed through level control potentiometers R18 and R19,

respectively, to input switch SW1. The level-adjust devices are located internal to the

preamplifier.

The tone-control amplifier (V3) uses a 6EU7 low-noise twin triode in a two-stage cascade that

consists of an input cathode-follower connected to a triode voltage amplifier through a frequency-

sensitive (tone control) interstate coupling network. The network consists of the following

components:

• R25, R26, R27, R28, and R30

• C16, C18, C19, C21, and C22

1. This value can be modified if needed by changing R1.

8

9


Phon

ogra

phin

put

0 V

V1, 7

025

(12A

X7)

C2

C5

C1

C4

C7

C6

C9

C8

C3

R1

R2R3

R4

R5

R9

R6

R10

R8

R7

+275

V

+260

V+2

35 V

+175

V

+1.5

V+1

.5 V0 V

++

++

1

2

3

45

6

7

8

9+195

V

Mic

roph

one

Inpu

t

V2, 5

879

+250

V

+110

V

+70

V0

V

+1.7

5 V

R11

R12

R13

R14

R15

C10 C

11C

12C13

+

+

1

37

8

9

4

5

Out

put

+245

V

0 V +1 V

+140

V

+140

V

C26

+

C23

C25

C24

R33 R

34

R35

R37

R36

R38

V4, 7

025

(12A

X7)

16

3

4

59

R32

Volu

me

Con

trol

Tune

rIn

put

Auxi

liary

Inpu

t

Prea

mp

inpu

t: P

hono

Mic

T

uner

Aux

Fro

nt A

ux

R16

R17

R18

R19

SW1a

/b

V3, 6

EU7

+250

V+2

20 V

+140

V

+12

V

+1 V

+17

V

0 V

C14

C15

C16

C17 C18

C19

C20

C21

C22

R20

R21

R22

R23

R31

R25 R27

R28

R29

R30

Treb

leC

ontr

ol

R26

Bas

sC

ontr

ol

1

2

45

67

8

9

R24

++

CP

1

C27

+C

28

+27 8

Fron

t Aux

Inpu

t

+27 8

Tape

Out

+245

V

0 V +1 V

+140

V

+140

V

C29

+

C30

C32

C31

R43 R44

R45

R39

R46

R41

V5, 7

025

(12A

X7)

16

3

4

59

R42

Hum

Bal

ance

To h

eate

rco

nnec

tions

of

V1 –

V5

23 4

Gro

und6.3

V he

ater

R40

Line

Out

Leve

l

H H

SW2

To m

icin

put o

not

her c

hann

el

Mic

mix

D1

D2

R47

C33

CR

-1

C28

+

ac in

ac in

—

+

Inpu

t Pre

amlif

iers

Tone

Con

trol A

mpl

ifier

Out

put B

uffe

r

Opt

iona

l6.

3 V

dc

reci

fier

outp

ut

6.3

V he

ater

Figure 1.1 Schematic diagram of the preamplifier.

WhitakerAudio

The bass and treble controls in the coupling network can be adjusted to provide for up to

approximately 15 dB of boost or attenuation (cut) at 20 Hz and 20 kHz. With the bass and treble

controls set at the mid-range positions, the amplifier provides a reasonably flat response curve and

a nominal voltage gain of 3.

A two-stage buffer amplifier in the form of V4 is used to transform the high-impedance output

of the tone-control circuit to a lower impedance that is less susceptible to capacitive loading from

interconnecting cables, thereby simplifying the connection to external devices (usually an audio

power amplifier). The first stage of the 7025/12AX7 is a conventional voltage amplifier, with the

plate connected directly to the grid of the second stage. The cathode-follower output is taken

across a 100 kΩ load. Voltage gain is nominally 30.

The V4 buffer stage is duplicated in V5 and serves as a buffer amplifier for the Tape Out jack.

This stage separates the two signal paths and ensures that actions on one will not impact the other.

The input B+ voltage is filtered and reduced in potential as needed for the various circuits by

the following components:

• R4 and C2, and R9 and C5 for the phono preamp

• R15 and C13 for the mic preamp

• R23 and C15, and R29 and C17 for the tone-control circuit

• R38 and C26 for the output buffer amplifier

Volume Input Power Bass TrebelPhonograph

Microphone

Tuner

Auxiliary

Front Aux

Stereo Preamp J C Whitaker 2012

Front Aux

Figure 1.2 Preamplifier front panel.

10


• R41 and C29 for the Tape Out buffer amplifier

• C27 and C28 on the B+ input line, with bleeder resistor R47

Typical operating voltages are shown in Figure 1.1 for a maximum input B+ of 275 V dc.

Zener diodes D1 and D2 limit the B+ input voltage to 400 V dc. These devices are intended to

protect the preamplifier against excessive voltage input due to a power supply problem. Note that

the range of acceptable input power supply voltages is 250 V ± 25 V.

The circuit boards are designed to accommodate either ac or dc heater voltages. To minimize

hum, a dc heater supply is used in the stereo preamplifier. The external heater supply of 6.3 V ac

is applied to a chassis-mounted bridge rectifier and filtered by a large-value electrolytic capacitor.

The 6.3 V dc (approximate) supply is then distributed to both PWBs and all heaters. The Hum

Balance control (R42) provides a means to vary the resistance to ground of each leg of the heater

supply. Note that only one Hum Balance control may be installed on a stereo circuit.

It is important to remember that a capacitor-input dc power supply, such as that used for the

heater circuit, will rise in output as the load is reduced. For this reason, do not operating the

preamplifier with any of the tubes removed as this may allow the rectifier circuit output to

increase beyond the recommended 6.3 V dc.

The front panel controls are shown in Figure 1.2. Back panel connections are shown in Figure

1.3.

Right Left Right Left Right Left Right Left Right Left Right Left

Auxiliary Power ConnectorDanger: High Voltages

Do Not Probe

WhitakerAudio Stereo PreamplifierBuilt in California, USA.Phonograph Microphone Tuner Auxiliary Tape Output Line Output

MicMix

StereoMic

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

TE shell 17

Center

0.15-in

1.12-in

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix Combo

0.88-in

0.13-in

Center

Neutrix Combo

0.88-in

0.13-in

Center

Figure 1.3 Preamplifier rear panel.

11

WhitakerAudio

2 Parts List

A complete parts list for the stereo preamplifier is given in Table 2.1. The table includes

manufacturer part numbers and stock numbers from one of the major parts houses2. These parts

are, of course, available from a number of manufactures and suppliers. The details given here are

for the convenience of the user. Substitutions of certain components may be made based on

availability..

2. Allied Electronics, 7151 Jack Newell Blvd. S., Fort Worth, Texas, 76118, USA,. www.alliedelec.com.

Table 2.1 Parts List for Stereo PreamplifierPart Description Quantity Manufacturer Part No. Allied

Stock No.

C1, C4, C11, C24, C31

25 μF, 50 V, electrolytic

10 Vishay TVA1306-E3 507-1089

C2, C5, C15, C17, C26, C28, C29

22 μF, 450 V, electrolytic

14 Illinois Capacitor 226TTA450M 613-0367

C3, C16 0.1 μF, 400 V 4 Vishay 225P10494XD3 507-0247

C6 0.0033 μF, 600 V 2 Vishay 715P33256JD3 507-0337

C7 0.01 μF, 400 V 2 Vishay 225P10394XD3 507-0642

C8 180 pF, 500 V, mica 2 Cornell-Dubilier CD15FD181JO3F 862-0547

C9, C12, C20, C25, C32

0.22 μF, 400 V 10 Vishay 225P22494YD3 507-0251

C10, C14, C33

0.047 μF, 400 V 6 Vishay 715P47354LD3 507-0342

C13 47 μF, 450 V, electrolytic

2 Illinois Capacitor 476TTA450M 613-0369

C18, C22 0.0022 μF, 600 V 4 Vishay 715P22256JD3 507-0332

C19 0.022 μF, 400 V 2 Vishay 225P22394XD3 507-0643

C21 220 pF, 600 V, ceramic

2 Vishay 562R5GAT22 507-0815

C23, C30 100 pF, 1000 V disc 4 Vishay 561R10TCCT10 507-0889

C27 0.01 μF disc, 1 kV 2 Vishay 562R5GAS10 507-0721

C28 4700 μF, 50 V, electrolytic

1 Illinois Capacitor 478TTA050M 70112239

CP 25 pF, 1000 V ceramic

2 Vishay 561R10TCCQ25 507-0725

R1 47 kΩ, 0.5 W 2 Ohmite OL4735E 296-4788

R2, R7 2.7 kΩ, 0.5 W 4 Ohmite OL2725E 296-6170

12


R3, R5, R12, R25, R28, R31, R35, R37, R39, R45

100 kΩ, 0.5 W 20 Ohmite OL1045E 296-4773

R4 39 kΩ, 0.5 W 2 Ohmite OL3935E 296-4784

R6, R14, R20

470 kΩ, 0.5 W 6 Ohmite OL4745E 296-4789

R8 680 kΩ, 0.5 W 2 Tyco LR1F680K 437-0431

R9, R15, R29, R38, R41

15 kΩ, 2 W 10 Ohmite OY153KE 296-2333

R10, R23 22 kΩ, 0.5 W 4 Ohmite OL2235E 296-4779

R11 2.2 mΩ, 0.5 W 2 Ohmite OL2255E 296-6466

R13, R24, R36, R46

1 kΩ, 0.5 W 8 Ohmite OL1025E 296-4772

R16, R17, R40

250 kΩ potentiometer, linear

6 Honeywell/Clarostat 308N250K 70153217

R18, R19 100 kΩ potentiometer, linear

4 Honeywell/Clarostat 308NPC100K 70153229

R21 1.5 kΩ, 0.5 W 2 Ohmite OL1525E 296-4774

R22 15 kΩ, 0.5 W 2 Ohmite OL1535E 296-4775

R26, R30 1 mΩ potentiometer, audio taper, dual gang

2 Precision Electronic Components

KKA1051S28-ND DigiKey1

R32 250 kΩ potentiometer, audio taper, dual gang

1 Precision Electronic Components

KKA2541S28 DigiKey

R27 10 kΩ, 0.5 W 2 Ohmite OL1035E 296-1483

R33, R34, R43, R44

220 kΩ, 0.5 W 8 Ohmite OL2245E 296-4780

R42 100 Ω, 1 W, potentiometer3

1 Honeywell/Clarostat 381L100 753-8428

R47 100 kΩ, 2W 2 Ohmite OY104KE 296-2322

D1, D2 Zener, 200 V, 5 W 4 ON Semiconductor 1N5388BG Newark

CR-1 Bridge rectifier, 25 A, 50 V

1 Vishay 26MB05A 70078708

SW1 Rotary switch 1 Electroswitch C4D0206N-A 747-6690

SW2 Toggle switch 1 Cannon 7101SYZQE 676-3000

Table 2.1 Parts List for Stereo Preamplifier

13

WhitakerAudio

4 terminal barrier strip

2 Molex 38720-6204 607-0071

V1, V4, V5 7025/12AX7 tube 6 Electro-Harmonix, Genalex, others

Tube Depot2

V2 5879 tube 2 NOS Tube Depot

V3 6EU7 tube 2 Sovtek, others Tube Depot

1/2-inch #6 threaded standoff

4 Keystone Electronics 2210 70181896

3/4-inch #4 threaded standoff

20 Keystone Electronics 1895 839-0781

9-pin tube socket 10 Belton P-ST9-601 AES3

Main chassis, 10-in x 17-in x 3-in, steel

1 Hammond 1441-32BK3 806-0534

Main chassis baseplate, 10-in x 17 in, steel

1 Hammond 1431-30BK3 806-0544

Front panel, 19-in x 3.5-in

1 Front Panel Express custom

Rear panel overlay 3 FastFrame custom

Front panel handles 2 RAF 8128-832-A-24 219-8007

Chassis feet 4 Bud F-7264-A 736-7264

Large knob 4 Davies 1110 543-1110

Handle, side 2 Bud H-9174-B 736-4374

RCA connector 10 Neutrik NF2D-B-0 514-0004

Mic connector 2 Neutrix NJ3FP6C-BAG 70088266

3.5 mm connector 1 Switchcraft 142AX Mouser

Power connector, pin

1 Tyco 211767-1 512-8561

Power connector, receptacle

1 Tyco 211766-1 512-8869

Power connector, shell

1 Tyco 206070-8 374-1260

Power connector, contact pin

10 Tyco 1-66099-5 374-1075

Power connector, contact socket

10 Tyco 1-66101-9 374-1076

12 terminal Molex header

4 Molex 22-03-2121 863-0352

12 terminal Molex housing

4 Molex 22-01-2127 863-0334


14


Consumables for the stereo preamp are listed in Table 2.2.

4 terminal Molex header

16 Molex 22-03-2041 863-0309

4 terminal Molex housing

16 Molex 22-01-2041 863-0424

Molex crimp terminal

128 Molex 08-50-0114 863-0292

Pilot light 1 Dialight 249-7841-1431-574 511-0276

PWB 2 ExpressPCB custom

Top chassis decal 1 custom

Plexiglas overlay 1 TAP Plastics custom

9-pin tube socket shield, black

4 Antique Electronic Supply

KU:P-SS9-325-BK

Notes:

1 Digi-Key Electronics (www.digikey.com).

2 Vacuum tubes are available from a number of suppliers, including: Tube Depot (www.tubedepot.com), The Tube Store (www.thetubestore.com), Tube World (www.tubeworld.com), and other suppliers.

3 Antique Electronic Supply (www.tubesandmore.com/).

Table 2.2 Consumables for the 20 W Stereo AmplifierQuantity Description

6 ft Hookup wire, #18 black, solid

2 ft Hookup wire, #18 green, solid

2 ft Hookup wire, #22 black, solid

20 ft Hookup wire, #22 green, solid

2 ft Hookup wire, #22 red, solid

30 ft Audio cable, 1 conductor shielded, stranded



2 ft Expandable sleeving

4 ft Heat-shrink tubing, small

1 ft Heat shrink tubing, large

10 Spade terminal, #6 screw, #22 wire

4 Quick-disconnect terminals, female, #18 wire

1 Ground lug terminal, #6 screw, 3/4-inch

68 Phillips head screw, #4, 3/8-inch

28 Nut, #4 with captive lock washer

16 Lockwasher, #4




15

WhitakerAudio

Because of the large number of interconnecting wires needed for this preamplifier, expanded

sleeving is used where practical to organize the cabling. This tends to simplify cable routing and

provides for a cleaner appearance. Wherever possible, cables should be dressed either tight to the

chassis, or well above the chassis so as to stay as far as possible away from signal-carrying lines

and to rest against the bottom cover plate, which further improves shielding.

1 Nut, #6, fiber locking

1 Wing nut, #6

10 Flat washer, #6

4 Lockwasher, #6

4 Standoff, #6 female-female, aluminum

4 Sheet metal screw, Phillips head, #6

4 Standoff, chrome, 0.75-in. tall, 0.62-in. outside diameter, 0.32-in. inside diameter



5 Nut, #8 with captive lock washer

8 Lockwasher, #8

11 Tie wrap, small

10 ft Rosin core solder

4 Cable clamp, 1/2-inch

1 Paint, black, hammered

3 ft Hookup wire, #16 stranded, red (for external power supply cable)

6 ft Hookup wire, #16 stranded, black (for external power supply cable)

6 ft Hookup wire, #16 stranded, white (for external power supply cable)

6 ft Hookup wire, #16 stranded, green (for external power supply cable)

3 ft Large expandable sleeving (for external power supply cable)

6 inch Large heat shrink tubing (for external power supply cable)

Table 2.2 Consumables for the 20 W Stereo Amplifier

16


3 PWB Design

The PWB implementation incorporates all components for each channel, except for the front

panel controls and the rear panel connectors. The overall component layout is shown in Figure

3.1. A ground plane is used on the component side of the board to reduce hum and stray noise.

Input level control potentiometers R16, R17, R18, and R19 are mounted on the board and

accessible from underneath the chassis. These controls are typically adjusted during initial setup

and are not changed thereafter. The vacuum tubes are mounted on the chassis and connected with

short jumpers to the PWB. Connections from the boards to the chassis-mounted components are

accomplished with removable connectors.

Figure 3.1 Component layout for the preamplifier PWB.

17

WhitakerAudio

The wire connection codes from the PWB to the chassis components are given in Table 3.1

Table 3.1 PWB Connection Wiring CodesConnector # PWB Code Function Connection Color Code

Conn1 GND Phonograph input ground Shielded cable, shield

PHONO Phonograph input Shielded cable, white

GND Phonograph input ground

GND Tuner input ground Shielded cable, shield

TUNER Tuner input Shielded cable, white

GND Tuner input ground

GND Auxiliary input ground Shielded cable, shield

AUX Auxiliary input Shielded cable, white

GND Auxiliary input ground

GND Microphone input ground Shielded cable, shield

MIC Microphone input Shielded cable, white

GND Microphone input ground

Conn2 GND Tape Out ground Shielded cable, shield

TAPE2 Tape Output to back panel

Shielded cable, white

GND Tape Out ground

GND Ground

Conn3 GND Phono preamp output ground

Shielded cable, shield

PH-OUT Phono preamp output to SW1


GND Phono preamp output ground

GND Ground

Conn4 GND Tuner output ground Shielded cable, shield

TUN-OUT Tuner output to SW1 Shielded cable, white

GND Tuner output ground

GND Ground

Conn5 GND Auxiliary output ground Shielded cable, shield

AUX-OUT Auxiliary output to SW1 Shielded cable, white

GND Auxiliary output ground

GND Ground

18


Conn6 GND Microphone preamp output ground


MIC-OUT Microphone preamp output to SW1


GND Microphone preamp ground

GND Ground

Conn7 GND Tape Out ground Shielded cable, shield

TAPE1 Tape Out input from SW1 Shielded cable, white

GND Tape Out ground

GND Ground

H 6.3 V ac heater to tube sockets

Green

H 6.3 V ac heater to tube sockets

Green

Conn8 GND Tone control stage ground


INPUT Tone control stage input from SW1


GND Tone control stage ground

GND Ground

Conn9 R26a High terminal of R26 Shielded cable, red

R26arm R26 wiper Shielded cable, white

R26b Low terminal of R26 Shielded cable, black

GND Shield for R26 cable Shielded cable, drain wire

R30a High terminal of R30 Shielded cable, red




R32a High terminal of R32 Shielded cable, red




Conn10 GND Buffer amplifier output ground


BUF-OUT Buffer amplifier output to back panel


GND Buffer amplifier ground

GND Buffer amplifier ground

Table 3.1 PWB Connection Wiring Codes

19

WhitakerAudio

The PWB is connected to the chassis through the mounting holes for the vacuum tube sockets.

The tube sockets are conventional chassis-mounted devices, with jumper leads extending from the

active pins to the PWB. The sockets are held away from the PWB using standoffs. This approach

permits the sockets to be firmly mounted on the chassis, rather than being physically supported by

the PWB. This mounting method also takes advantage of the shielding capability of the steel

chassis.

Heater connections to the vacuum tubes are made directly on the sockets using

interconnecting wire tightly twisted together and dressed against the chassis to minimize hum.

The pinout for the back panel power connector is given in Table 3.2.

One channel of the stereo preamplifier is shown in Figure 3.2.

Table 3.2 Back Panel Power Connector PinoutPin No. Function Connects To Notes

1 Ground Terminal 2 of PWB (black)

2 Pilot lamp PL1 115 V ac (white)

3 Pilot lamp PL1 115 V ac (white)

4 Auxiliary B+ Terminal 1 of PWB Approximately +275 V dc (red)

5 Filament Heater 6.3 V ac for heaters (green)

6 Filament Heater 6.3 V ac for heaters (green)

7 Not used

8 Not used

9 Ground Chassis ground (black)

Figure 3.2 The right channel preamplifier PWB.

20


4 Construction Techniques

Most of the construction work on this amplifier involves populating the two PWBs, and then

connecting the chassis-mounted devices to the right and left channel PWBs.

There is no mystery to populating a PWB. Having said that, some general guidelines are

worth noting:

• Proceed in a logical manner, usually beginning with the smallest components; these are

typically resistors. Begin at R1 and move through to Rx.

• As each component is installed, check it off the list before moving on.

• Confirm the value of each component before it is installed on the PWB. While rare,

incorrect packaging or labeling of components can occur. Therefore, verify the marked

resistor and capacitor values before installation. If you are a bit rusty on the resistor color

codes, check the resistors with an ohmmeter. Capacitors can be checked with a

capacitance measurement bridge. While this is perhaps overkill, it is instructive to see the

variation in component values, which measurement before installation will reveal. The

resistor color code is shown in Figure 4.1.

• Observe proper polarity of devices such as electrolytic capacitors and diodes. A distinctive

pad usually denotes the positive terminal, in the case of an electrolytic. The silk screen

legend may also provide guidance.

• For devices that may dissipate some amount of heat, such as 5 W resistors, allow extra

lead space between the device and the board so as to keep the device above the board. This

will minimize heat-caused damage to the PWB and provide for better cooling of the

Figures Multiplier Tolerance Temp. Coeff.(Ω) (0.10 /K)-6

10

23456789

0.010.11101001 k10 k100 k1 M10 M

10%5%

1%2%

0.5%0.25%0.1%

200100501525

1051

SilverGoldBlackBrownRedOrangeYellowGreenBlueVioletGreyWhite

Figure 4.1 Resistor color code.

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device. For resistors in the 2–5 W range, 1/4-inch should be sufficient. For higher-power

devices, more clearance should be provided.

• Use only the amount of heat and solder necessary to do the job. Once a PWB is damaged

by excessive heat it is very difficult to repair. Excessive solder flux on the board will tend

to attract dust, which—at high voltages—can lead to arc-over problems.

• Inspect each solder connection to be certain that the mounting pad or via has been filled.

• Mount large components last. These typically include barrier strips and high-voltage

electrolytic capacitors.

All audio cables of length greater than about 6 inches should use shielded cable. Generally

speaking, the shield should be connected to ground at the load end. Grounding both ends of a

shielded cable to the same ground plane is not recommended as it may lead to circulating currents

that will adversely impact the noise floor.

Efficiently making the socket connections to the PWB requires a bit of practice. The

recommended procedure is detailed in Section 5. Be certain to wear protective eye glasses during

this process, and at other steps in PWB assembly.

22


5 Step-by-Step Instructions

The following steps are recommended for assembly of the stereo preamplifier. As each step is

completed, check it off the list. The project is divided into three major elements—assembly of the

right channel PWB, assembly of the left channel PWB, and assembly of the chassis. Builders are

encouraged to follow the specific instructions provided in the following sections.

An examination of the shipping box will show that the right channel and left channel parts are

contained in separate boxes. It is recommended that only one set of components be opened at a

time as this will reduce the possibility of component mix-ups.

Each section heading that follows marks a logical start/stop point in construction of the

amplifier. Do not rush through the project; instead, consider setting modest goals for progress

over a period of some days. A more relaxed pace often results in a better end-product, and a more

enjoyable overall experience.

Organize a work space that provides ample room and light. The following tools are minimally

required for assembly:

• Common screwdriver

• Phillips screwdriver

• Wire cutters

• Pliers

• Long-noise pliers

• Solderless terminal crimp tool

• 25–35 W soldering iron

• Solder

• Heat-shrink gun is recommended, although not strictly required.

While there are certainly a number of approaches to building a vacuum tube product, the

following procedure has proven to be efficient and repeatable.

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5.1 Assembly of Right Channel PWB

Check the shipping box for the major component groups. Find the right channel preamplifier box.

Set the other boxes aside for now. Organize the components for the right channel preamp in a

logical manner, grouping resistors together, capacitors together, and so on. Note that a schematic

diagram of the right channel preamplifier block is provided, along with a copy of the PWB

component layout. The component layout sheet is a useful reference as many of the part numbers

are obscured when the components are installed. The estimated time required to complete

assembly of the right channel PWB is 4 hours.

After reviewing the Construction Techniques section of this manual, begin populating the

right channel PWB. As the packing envelopes are used, set them aside for recycling. Before

beginning, clean the soldering iron tip.

– Install R1 – 47 kΩ, 0.5 W. Solder in place, leaving some extra lead space so as to place

the resistor about 1/4-inch above the board. This will facilitate replacement should it be

necessary to use a different value of magnetic cartridge load resistor.

Caution: leads may tend to fly toward the face when they are cut. Wear protective glasses. For

all steps that follow, excess lead length should be trimmed after the component is soldered in

place.

– Install R2 and R7 – 2.7 kΩ, 0.5 W. Solder in place.

– Install R3, R5, R12, R25, R28, R31, R35, R37, R39, and R45 – 100 kΩ, 0.5 W. Solder in

place.

– Install R4 – 39 kΩ, 0.5 W. Solder in place.

– Install R6, R14, and R20 – 470 kΩ, 0.5 W. Solder in place.


– Install R9, R15, R29, R38, and R41 – 15 kΩ, 2 W. Position the components about 1/2-

inch above the board and solder in place.

– Install R10 and R23 – 22 kΩ, 0.5 W. Solder in place.

– Install R11 – 2.2 mΩ, 0.5 W. Solder in place.

24


– Install R13, R24, R36, and R46 – 1 kΩ, 0.5 W. Solder in place.

– Install R21 – 1.5 kΩ, 0.5 W. Solder in place.




Find the potentiometers (R16, R17, R18, R19, and R40). Remove the mounting nut and

lockwasher from each device. Inspect one of the devices. Note that a mounting “key” tab is

included on one side of the component. Using a pair of wire cutters, clip-off the tab so that the

device will mount flush against the circuit board. Note also that the leads from the device will

need to be reoriented in order to mount properly on the board. Bend the three leads straight down

toward the shaft of the device. Using the long-noise pliers, spread the outer two leads slightly

away from the center lead. Fit the shaft of the device through the mounting hole and then guide

the three leads through the PWB pads. When viewed from the component side of the board, the

leads of the device should protrude a short distance above the board.

– Install R16, R17, and R40 – 250 kΩ potentiometer, linear. Secure with the hardware

provided. Solder the leads in place from the component side of the board.

– Install R18 and R19 – 100 kΩ potentiometer, linear. Secure with the hardware provided.

Solder the leads in place from the component side of the board.

Find the 100 Ω potentiometer. Inspect the body of the device and find the mounting tab on one

side. Using a pair of long-nose pliers, bend the tab down toward the device. Remove the mounting

hardware.

– Install R42 – 100 Ω, 1 W, potentiometer, linear. Connect a 1-inch piece of bare wire to

each terminal of the device and solder. Fit the leads through the PWB pads and then fit the

shaft of the device through the mounting hole. Secure with the hardware provided (the

locking ring nut should be finger-tight). Solder the leads in place from the component side

of the board.

– Install R47, 100 kΩ, 2W. Solder in place, leaving approximately 1/2-in spacing above the

board.

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All resistors have now been placed on the right channel PWB. Clean the solder iron tip before

proceeding with placement of the capacitors

– Install C1, C4, C11, C24, and C31 – 25 µF, 50 V, electrolytic. Solder in place. Be certain

to observe proper polarity.

– Install C2, C5, C15, C17, C26, C28, and C29 – 22 µF, 450 V, electrolytic. Solder in

place.

– Install C3 and C16 – 0.1 µF, 400 V. Solder in place.

– Install C6 – 0.0033 µF, 600 V. Solder in place.


– Install C8 – 180 pF, 500 V, mica. Solder in place.

– Install C9, C12, C20, C25, C32 – 0.22 µF, 400 V. Solder in place.

– Install C10, C14, C33 – 0.047 µF, 400 V. Solder in place.

– Install C13 – 47 µF, 450 V, electrolytic. Solder in place. Be certain to observe proper

polarity.



– Install C21 – 220 pF, 600 V, ceramic. Solder in place.

– Install C23 and C30 – 100 pF, 1000 V disc. Solder in place.

– Install C27 – 0.01 µF disc, 1 kV. Solder in place.

– Find CP – 25 pF, 1000 V ceramic. Set aside for now. This capacitor may be installed

following frequency response tests to pad out the high frequency response of the phono

preamp to more closely match the RIAA transfer curve.

All capacitors have now been installed on the board. Clean the soldering iron tip before

proceeding.

26


Note that D1 and D2 diodes installed in the steps that follow are sensitive to static discharge.

Take appropriate measures to minimize the possibility of component damage due to static

electricity. Note that the electrostatic protection packaging is not recyclable.

– Install D1 and D2 – zener diode, 200 V, 5W. Position each device about 1/2-inch above

the board. Solder in place. Be certain to observe proper polarity.

– Install the 4-terminal Molex terminal strip. Solder in place.

– Install the 12 pin miniature Molex connector headers (male) at the following positions:

Conn1 and Conn9. Note that the pins are closely spaced. Be careful to not create a solder

bridge between pads. Solder in place.

– Install the 4-pin miniature Molex connector headers (male) at the following positions:

Conn2, Conn3, Conn4, Conn5, Conn6, Conn7, Conn8, and Conn10. Solder in place. Note

that it will be necessary to hold the headers in place from the component side of the board

until the first connection point is soldered. Be careful to not create a solder bridge between

pads.

– Mount the standoffs on the foil side of the board using 4-40 hardware provided. Use a

lock-washer on the screw head (component side) of the board. Standoffs are placed on

each side of the five tube sockets. The screws should be finger-tight for now.

– Find the five tube sockets. On three of the sockets, use a short piece of bare #22 hookup

wire (found in the Tube Socket Hookup Wire package) to connect pins 4 and 5. Crimp in

place. Solder only the pin 5 connections.

– Mount the three sockets with the jumper wires between pins 4 and 5 on the standoffs for

tubes V1, V4, and V5 (the 7025 devices). Pin 1 of the socket can be identified by a number

stamped on the bottom side of the socket. Pin 1 on the PWB is identified by a square pad.

Place the socket in the proper orientation over the PWB so it rests on the standoffs. The

top of the socket (the side the tube plugs into) should face outward. Loosely tighten the

mounting screws. See Figure 5.1a.

– Repeat this procedure for the remaining tube sockets (those with no jumpers between pins

4 and 5), which are V2 and V3.

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Note that V3 does not have a square pad on the board denoting pin #1 because for the 6EU7

pin #1 is one of the heater connections (which are wired off the board). Use the same orientation

for V3 as for the other tubes on the board.

In the following steps the tube sockets will be connected. These steps will require a

considerable amount of time and patience. Successful completion is critical to proper operation of

the amplifier.

– Install connecting wires from the PWB pads to the tube socket pins. Solder in place.

Materials for these steps can be found in the “Tube Socket Hookup” package. Note that

the PWB contains pads only for the active pins used on the socket. The heater connections

are made separately. The recommended procedure for installing the connecting wires is as

follows:

(a) (b) (c)

(d)

Figure 5.1 Installation of sockets on the PWB: (a) mount the socket on the standoffs, (b) insert connecting wires from the PWB to the socket, (c) board-to-socket connections completed, (d)

finished job with heater wires installed.

28


1) Feed a bare wire from the component side through the pad and into the socket

soldering tab.

2) Crimp in place (see Figure 5.1b).

3) From the component side, pull the wire tight and solder in place.

4) Trim the excess lead length.

5) Solder the socket tab.

Table 5.1 shows the pins that should be connected to the PWB.

– Repeat the connection process for each PWB-to-socket connection listed in Table 5.1,

being careful to match up the right pad on the board with the right connection on the

socket. (See Figure 5.1c.) This is best done by starting at pin 1 on one side of the socket,

and on pin 9 on the other side. Check to make sure sufficient clearance is provided for

wires and connecting points on the sockets.

After all of the connecting wires from the PWB to the sockets have been installed, double-

check your work. Make sure all of the appropriate pins have been soldered. Note that no

connections are made to the PWB pad in the center of the tube sockets.

The final step in assembly of the right channel PWB is connection of the heater pins.

– Unscrew and remove the standoff adjacent to pins 4 and 5 on the V1 tube socket. This will

facilitate access to the pins.

– Open the “Heater Circuit Wire” package and cut two 7-inch pieces of #22 green heater

wire. Place one piece of heat-shrink tubing over one end of each wire.

Table 5.1 PWB to Tube Socket ConnectionsSocket No. V1, 7025 V2, 5879 V3, 6EU7 V4, 7027 V5, 7025

Pin 1 yes yes no, heater yes yes

Pin 2 yes no, NC no, heater yes yes

Pin 3 yes yes no, NC yes yes

Pin 4 no, heater no, heater yes no, heater no, heater

Pin 5 no, heater no, heater yes no, heater no, heater

Pin 6 yes no, NC yes yes yes

Pin 7 yes yes yes yes yes

Pin 8 yes yes yes yes yes

Pin 9 no, heater ct. yes yes no, heater ct. no, heater ct.

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– Connect one wire to pin 4 of V1. Crimp and then solder (note there is already one wire on

this pin, placed there before the socket was installed). When soldering, make sure the heat-

shrink tubing is pushed back on the wire so it is away from the soldering point. If the heat-

shrink tubing gets hot it will be very difficult to slide into position later.

– Move the heat-shrink tubing along the wire to the base of the pin. Apply a stream of hot air

to shrink the tubing over the wire. Be careful to not apply too much heat.

– Replace the standoff removed from the V1 socket. The screws should be finger-tight.

Remember to include the lockwasher on the component side of the board.

– Crimp the other green wire on pin 9 of V1. Solder in place, keeping in mind the caution

about the heat-shrink tubing.

– Slide the heat-shrink tubing into position at the base of the socket. Apply a stream of hot

air to shrink the tubing over the pins. Be careful to not apply too much heat.

– Twist the wires together and route to socket V2. The wires should be placed at socket-

level. In this way they will be resting against the chassis when the board is inserted in the

chassis. Do not dress the wires against the PWB as this may increase hum in the output of

the preamplifier.



– Trim the wire lengths as needed to comfortably reach V2. As before, slip a piece of heat-

shrink tubing over each wire. Crimp one wire on pin 5, and then crimp the other on pin 4.

Do not solder.

– Cut two 7-inch pieces of #22 green heater wire. Slip one through the heat-shrink tubing on

the lead already connected to pin 5 of V2. Route the wire to the pin and crimp in place.

Solder, keeping the heat-shrink tubing away from the hot wires and the soldering iron.

Slip the other wire through the heat-shrink tubing on the lead already connected to pin 4 of

V2. Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing

away from the hot wires and the soldering iron.

30



air to shrink the tubing over the pins. Be careful to not use too much heat.

– Replace the standoff removed from the V2 socket. The screws should be finger-tight. The

heater leads should straddle the standoff. Remember to include the lockwasher on the

component side of the board.


level. Do not dress the wires against the PWB.



– Trim the wire lengths as needed to comfortably reach V5. Note that one lead goes to pin 4

and the other to pin 9. As before, slip a piece of heat-shrink tubing over each wire.

– Connect one wire to pin 4 of V5. Crimp in place (note there is already one wire on this pin,

placed there before the socket was installed). Crimp the other green wire to pin 9. Do not

solder.

– Cut two 7-inch pieces of green heater wire. Slip one through the heat-shrink tubing on the

lead already connected to pin 4 of V5. Route the wire to the pin and crimp in place. Solder,

keeping the heat-shrink tubing away from the hot wires and the soldering iron. Slip the

other wire through the heat-shrink tubing on the lead already connected to pin 9 of V5.

Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing away

from the hot wires and the soldering iron.





– Twist the wires together and route to the H pads on the PWB. The wires should be placed

at socket-level. Do not dress the wires against the PWB.

– Trim the wire lengths as needed to comfortably reach the pads on the foil side of the

board. Insert one wire in each H position. Do not solder.

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– Prepare two 7-inch pieces of #22 green heater wire. Place one piece of heat-shrink tubing

over one end of each wire.

– Connect one wire to pin 1 of V3. Crimp and then solder. When soldering, make sure the

heat-shrink tubing is pushed back on the wire so it is away from the soldering point. If the

heat-shrink tubing gets hot it will be very difficult to slide into position later. Connect the

other green wire to pin 2 of V3. Crimp and then solder.

– Move the heat-shrink tubing along the wire to the base of the pins. Apply a stream of hot

air to shrink the tubing over the wire. Be careful to not apply too much heat.




the preamplifier.




shrink tubing over each wire. Crimp one wire on pin 4 (note there is already one wire on

this pin, placed there before the socket was installed). Crimp the other wire on pin 9. Do

not solder.











32





board. Insert one wire in each H position (there will be two green wires in each H pad.

– Solder both H pads from the foil side of the board. Time off any excess lead length.

– Find tubes V1, V2, V3, V4, and V5. Set them aside in a safe place. The tubes will be

installed during the final checkout process.

– Check the PWB for proper installation of all components. Look for bad solder connections

or a solder splash that might cause a short-circuit.

Assembly of the right channel PWB has now been completed. The finished board is shown in

Figure 5.2. Set the board aside; it will be used later.

33

34

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Figure 5.2 Preamp PWB: (a) component side, (b) foil side.

a

35


b

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5.2 Assembly of the Left Channel PWB

Check the shipping box for the major component groups. Find the left channel preamplifier box.

Set the other boxes aside for now. Organize the components for the left channel preamp in a

logical manner, grouping resistors together, capacitors together, and so on. Note that a schematic

diagram of the left channel preamplifier block is provided, along with a copy of the PWB

component layout. The component layout sheet is a useful reference as many of the part numbers

are obscured when the components are installed. The estimated time required to complete

assembly of the left channel PWB is 4 hours.

After reviewing the Construction Techniques section of this manual, begin populating the left

channel PWB. As the packing envelopes are used, set them aside for recycling. Before beginning,

clean the soldering iron tip.

– Install R1 – 47 kΩ, 0.5 W. Solder in place, leaving some extra lead space so as to place

the resistor about 1/4-inch above the board. This will facilitate replacement should it be

necessary to use a different value of magnetic cartridge load resistor.

Caution: leads may tend to fly toward the face when they are cut. Wear protective glasses. For

all steps that follow, excess lead length should be trimmed after the component is soldered in

place.

– Install R2 and R7 – 2.7 kΩ, 0.5 W. Solder in place.

– Install R3, R5, R12, R25, R28, R31, R35, R37, R39, and R45 – 100 kΩ, 0.5 W. Solder in

place.


– Install R6, R14, and R20 – 470 kΩ, 0.5 W. Solder in place.


– Install R9, R15, R29, R38, and R41 – 15 kΩ, 2 W. Position the components about 1/2-

inch above the board and solder in place.

– Install R10 and R23 – 22 kΩ, 0.5 W. Solder in place.

– Install R11 – 2.2 mΩ, 0.5 W. Solder in place.

36



– Install R21 – 1.5 kΩ, 0.5 W. Solder in place.




Find the potentiometers (R16, R17, R18, R19, and R40). Remove the mounting nut and

lockwasher from each device. Inspect one of the devices. Note that a mounting “key” tab is

included on one side of the component. Using a pair of wire cutters, clip-off the tab so that the

device will mount flush against the circuit board. Note also that the leads from the device will

need to be reoriented in order to mount properly on the board. Bend the three leads straight down

toward the shaft of the device. Using the long-noise pliers, spread the outer two leads slightly

away from the center lead. Fit the shaft of the device through the mounting hole and then guide

the three leads through the PWB pads. When viewed from the component side of the board, the

leads of the device should protrude a short distance above the board.

– Install R16, R17, and R40 – 250 kΩ potentiometer, linear. Secure with the hardware

provided. Solder the leads in place from the component side of the board.

– Install R18 and R19 – 100 kΩ potentiometer, linear. Secure with the hardware provided.

Solder the leads in place from the component side of the board.

– Install R47, 100 kΩ, 2W. Solder in place, leaving approximately 1/2-in spacing above the

board.

All resistors have now been placed on the right channel PWB. Clean the solder iron tip before

proceeding with placement of the capacitors

– Install C1, C4, C11, C24, and C31 – 25 µF, 50 V, electrolytic. Solder in place. Be certain

to observe proper polarity.

– Install C2, C5, C15, C17, C26, C28, and C29 – 22 µF, 450 V, electrolytic. Solder in

place.


37

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– Install C8 – 180 pF, 500 V, mica. Solder in place.

– Install C9, C12, C20, C25, C32 – 0.22 µF, 400 V. Solder in place.

– Install C10, C14, C33 – 0.047 µF, 400 V. Solder in place.

– Install C13 – 47 µF, 450 V, electrolytic. Solder in place. Be certain to observe proper

polarity.



– Install C21 – 220 pF, 600 V, ceramic. Solder in place.

– Install C23 and C30 – 100 pF, 1000 V disc. Solder in place.

– Install C27 – 0.01 µF disc, 1 kV. Solder in place.

– Find CP – 25 pF, 1000 V ceramic. Set aside for now. This capacitor may be installed

following frequency response tests to pad out the high frequency response of the phono

preamp to more closely match the RIAA transfer curve.

All capacitors have now been installed on the board. Clean the soldering iron tip before

proceeding.

Note that D1 and D2 diodes installed in the steps that follow are sensitive to static discharge.

Take appropriate measures to minimize the possibility of component damage due to static

electricity. Note that the electrostatic protection packaging is not recyclable.

– Install D1 and D2 – zener diode, 200 V, 5W. Position each device about 1/2-inch above

the board. Solder in place. Be certain to observe proper polarity.

– Install the 4-terminal Molex terminal strip. Solder in place.

– Install the 12 pin miniature Molex connector headers (male) at the following positions:

Conn1 and Conn9. Note that the pins are closely spaced. Be careful to not create a solder

bridge between pads. Solder in place.

38


– Install the 4-pin miniature Molex connector headers (male) at the following positions:

Conn2, Conn3, Conn4, Conn5, Conn6, Conn7, Conn8, and Conn10. Solder in place. Note

that it will be necessary to hold the headers in place from the component side of the board

until the first connection point is soldered. Be careful to not create a solder bridge between

pads.

– Mount the standoffs on the foil side of the board using 4-40 hardware provided. Use a

lock-washer on the screw head (component side) of the board. Standoffs are placed on

each side of the five tube sockets. The screws should be finger-tight for now.

– Find the five tube sockets. On three of the sockets, use a short piece of bare #22 hookup

wire (found in the Tube Socket Hookup Wire package) to connect pins 4 and 5. Crimp in

place. Solder only the pin 5 connections.

– Mount the three sockets with the jumper wires between pins 4 and 5 on the standoffs for

tubes V1, V4, and V5 (the 7025 devices). Pin 1 of the socket can be identified by a number

stamped on the bottom side of the socket. Pin 1 on the PWB is identified by a square pad.

Place the socket in the proper orientation over the PWB so it rests on the standoffs. The

top of the socket (the side the tube plugs into) should face outward. Loosely tighten the

mounting screws. See Figure 5.1a.

– Repeat this procedure for the remaining tube sockets (those with no jumpers between pins

4 and 5), which are V2 and V3.

Note that V3 does not have a square pad on the board denoting pin #1 because for the 6EU7

pin #1 is one of the heater connections (which are wired off the board). Use the same orientation

for V3 as for the other tubes on the board.

In the following steps the tube sockets will be connected. These steps will require a

considerable amount of time and patience. Successful completion is critical to proper operation of

the amplifier.

– Install connecting wires from the PWB pads to the tube socket pins. Solder in place.

Materials for these steps can be found in the “Tube Socket Hookup” package. Note that

the PWB contains pads only for the active pins used on the socket. The heater connections

39

WhitakerAudio

are made separately. The recommended procedure for installing the connecting wires is as

follows:

1) Feed a bare wire from the component side through the pad and into the socket

soldering tab.

2) Crimp in place (see Figure 5.1b).

3) From the component side, pull the wire tight and solder in place.

4) Trim the excess lead length.

5) Solder the socket tab.

Table 5.1 shows the pins that should be connected to the PWB.

– Repeat the connection process for each PWB-to-socket connection listed in Table 5.1,

being careful to match up the right pad on the board with the right connection on the

socket. (See Figure 5.1c.) This is best done by starting at pin 1 on one side of the socket,

and on pin 9 on the other side. Check to make sure sufficient clearance is provided for

wires and connecting points on the sockets.

After all of the connecting wires from the PWB to the sockets have been installed, double-

check your work. Make sure all of the appropriate pins have been soldered. Note that no

connections are made to the PWB pad in the center of the tube sockets.

The final step in assembly of the right channel PWB is connection of the heater pins.



– Open the “Heater Circuit Wire” package and cut two 7-inch pieces of #22 green heater

wire. Place one piece of heat-shrink tubing over one end of each wire.

– Connect one wire to pin 4 of V1. Crimp and then solder (note there is already one wire on

this pin, placed there before the socket was installed). When soldering, make sure the heat-

shrink tubing is pushed back on the wire so it is away from the soldering point. If the heat-

shrink tubing gets hot it will be very difficult to slide into position later.

– Move the heat-shrink tubing along the wire to the base of the pin. Apply a stream of hot air

to shrink the tubing over the wire. Be careful to not apply too much heat.

40




– Crimp the other green wire on pin 9 of V1. Solder in place, keeping in mind the caution

about the heat-shrink tubing.


air to shrink the tubing over the pins. Be careful to not apply too much heat.




the preamplifier.




shrink tubing over each wire. Crimp one wire on pin 5, and then crimp the other on pin 4.

Do not solder.









– Replace the standoff removed from the V2 socket. The screws should be finger-tight. The

heater leads should straddle the standoff. Remember to include the lockwasher on the

component side of the board.


level. Do not dress the wires against the PWB.

41

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– Trim the wire lengths as needed to comfortably reach V5. Note that one lead goes to pin 4

and the other to pin 9. As before, slip a piece of heat-shrink tubing over each wire.

– Connect one wire to pin 4 of V5. Crimp in place (note there is already one wire on this pin,

placed there before the socket was installed). Crimp the other green wire to pin 9. Do not

solder.

– Cut two 7-inch pieces of green heater wire. Slip one through the heat-shrink tubing on the

lead already connected to pin 4 of V5. Route the wire to the pin and crimp in place. Solder,

keeping the heat-shrink tubing away from the hot wires and the soldering iron. Slip the

other wire through the heat-shrink tubing on the lead already connected to pin 9 of V5.

Route the wire to the pin and crimp in place. Solder, keeping the heat-shrink tubing away

from the hot wires and the soldering iron.








board. Insert one wire in each H position. Do not solder.

– Prepare two 7-inch pieces of #22 green heater wire. Place one piece of heat-shrink tubing

over one end of each wire.

– Connect one wire to pin 1 of V3. Crimp and then solder. When soldering, make sure the

heat-shrink tubing is pushed back on the wire so it is away from the soldering point. If the

heat-shrink tubing gets hot it will be very difficult to slide into position later. Connect the

other green wire to pin 2 of V3. Crimp and then solder.

42


– Move the heat-shrink tubing along the wire to the base of the pins. Apply a stream of hot

air to shrink the tubing over the wire. Be careful to not apply too much heat.




the preamplifier.




shrink tubing over each wire. Crimp one wire on pin 4 (note there is already one wire on

this pin, placed there before the socket was installed). Crimp the other wire on pin 9. Do

not solder.














board. Insert one wire in each H position (there will be two green wires in each H pad.

– Solder both H pads from the foil side of the board. Time off any excess lead length.

43

WhitakerAudio

– Find tubes V1, V2, V3, V4, and V5. Set them aside in a safe place. The tubes will be

installed during the final checkout process.

– Check the PWB for proper installation of all components. Look for bad solder connections

or a solder splash that might cause a short-circuit.

Assembly of the left channel PWB has now been completed. The finished board is shown in

Figure 5.2. Note that the right channel and left channel PWBs are identical, except that the hum

balance potentiometer is not installed on the left channel PWB. Set the board aside; it will be used

later.

44


5.3 Chassis and Final Assembly

Unpack the chassis components and position on the work surface. Place a towel or other soft

covering on the work surface to avoid scratching the finish. Note there may be some rough edges

on the drill holes and cutouts. Be careful to avoid cuts or nicks.

– Find the chassis bottom plate and feet hardware. Install using the hardware provided. Note

that four self-tapping #6 screws are provided. Save these for the final step of assembly—

mounting the bottom plate to the chassis.

– Find the side chassis handles and install using the #8 hardware provided. Use the handles

during the following construction steps to ensure a good grip on the chassis.

The estimated time for completion of the following steps is about 6 hours. The chassis and

final assembly process is divided into several sub-sections, allowing the builder to proceed at a

measured pace. Figure 5.4 shows the overall placement of chassis components.

5.3.1 Front Panel Components

Collect the front panel components, which include the following:

• Front panel handles

• Three dual potentiometers (R26, R30, and R32)

• Rotary switch (SW1)

• Pilot lamp (PL1)

• 3.5 mm jack

• Front panel knobs

– Install the front panel handles using the #8 hardware provided. Be careful to not scratch

the finish. Note that the front panel handles are decorative and should not be used to move

the preamplifier after it is has been completed.

– Find the two 1 mΩ dual potentiometers (Bass, Treble). Using a pair of long-nose pliers,

bend the three terminal groups slightly toward the back of the potentiometer. Note that

there is a key tab at the base of the potentiometer. Using pliers, bend this tab down toward

the face of the component.

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WhitakerAudio

– Mount the two 1 mΩ dual potentiometers as follows. Place the lock washers provided on

each shaft. Feed the component through the chassis/front panel. Add a flat washer and

secure with the nut provided. Do not tighten. The terminals of the potentiometers should

face downward toward the chassis bottom plate.

– Find the 250 kΩ dual potentiometer (Volume). Using a pair of long-nose pliers, bend the

three terminals slightly toward the back of the potentiometer. Note that there is a key tab at

the base of the potentiometer. Using pliers, bend this tab down toward the face of the

component.

VolumeInputPowerBassTrebelPhonograph

Microphone

Tuner

Auxiliary

Front Aux

Stereo Preamp J C Whitaker 2012

Front Aux

Right Left Right Left Right Left Right Left Right Left Right Left

Auxiliary Power ConnectorDanger: High Voltages

Do Not Probe

WhitakerAudio Stereo Preamplifier Built in California, USA.Phonograph Microphone Tuner Auxiliary Tape Output Line Output

MicMix

StereoMic

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

TE shell 17

Center

0.15-in

1.12-in

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix Combo

0.88-in

0.13-in

Center

Neutrix Combo

0.88-in

0.13-in

Center

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

TE shell 17

Cable clamp

Cable clamp

Cable clamp

Cable clamp

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

Neutrix N

F2D

Side view

R30 F26SW1

R32Pilotlamp

a arm b a arm b a arm b

Figure 5.3 Chassis layout (bottom view).

46


– Mount the 250 kΩ dual potentiometer as follows. Place the lock washers provided on the

shaft. Feed the component through the chassis/front panel. Add the flat washer and secure

with the nut provided. Do not tighten. The terminals of the potentiometer should face

downward toward the chassis bottom plate.

Find rotary switch SW1. Find the front panel knobs. Remove one of the knobs from the

package and attach to the shaft of SW1 (the set screw should work against the flattened side of the

shaft). Turn the switch shaft and observe the number of positions the switch is set for. (Hold the

body of the switch with a rag to avoid the sharp edges of the terminal posts.) If five positions are

found, then the switch is configured correctly. For some number of positions other than five,

remove the knob and remove the mounting nut on the shaft of SW1. You will note there is a keyed

“stop” on the shaft. This stop determines the number of positions that the switch has. Pull the stop

up and move it to a higher position. Replace the nut and knob and recheck.

After the switch position stop has been properly set, remove the knob and observe the body of

the switch. There is mounting tab on one side of the body. Using a pair of wire cutters, remove the

tab.

– Place the lockwasher on the shaft of SW1 and mount the switch using the flat washer and

nut provided. The nut should be finger-tight for now.

– Mount pilot lamp PL1 as follows. Thread the large mounting nut on the pilot lamp and run

down to the base of the threads. Place the lock washer provided on the shaft. Mount the

pilot lamp to the chassis/panel and secure with the decorative nut provided. Run the

decorate nut down to be approximately flush with the mounting shaft. From the underside

of the chassis, spin the large mounting nut up towards the chassis. Do not tighten.

– Find the 3.5 mm Front Panel Aux jack. There is a terminal at the base of the jack near the

mounting shaft. This is the ground terminal. Bend this terminal back slightly toward the

rear of the device. This will make it easier to connect the proper wires to the terminal later

in the assembly process.

– Mount the Front Panel Aux jack to the chassis/panel and secure using the washer and nut

provided. Tighten by carefully rotating the jack body. Position the ground terminal so it

faces one side of the chassis. This will make it easier to connect later. When properly

47

WhitakerAudio

mounted, the nut will be flush with the top of the shaft. Do not over-tighten as this may

damage the body of the jack.

– Using caution to not scratch the finish, tighten the three potentiometer mounting nuts. Be

certain that the terminals face downward toward the chassis bottom plate.

– Working from the chassis side, tighten the pilot lamp mounting nut. Position the terminals

of the device so they are parallel with the chassis top. This will make it easier to connect

wires later in the process.

– Move switch SW1 to the extreme counter-clockwise position using pliers while holding

the body of the device. Note the flattened side of the shaft. The switch should be oriented

so the flat side of the shaft points the knob to the first position on the front panel Input

switch (Phonograph). Once in position, tighten the mounting nut. Attach the knob and

check the positioning through the range of inputs. Adjust the switch as needed. Be careful

to not scratch the finish.

– Install the Volume Control knob. Using a small common screwdriver, run the set screw

out so the knob will fit on the shaft. Set the potentiometers to its full counterclockwise

position. Place the knob on the shaft and set to the 7 o’clock position (approximately).

Tighten the set screw.

– Install the Bass and Treble control knobs. Using a small common screwdriver, run the set

screws out so the knobs will fit on the shaft. Set the potentiometers to their approximate

center point. Place the knobs on the shafts and set to the 12 o’clock position

(approximately). Tighten the set screws.

Figure 5.4 Front panel of the stereo preamplifier.

48


The 12 o’clock position denotes flat response for the Bass and Treble controls. A final

adjustment of these knobs will be made during performance testing.

The front panel components have been installed. The completed front panel is shown in

Figure 5.4.

5.3.2 Back Panel Components

Collect the back panel components, which include the following:

• RCA input jacks (quantity = 10)

• Microphone (1/4-inch) input jacks (quantity = 2)

• Auxiliary power connector

• Microphone mix switch

• Ground lug hardware

• Back panel decals

Examine the back panel cutouts. Note there are 13 large holes. One is for the power connector,

two are for the microphone inputs and ten are for the RCA jacks. The power connector and

microphone input jacks are larger in size than the RCA jacks.

– Mount the ten RCA input jacks using the hardware provided. Orient the components so

the ground lug faces downward toward the chassis bottom plate. Leave the mounting

screws loose for now.

Open the Power Connector Hardware package and remove the chassis-mounted connector

body and associated mounting screws. Reseal the package and set it aside for now.

– Mount the external power supply connector using the hardware provided. The socket

mounting pad rests against the outside of the back panel. Note the pin numbers printed on

the connector side of the socket. Orient the socket so the lettering is upright when viewed

from the rear panel.

– Mount the two microphone jacks using the hardware provided. Leave the mounting

screws loose for now.

Note that the standard connector jack for the microphone inputs is a 1/4-inch device. This

connector may optionally be supplied as an RCA input. For the optional device, mount the

connector in the same manner as the other RCA jacks.

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WhitakerAudio

– Mount the ground lug terminal. Place the three ground lugs on to the 3/4-inch long #6

screw provided. Using a pair of long-nose pliers, bend the ends of the three lug upward

(toward the head of the screw). Insert the screw into the back panel hole so the screw

protrudes out of the back chassis. Install a lock washer on the screw, and then the lock-

tight nut. Spread the ground lugs on the inside of the chassis so they can be used

individually. Tighten the lock-tight nut. Install two flat washers on the screw and then the

wing nut. Tighten the wing nut finger-tight. The finished installation is shown in Figure

5.5.

Find toggle switch SW2. Open the package and remove the mounting hardware. Thread one

mounting nut on the shaft and run it down about 1/4-inch toward the body of the switch. Place the

lock washer on the shaft.

– Install switch SW2 and secure with the available washer and mounting nut. Note there is a

tab on one end of the washer. The tab should face up away from the chassis. Orient the

switch so the switch handle throws from side-to-side.

– Install the 17-inch long back panel decal. The top of the decal is aligned with the top of the

chassis.

– Install the two small decals adjacent to the Mic Mix switch, SW2. The “GND / Mix

Stereo” decal should be placed on the left hand side of the switch. The “Mic Mix (Mono)”

decal goes on the right.

– Tighten the mounting screws on the power connector, RCA connectors, and mic

connectors. Note there is some amount of adjustment range for each connector. To the

extent possible, align the top edge of the connector so it is parallel (or just touches) the top

(a) (b)

Figure 5.5 Installation of the ground lug terminal: (a) inside chassis, (b) outside chassis.

50


decal. For the best fit against the chassis, tighten alternate screws in stages; i.e., tighten

one side by a small amount, then tighten the other side, etc.

The back panel components have now been installed. The back panel is shown in Figure 5.6.

5.3.3 Cable Clamps and Related Hardware

The cable clamp mounting screws will not be accessible after the PWBs have been installed.

Therefore, they need to be installed first. The screws that hold the cable clamps also serve as

mounting posts for the Plexiglas cover. See the chassis layout drawing in Figure 5.3. Note the

locations of the cable clamps.

– Find the Cable Management Hardware package. There are four sets of mounting

hardware. Install using the hardware provided. Orient the cable clamps as shown in Figure

5.3. The proper order of hardware is as follows (from bottom of chassis to top): screw

head / flat washer / cable clamp / chassis / lock washer / standoff / chrome spacer / flat

washer / screw head. The Plexiglas cover will be installed later.

– Apply the top chassis decal. The decal should be placed adjacent to the front panel in the

center of the chassis.

5.3.4 Install the PWBs

This step requires care and patience. Do not rush installation of the PWBs. Check the foil side of

the boards. Make sure the green heater wires are placed at socket level and are clear of the sockets

so they are not pinched when the boards are mounted to the chassis.

Figure 5.6 Back panel of the stereo preamplifier.

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WhitakerAudio

– Locate the right channel preamp PWB. Remove the screws that secure the sockets to the

standoffs. Set these in a convenient place.

– Carefully place the right channel preamplifier PWB in position from the underside of the

chassis.

– Install the 4-40 x 3/8-inch mounting screws from the top of the chassis to the socket

standoffs on the circuit board. Some adjustment may be needed for the sockets to achieve

proper alignment. Do not tighten the screws until all screws have been threaded and the

sockets properly extend through the chassis cutouts so they are flush with the top of the

chassis. Work carefully to guide the sockets and mounting screws into position. Be careful

to avoid scratching the chassis top finish.

Note that tube shields may optionally be used on the turntable and microphone preamp tubes,

V1 and V2, respectively. Improved noise performance will be realized with the sockets in place.

The performance measurements detailed in Section 7 of this manual assume that shields are not

used. If the user wishes to use the shields, they are installed at this point on the V1 and V2

sockets.

– Repeat this procedure for the left channel PWB.

Sockets properly mated to the chassis are shown in Figure 5.7.

5.3.5 Back Panel Audio Connections

Find the shielded audio cable. There are two types of cables: one has one conductor plus the

shield and the other has three conductors plus the shield. Unless otherwise noted, use the one-

conductor cable. For the single-conductor cable, the white wire is the signal-carrying wire, and

the shield wire is ground.

Figure 5.7 Mounting of the PWB sockets to the chassis.

52


In the following steps, 4-pin or 12-pin Molex connectors will be used. After stripping the

cable, place a short (approximately 3/4-inch long) piece of heat-shrink tubing on each end of the

audio cable. For the end of the cable inserted into the Molex connector, process (apply heat) after

the wires have been inserted into the connector but before the cable is installed in the amplifier.

The length of the shielded audio cable used in the preamplifier will impact the high frequency

response of the amplifier under certain conditions. For this reason, the cable lengths are kept as

short as possible. Also, to ensure that the frequency response of the channels track closely, the

cable lengths are specified to be identical between channels for similar functions.

Hardware for the following steps can be found in the following packages: 4 Terminal Molex

Housing, 12 Terminal Molex Housing, and Molex Crimp Terminal.

Examine the Molex socket housing. The connector pins insert one way only. As each

connector pin is attached, check for a good crimp. Builders should note that the housing is held

against the PWB-mounted pins by the friction of the connector elements. In order to provide for a

good physical mating, it is recommended that any positions on the header that are not used for

wires (listed as “vacant” in the instructions that follow) be filled with a pin.

Before proceeding, clean the soldering iron tip.

– Prepare a 12-inch long piece of audio cable. Attach crimp terminals to the white and shield

wires on one end of the cable. Insert the crimped terminals into a 12 terminal Molex

header as follows:

• Shield to pin #1

• White to pin #2

• Position #3 is vacant

– Cut two 3/4-inch long pieces of heat-shrink tubing. Slip one piece over each end of the

audio cable. Process, being careful to not use too much heat.


wires on one end of the cable. Insert the crimped terminals into the 12 terminal Molex

header as follows:


• White to pin #11

53

WhitakerAudio






header as follows:


• White to pin #5




– Prepare an 8-inch long piece of audio cable. Attach crimp terminals to the white and shield


header as follows:


• White to pin #8

• Position #9 vacant



– Prepare a 2-inch long yellow wire. Connect one end of the wire to the terminal of SW2

(Mic Mix) closest to the ground lugs. Solder.

Examine the microphone input jacks. Note there are three connections, which are labeled as

follows:

• GR – ground or sleeve. This is the ground connection.

• TP – tip. This is the hot (input) connection.

• RI – ring. This connection is not used.

In the steps that follow, be certain to match the channel inputs with the correct jacks.

54


– Find the 9-inch audio cable connected to pins 10–12 of the Molex header. This is the input

to the microphone channel. Connect the free end of the cable to the right channel

Microphone input. Connect the wires as follows: shield to GR and white to TP. Solder

the GR connection. Do not solder the TP connection.

– Connect the free end of the 2-inch yellow wire to the TP pin of the right channel

Microphone input. Solder (2 wires).

– Identify the right channel Conn1 connector. Insert the 12 terminal Molex header into the

connector on the board, being careful to orient the header properly. Use the printed legend

on the PWB as a guide.

– Find the audio cable connected to pins 1–2 of Conn1 (Phono). Connect the free end of the

cable to the right channel Phonograph input jack. Solder the white wire to the center

conductor of the RCA jack. Solder the shield wire to the outer conductor. Dress the cable

against the chassis.

– Find the audio cable connected to pins 4–5 of Conn1 (Tuner). Connect the free end of the

cable to the right channel Tuner input jack. Solder the white wire to the center conductor

of the RCA jack. Solder the shield wire to the outer conductor.

– Find the audio cable connected to pins 7–8 of Conn1 (Aux). Connect the free end of the

cable to the right channel Auxiliary input jack. Solder the white wire to the center

conductor of the RCA jack. Solder the shield wire to the outer conductor.

Be certain to dress the wires to the phonograph and microphone inputs close to the chassis and

away from the Conn1 cable bundle. Unless sufficient separation is maintained, it is possible for

crosstalk from the higher-level Tuner and Auxiliary inputs to the lower-level phone and mic input

circuits.


wires on one end of the cable. Insert the crimped terminals into a 4 terminal Molex header

as follows:


• White to pin #2

• Positions #3 and #4 are vacant

55

WhitakerAudio



– Insert the connector/cable just assembled into the Conn2 (TAPE2) pins on the right

channel PWB. Be careful to properly orient the cable, using the legend printed on the

board as a guide. Connect the free end of the cable to the right channel Tape Output jack.

Solder the white wire to the center conductor of the RCA jack. Solder the shield wire to

the outer conductor. Dress the cable against the chassis.



as follows:


• White to pin #2




– Insert the connector/cable just assembled into the Conn10 (BUF-OUT) pins on the right


board as a guide. Route the free end of the cable to the back panel right channel Line

Output jack, using the two available cable clamps adjacent to the left channel PWB.

Solder the white wire to the center conductor of the RCA jack. Solder the shield wire to

the outer conductor.


wires on one end of the cable. Insert the crimped terminals into a 12 terminal Molex

header as follows:


• White to pin #2




56




header as follows:


• White to pin #11






header as follows:


• White to pin #5






header as follows:


• White to pin #8




– Prepare a 2-inch long yellow wire. Connect one end of the wire to the center terminal of

SW2 (Mic Mix). Solder.

In the steps that follow, be certain to match the channel inputs with the correct jacks.

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WhitakerAudio

– Find the 9-inch audio cable connected to pins 10–12 of the Molex header. This is the input

to the microphone channel. Connect the free end of the cable to the left channel

Microphone input. Connect the shield to the GR pin and the white to the TP pin. Solder

the shield. Do not solder the white wire. Route the wire against the chassis top/back panel

corner.

– Connect the free end of the 2-inch yellow wire to the TP pin of the left channel

Microphone input. Solder. (2 wires).

– Identify the left channel Conn1 connector. Insert the 12 terminal Molex header into the

connector on the board, being careful to orient the header properly. Use the printed legend

on the PWB as a guide.

– Find the audio cable connected to pins 1–2 of Conn1 (Phono). Connect the free end of the

cable to the left channel Phonograph input jack. Solder the white wire to the center

conductor of the RCA jack. Solder the shield wire to the outer conductor. Route the wire

against the chassis.

– Find the audio cable connected to pins 4–5 of Conn1 (Tuner). Connect the free end of the

cable to the left channel Tuner input jack. Solder the white wire to the center conductor of

the RCA jack. Solder the shield wire to the outer conductor.

– Find the audio cable connected to pins 7–8 of Conn1 (Aux). Connect the free end of the

cable to the left channel Auxiliary input jack. Solder the white wire to the center

conductor of the RCA jack. Solder the shield wire to the outer conductor.



as follows:


• White to pin #2




58


– Insert the connector/cable just assembled into the Conn2 (TAPE2) pins on the left channel

PWB. Be careful to properly orient the cable, using the legend printed on the board as a

guide. Connect the free end of the cable to the left channel Tape Output jack. Solder the

white wire to the center conductor of the RCA jack. Solder the shield wire to the outer

conductor. Dress the wire against the chassis.



as follows:


• White to pin #2




– Insert the connector/cable just assembled into the Conn10 (BUF-OUT) pins on the left


board as a guide. Route the free end of the cable to the back panel left channel Line

Output jack, using the two available cable clamps adjacent to the left channel PWB.

Solder the white wire to the center conductor of the RCA jack. Solder the shield to the

outer conductor. Dress the excess length of cable between the rear panel and the left

channel PWB.

– Prepare a 24-inch long piece of #18 black hookup wire. Connect one end to one of the

available ground lugs on the back panel. Solder. Route the wire toward the front panel

using the two cable clamps alongside the right channel PWB. Route to the #2 pin on the

right channel PWB barrier strip. It will be connected later.

– Prepare a 32-inch long piece of #18 black hookup wire. Connect one end to one of the

available ground lugs on the back panel. Solder. Route the wire toward the front panel

using the two cable clamps alongside the right channel PWB. Route to the #2 pin on the

left channel PWB barrier strip. It will be connected later.

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WhitakerAudio

– Prepare an 19-inch long piece of #18 black hookup wire. Connect one end to one of the

available ground lugs on the back panel. Solder. Route the wire toward the back panel

power connector. It will be connected later.

Assembly of the back panel audio connectors has now been completed. Use tie-wraps as

appropriate to dress the cabling.

5.3.6 Front Panel Connections

The potentiometers on the front panel are dual-ganged units. In all cases, the front section (the one

nearest the front panel) is used for the right channel and the back section is used for the left

channel. Each potentiometer has three terminals, “a”, “arm”, and “b”. See Figure 5.8.

The length of the shielded audio cable used in the preamplifier will impact the high frequency

response of the amplifier under certain conditions. For this reason, the cable lengths are kept as

short as possible. Also, to ensure that the frequency response of the channels track closely, the

cable lengths are specified to be identical between channels for similar functions.

Before proceeding, clean the soldering iron tip.



as follows:


• White to pin #2

• Positions #3 and #4 are is vacant



– Insert the connector/cable just assembled into the Conn7 (TAPE1) pins on the right


board as a guide. Route the free end of the cable to the front panel Input switch (SW1).

Clip off the shield wire; it is not used. Leave the free end of the cable unconnected for

now.

60




as follows:


• White to pin #2

• Positions #3 and #4 are is vacant



– Insert the connector/cable just assembled into the Conn7 (TAPE1) pins on the left channel


guide. Route the free end of the cable to the front panel Input switch (SW1), using the

available cable clamp adjacent to the left channel PWB. Clip off the shield wire; it is not

used. Leave the free end of the cable unconnected for now.

In the following steps the Input selector switch, SW1, will be wired. Note there are two

sections to the switch, divided along a centerline down the middle of the device. The section

facing the right channel PWB will be used for the right channel inputs and the section facing the

left channel PWB will be used for the left channel inputs. The wiper arm is the connection post

nearest the front panel. Note also the position of the connection arm when the switch is in the

“Phonograph” position (check with an ohmmeter). This is position #1. Other positions go in

logical order.

Note that in order to gain access to some connection posts it may be necessary to temporarily

remove the Input switch knob and loosen the mounting nut. This will allow the component to be

rotated slightly.

– Find the 12-inch long audio cable from the Conn7 (TAPE1) connector on the right

channel PWB. Connect the clear conductor to the wiper arm of the right channel input

switch (SW1). Do not solder.

– Find the 12-inch long audio cable from the Conn7 (TAPE1) connector on the left channel

PWB. Connect the clear conductor to the wiper arm of the left channel input switch

(SW1). Do not solder.

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WhitakerAudio



as follows:


• White to pin #2

• Positions #3 and #4 position are vacant



– Insert the connector/cable just assembled into the Conn8 (INPUT) pins on the right



Clip off the shield wire; it is not used. Connect the white conductor to the wiper arm of

the right channel input switch (SW1). Solder (2 wires).



as follows:


• White to pin #2




– Insert the connector/cable just assembled into the Conn3 (PH-OUT) pins on the right



Clip off the shield wire; it is not used. Connect the clear (signal) conductor to the #1

position on the right channel input switch (SW1). Solder.



as follows:

62



• White to pin #2




– Insert the connector/cable just assembled into the Conn4 (TUN-OUT) pins on the right



Clip off the shield wire; it is not used. Connect the white conductor to the #3 position on

the right channel input switch (SW1). Solder.



as follows:


• White to pin #2




– Insert the connector/cable just assembled into the Conn5 (AUX-OUT) pins on the right



Clip off the shield wire; it is not used. Connect the white conductor to the #4 position on

the right channel input switch (SW1). Solder.



as follows:


• White to pin #2


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WhitakerAudio



– Insert the connector/cable just assembled into the Conn6 (MIC-OUT) pins on the right



Clip off the shield wire; it is not used. Connect the clear (signal) conductor to the #2

position on the right channel input switch (SW1). Solder.



as follows:


• White to pin #2



– Prepare a 12-inch long piece of #22 black wire. Attach a crimp terminal to one end. Insert

the black wire into the 4 terminal Molex header just assembled as follows:

• Black to pin #3


– Insert the connector/cable just assembled into the Conn8 (INPUT) pins on the left channel


guide. Route the free end of the cable to the front panel Input switch (SW1). Clip off the

shield wire; it is not used. Connect the white conductor to the wiper arm of the left

channel input switch (SW1). Solder (2 wires). Connect the black wire to the last terminal

position on the right channel side of SW1. (This pin is adjacent to the left channel wiper

arm connection.) This connection is not used by the switch and will serve as a ground

point for the Front Aux connector.

Examine the Front Aux input jack mounted on the front panel. This connector carries three

signals: right, left, and ground. Connections for such jacks are also commonly described as tip,

ring, and sleeve, which correspond to the right, left, and ground connections, respectively.

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– Prepare an 11-inch long piece of 3-conductor audio cable. Connect the black and shield

wires to the ground point just established on the unused terminal on SW1. Solder (3

wires). Connect the red wire to the #5 position on the right channel side of Input switch

SW1. Solder. Connect the white wire to the #5 position on the left channel side of Input

switch SW1. Solder. Note: it may be helpful to temporarily remove the connector to

Conn10 on the left channel PWB in order to better access position #5 of SW1.

– Route the 11-inch 3-conductor audio cable to the Front Aux jack. Connect the black wire

to the common (sleeve) terminal, which is adjacent to the front panel. Solder. Connect the

white wire to the ring (middle) terminal. This terminal can usually be identified visually or

by use of an ohmmeter. Solder. Connect the red wire to the tip terminal. Solder.



as follows:


• White to pin #2




– Insert the connector/cable just assembled into the Conn3 (PH-OUT) pins on the left


board as a guide. Route the free end of the cable to the front panel Input switch (SW1),

using one of the cable clamps adjacent to the left channel PWB. Clip off the shield wire; it

is not used. Connect the white conductor to the #1 position on the left channel input

switch (SW1). Solder.



as follows:


• White to pin #2

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– Insert the connector/cable just assembled into the Conn4 (TUN-OUT) pins on the left








as follows:


• White to pin #2




– Insert the connector/cable just assembled into the Conn5 (AUX-OUT) pins on the left








as follows:


• White to pin #2


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– Insert the connector/cable just assembled into the Conn6 (MIC-OUT) pins on the left






If the input switch (SW1) was moved in order to make connections to the device, reposition

the switch and secure the nut. Reinstall the knob and check positioning.

– Prepare a 12-inch long piece of 3-conductor audio cable. Attach crimp terminals to the

red, white, black, and shield wires on one end of the cable. Insert the crimped terminals

into a 12 terminal Molex header as follows:

• White to pin #1 (R26a)

• Red to pin #1 (R26arm)

• Black to pin #3 (R26b)

• Shield to pin #4 (GND)





into the 12 terminal Molex header as follows:







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• Red to pin #10 (R32arn)





– Insert the connector/cable just assembled into the Conn9 pins on the right channel PWB.

Be careful to properly orient the connector, using the legend printed on the board as a

guide. Route the free end of the cables to the front panel. For the cable from the #5 to #8

pins, route through the cable clamp adjacent to the right channel PWB. Clip off the shield

drain wires; they are not used.

– Find the right channel Conn9 R26 cable (R26a, R25arm, R26b). Solder the white wire to

terminal “a” on the front section of R26 (Bass). Solder the red wire to the “arm” terminal.

Solder the black wire to the “b” terminal.


terminal “a” on the front section of R30 (Treble). Solder the red wire to the “arm”

terminal. Solder the black wire to the “b” terminal.


terminal “a” on the front section of R32 (Volume). Solder the red wire to the “arm”


Inside chassis view

Potentiometer

Term 1, low side(”b”)

Term 2, wiper(”arm”)

Term 3, high side(”a”)Figure 5.8 Terminal numbering for chassis-mounted potentiometers.

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Dress the cables under the lip of the front chassis edge.



into a 12 terminal Molex header as follows:




















• Red to pin #10 (R32arn)





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– Insert the connector/cable just assembled into the Conn9 pins on the left channel PWB.

Be careful to properly orient the cable, using the legend printed on the board as a guide.

Route the free end of the cable to the front panel. Clip off the shield drain wires; they are

not used.

– Find the left channel Conn9 R26 cable (R26a, R26arm, R26b). Solder the white wire to

terminal “a” on the rear section of R26 (Bass). Solder the red wire to the “arm” terminal.



terminal “a” on the rear section of R30 (Treble). Solder the red wire to the “arm” terminal.



terminal “a” on the back section of R32 (Volume). Solder the red wire to the “arm”


Dress the wires under the front chassis lip. Secure with tie wraps as needed.

5.3.7 Install Interconnecting Wiring

The final step in construction of the preamplifier is to install the filament voltage rectifier, board-

to-board wiring and the rear panel power connector wiring. To facilitate cable organization, it is

advisable to install expandable flexible sleeving on these wires. A supply of this material is

provided. To properly perform the following steps, it will be necessary to have a heat gun on

hand. (A common hair dryer may be sufficient.) In addition, the preferred method of cutting the

tubing is to use a so-called “heat knife.” If such a tool is unavailable, scissors may be used.

When completed, the cable bundle will be placed above the circuit boards and audio wiring

and will rest against the chassis bottom plate (when installed). See Figure 5.9 for placement.

The parts needed for the following steps can be found in the Power Connector Hardware,

Chassis Hookup Wire, and Braided Sleeving/Heat-Shrink Tubing packages. The pinout for the

power connector is given in Table 3.2. Before proceeding, clean the soldering iron tip.

– Locate the #18 black wire from the rear panel ground lug that was previously routed to the

power connector. Attach a pin to the wire and insert in the #9 position on the power

connector.

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Two-conductor twisted-pair audio cable will be used to connect the back panel power socket

to the front panel pilot light. Because the pilot lamp is driven by 115 V ac from the power supply

on the amplifier chassis, shielded wire will be used to minimize hum in the low-level audio

circuits of the preamplifier. Note that the audio cable specified is rated for use at up to 300 V.

– Prepare an 18-inch long piece of 2-conductor twisted-pair audio cable. Attach a pin to

each of the clear and black wires.

– Prepare a 9-inch long piece of #22 black wire. Combine one end of this wire with the

shield drain wire of the 2-conductor cable prepared in the previous step. Attach a pin to the

two wires, being careful to securely crimp both in place.

– Insert the pins as follows into the back panel power connector:

• Shield and 9-inch long #22 black wire in the #1 position

• Clear in the #2 position

• Black in the #3 position

– Prepare a 9-inch long piece of #22 red wire. Attach a pin to one end of the wire and insert

in the #4 position on the power connector.

– Prepare a 6-inch long piece of #18 green wire. Attach a pin to one end of the wire and

insert in the #5 position on the power connector.

– Prepare a 6-inch long piece of #18 green wire. Attach a pin to one end of the wire and

insert in the #6 position on the power connector.

– Twist the two #18 green wires just installed, leaving about 1-inch free on the end.

All pins on the power connector have now been installed. Pins 7 and 8 are not used.

– Cut a 5-inch long piece of expandable sleeving. Route the 2-conductor cable, black wire,

and red wire from the power connector through the tubing. Place a short (3/4-inch long)

piece of heat-shrink tubing over each end. Process with the heat gun, being careful to not

use too much heat.

– Prepare a 10-inch long #22 red wire. Combine one end of this wire with the red wire from

the power connector harness. Place both wires in a spade terminal and crimp.

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– Prepare two 5-inch long #18 green wires. Attach spade terminals to one end of each wire

and crimp. Twist the wires, leaving about 1-inch free on each end.

– Prepare two 11-inch long #18 green wires. Attach spade terminals to one end of each wire

and crimp. Twist the wires, leaving about 1-inch free on each end.

– Find the #18 black wire from the back panel ground point routed previously to the left

channel PWB. Combine this wire with the #22 black wire from the back panel power

connector. Attach a spade terminal to the wires (two wires).

– Loosen the four screws on the left channel PWB barrier strip. Insert the spade terminals

just prepared as follows:

• Red wires to position #1. Tighten.

• Black wires to position #2. Tighten.

• One of the 5-inch long green wires and one of the 11-inch long green wires to position

#3. Tighten.

• The remaining 5-inch long green wire and the remaining 11-inch long green wire to

position #4. Tighten.

– Cut a 5-inch long piece of expandable sleeving. Route the wires from the left channel

PWB barrier strip and the pilot lamp cable through the tubing. Place a short (3/4-inch

long) piece of heat-shrink tubing over each end. Process with the heat gun, being careful

to not use too much heat.

– Connect the clear and black wires of the pilot lamp cable to the front panel pilot lamp.

Solder. Clip off the shield drain wire. It will not be used on this end.

– Find the #18 black wire from the back panel ground point routed previously to the right

channel PWB. Attach a spade terminal to the wire.

– Attach a spade terminal to the red wire from the harness.

– Attach spade terminals to the green wires from the harness.

– Loosen the four screws on the right channel PWB barrier strip. Insert the spade terminals

just prepared as follows:

• Red wire to position #1. Tighten.

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• Black wire to position #2. Tighten.

• Green wires to position #3 and position #4. Tighten.

Next, the filament voltage rectifier and filter will be installed. Locate C28 and CR-1. Examine

CR-1 and note the position of the positive and negative terminals.

– Find one of the #18 green wires connected to the left channel PWB barrier strip, position

#3 or #4. Find C28 and note the positive terminal. Add a piece of spaghetti to the bare

lead. (Insulation from a length of #18 wire should suffice.) Combine this wire with the

positive lead of C28 in a single large-gauge (blue) quick-disconnect terminal. Crimp.

– Find the other #18 green wire connected to the left channel PWB barrier strip. Combine

this wire with the negative lead of C28 in a single large-gauge (blue) quick-disconnect

terminal. Add a piece of spaghetti to the bare lead. Crimp.

– Insert the quick-disconnect terminal connected to the positive lead of C28 into the positive

(+) post on CR-1.

– Insert the quick-disconnect terminal connected to the negative lead of C28 into the

negative (–) post on CR-1.

– Double-check the two connections to CR-1.

If heat sink compound is available, place a small amount on the heat sink side of CR-1 before

mounting the component on the chassis.

– Mount CR-1 in the hole provided adjacent to the side handle using the #8 hardware

supplied.

– Find the two #18 green wires from the back panel power connector. Attach a quick-

disconnect terminal to each wire.

– Insert the remaining green wires on the “ac input” posts of CR-1. Use a pair of long-nose

pliers to seat the connectors.

– Dress the green wires from the rear panel power connector against the side of the chassis.

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5.3.8 Final Assembly Check

The stereo preamplifier has now been completed. Make a thorough visual inspection to make sure

everything looks OK. Use the tie-wraps provided to organize and cleanup the wiring. All parts

should be installed, with the exception of the vacuum tubes. If you have any parts left-over,

recheck the procedures documented here and install the part(s) as needed. Figure 5.9 shows the

chassis view of the stereo preamplifier.

– Check for any wiring debris inside the chassis by turning it over to remove any particles.

– Tighten the tube sockets. Use two Phillips screw drivers, one from underside the chassis

and the other from the top of the chassis. Turn in opposite directions to tighten. Caution:

do not use excessive force. Be careful to not scratch the chassis finish.

Before proceeding to Initial Checkout, it is recommended that builders clean up the

workspace and put away all tools. An occasional “workbench reset” is useful for any project.

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bFigure 5.9 The stereo audio preamplifier: (a) bottom view, (b) top view.

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6 Initial Checkout

After the preamplifier has been assembled, the following steps are recommended to confirm

proper construction:

1) Remove all power from the unit.

2) Remove all tubes.

3) Using an ohmmeter, confirm that the resistance values listed in Table 6.1 are observed on

the tube socket connections for the right and left channels. All readings are with reference to

ground. Readings within ±5 percent are considered normal. The resistance measurements are

made with the power supply disconnected.

Table 6.1 Typical Ohmmeter Readings with Power Removed and Tubes RemovedTube Pin

No.Typical Reading

Notes

V1, 7025, Phono Preamp

1 > 200 kΩ Varies due to filter capacitor in circuit

2 47 kΩ Measured with no input connected

3 2.7 kΩ

4 ~50 Ω With Hum Balance control at mid-point



7 680 kΩ

8 2.7 kΩ


V2, 5879, Microphone Preamp

1 2.2 mΩ With SW-2 in the Mic Stereo position

2 –

3 1 kΩ



6 –



9 1 kΩ

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4) If any deviations from the expected values listed in Table 6.1 are observed, recheck wiring

and components for correct installation. Do not advance to the next step until the resistance

measurements are within tolerance.

5) After the resistance checks have been successfully completed, connect the power supply and

apply power to the unit. Be very careful since high voltages are present. Observe the

preamplifier for any signs of abnormal behavior. When the power supply (power amplifier)

is switched on, the pilot lamp on the preamplifier should light.

V3, 6EU7, Tone Control Preamp



3 –

4 16.5 kΩ

5 480 kΩ



8 110 kΩ to 1.1 mΩ

Reading depends on setting of Bass tone control. Values shown are for extreme positions

9 1 kΩ

V4, 7025, Buffer Amplifier


2 108 kΩ to 180 kΩ

Varies depending on setting of input volume control

3 1 kΩ





8 100 kΩ


V5, 7025, Tape Out Amplifier


2 108 kΩ to 180 kΩ

Varies depending on setting of Tape Out Level control

3 1 kΩ





8 100 kΩ


Table 6.1 Typical Ohmmeter Readings with Power Removed and Tubes Removed

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6) Using extreme caution, check for proper voltages at the tube connection points listed in

Table 6.2 for the right and left channels. All voltages are measured with respect to ground

(unless otherwise noted) using a high impedance voltmeter. Variations within ±10 percent

are normal. These measurements can be taken from the top of the chassis by carefully

inserting the voltmeter probe into the socket connection points. This is safer than attempting

to make the measurements from underneath the chassis. Note that the dc heater voltages are

typical when the Hum Balance control is set for mid-point.

Table 6.2 Typical Voltmeter Readings with Tubes Removed and Power Applied(B+ supply of 275 V dc is assumed for these measurements.)

Tube Pin No.

Typical Reading

Notes

V1, 7025, Phono Preamp 1 270 V

2 0 V No input connected

3 0 V

4 3 to 4 V Heater (with Hum Balance set at center).

5 3 to 4 V Heater. There should be 0 V between pins 4 and 5.

6 270 V

7 0 V

8 0 V

9 3 to 4 V Heater. There should be ~7 V dc between pin 9 and pins 4 and 5.

V2, 5878, Microphone Preamp

1 0 V

2 0 V

3 0 V

4 3 to 4 V Heater

5 3 to 4 V Heater. There should be ~7 V dc between pins 4 and 5.

6 0 V

7 270 V

8 275 V

9 0 V

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7) If variations in the expected voltage readings are found, remove all power from the unit and

recheck components and wiring. Do not advance to the next step unless the voltages check

correctly.

8) Remove ac power from the unit.

9) Install all tubes. Install the tube shields on V1 and V2 (if used).

10) Set the tone controls to their center positions. Set the Volume control to the 9 o’clock

position. Set the Input switch to Auxiliary. Remove any connectors from the input jacks.

V3, 6EU7, Tone Control Preamp

1 3 to 4 V Heater

2 3 to 4 V Heater. There should be ~7 V dc between pins 1 and 2.

3 0 V

4 0 V

5 0 V

6 270 V

7 270 V

8 0 V

9 0 V

V4, 7025, Buffer Amplifier

1 270 V

2 0 V

3 0 V

4 3 to 4 V Heater


6 275 V

7 270 V

8 0 V


V5, 7025, Tape Out Amplifier

1 270 V

2 0 V

3 0 V

4 3 to 4 V Heater


6 275 V

7 270 V

8 0 V


Table 6.2 Typical Voltmeter Readings with Tubes Removed and Power Applied(B+ supply of 275 V dc is assumed for these measurements.)

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11) Power up the unit and very carefully confirm the circuit operating voltages shown on the

schematic diagram given in Figure 1.1. The readings should match the nominal values

within 10 percent. Note that the voltages given in the figure assume an input voltage of +275

V. For other values of input B+ scale appropriately.

12) Check the voltage of the filament supply. With all tubes installed the voltage between

terminals #3 and #4 on either PWB barrier strip should be 6.1 V dc.

Important: Never operate the stereo preamplifier unless all tubes are installed. The voltage of

the dc filament supply is not regulated and if multiple tubes are removed with power applied the

filament voltage can rise to the point that it could damage the remaining tubes.

13) Remove power from the preamplifier. Using a voltmeter, confirm that the power supply

filter capacitors have discharged to 0 V. Observe the circuit boards and components for any

signs of heating or other unexpected behavior. If problems are discovered, proceed no

further. Recheck the circuits for proper construction.

6.1 Functional Tests

The purpose of the following functional tests is to confirm that the preamplifier circuits are

working as expected. Detailed measurements will be taken later.

1) Set the front panel controls as follows:

• Volume control set to approximately 9 o’clock position

• Input switch to Phonograph

• Bass to center position

• Treble to center position

2) Set the gain adjust potentiometers on the PWBs as follows. The right and left channel

controls should be set the same.

• Phono Gain approximately 2/3 clockwise

• Tuner Gain fully clockwise

• Aux Gain fully clockwise

• Mic Gain approximately 2/3 clockwise

• Tape Out Level approximately 1/3 clockwise

• Hum Balance approximately center travel

3) Connect an audio amplifier to the left and right Line Output jacks.

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4) Apply power and observe the tubes for any signs of overheating or other unexpected

behavior. The filaments should be lit. Remove power if any problems are detected, or if

noticeable hum can be heard from the speaker. Recheck wiring and components as needed.

5) Rotate the Input switch through the various positions to confirm that no noise or hum can be

heard from the speakers. Advance the Volume control through each position. It is likely that

some noise and/or hum will be heard on the Phonograph and Microphone inputs, but not an

excessive amount.

6) Move the amplifier input cables from the Line Output jacks to the Tape Output jacks and

repeat Step #5 above.

After these preliminary tests have been successfully completed, operating levels should be set,

as detailed in the next section.

6.2 Setting Operating Levels

In order to achieve the best (lowest) noise performance from the preamplifier, it is necessary to

carefully set the operating points of the preamp stages. There are a number of variables involved.

The following steps are recommended prior to taking performance measurements3. Make these

adjustments on the right channel first, followed by the left channel.

Note that a reference point is needed when setting the operating levels. In the steps that

follow, the Auxiliary input is assumed to be the reference point. This is based on previous

experience with the preamplifier circuits. However, the selection of available source devices is

wide and varied and as such it may be necessary to select a different reference point, depending

on the specifics of a given application. Operational experience is the best guide here.

1) Set the front-panel Volume control for approximately 1/4 clockwise rotation. Set the front

panel Bass and Treble controls for flat response (center positions).

2) Connect an audio voltmeter to the output of the right channel preamplifier.

3) Switch the input to Auxiliary and apply a 1 kHz, 0.30 V rms signal to the right channel

Auxiliary input.

4) Adjust R19 (Auxiliary level control) to the full clockwise position.

3. Be extremely careful while making adjustments below the chassis as dangerous voltages exist. Use an insulated

adjustment tool; do not place hands near the circuit board or components.

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5) Readjust the front panel Volume control to produce 1 V rms output into a 1 MΩ load at the

output jack of the preamplifier. The setting of the Volume control should be within the 8 to

11 o’clock positions.

6) Switch the input to Tuner and apply a 1 kHz, 1.0 V rms signal to the right channel Tuner

input.

7) Adjust R18 (Tuner level control) to produce 1 V rms into 1 MΩ at the output terminal of the

preamplifier. The proper position for R18 should be approximately 2/3 clockwise. Do not

readjust the front panel Volume control.

8) Switch the input to Phonograph and apply a 1 kHz, 0.0072 V rms signal to the right channel

Phono input.

9) Adjust R16 (Phono level control) to produce 1 V rms output into a 1 MΩ load at the output

terminal of the preamplifier. Do not readjust the Volume control. The proper position for

R16 should be approximately 2/3 clockwise

10) Switch the input to Microphone and apply a 1 kHz, 0.0145 V rms signal to the right channel

Microphone input. Check to make sure the Microphone Mix switch on the back panel is set

to Stereo.

11) Adjust R17 (Microphone level control) to produce 1 V rms output into a 1 MΩ load at the

output terminal of the preamplifier. Do not readjust the Volume control. The proper position

for R17 should be approximately 2/3 clockwise.

Note that the output levels of different types of input devices varies widely. The setting

described here is a good starting point. Further adjustments may be needed depending on the type

of source material used.

12) Switch the preamplifier to Phonograph and apply a 1 kHz, 0.0072 V rms signal to the right

channel Phono input. Set the ac voltmeter to read decibels and adjust for a convenient

reference point.

13) Remove the input signal and connect a shorting connector to the Phono input.

14) While observing the residual noise level on the voltmeter, adjust R42 (Hum Balance control)

for a minimum reading. This setting can have a significant effect on the noise reading (up to

10 dB), so adjust carefully. At this point, a reading of –70 dB or so should be achieved.

15) Repeat steps #2 through #14 for the left channel of the preamplifier, with exception of step

#5. Do not readjust the front panel Volume control.

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Note that during adjustment of R42, the Hum Balance control, the optimal setting for one

channel may not coincide with the optimal setting for the other channel. The impact of this

adjustment can be significant. In the event that the minimum noise settings do not coincide, split

the difference between the two channels.

During adjustment of the input levels, the proper positions for the on-board potentiometers

should be nearly identical between channels. Minor gain variation between channels is normal.

For proper balance between channels it may be necessary to repeat the procedure outlined above

if the left channel output is noticeably less than the right channel, in which case the left channel

gain would be used as the reference point for setting the front panel Volume control.

16) Connect an audio voltmeter to the right channel Tape Output jack. Connect the signal

generator to the right channel Auxiliary input and switch the front panel Input control to

Auxiliary. Apply a 1 kHz, 0.30 V rms signal to the right channel Auxiliary input and adjust

the Tape Out Level control (R40) on the right channel PWB to produce 1 V rms at the Tape

Output jack. The proper position for R40 is about 1/3 clockwise rotation. Repeat this

procedure for the left channel.

The foregoing settings should provide a good starting point for use of the preamplifier. Touch-

up adjustments using program material can be made later.

After these adjustments have been made, remove power and install the bottom chassis plate.

After installing the bottom of the chassis, the measured noise floor should decrease by several

decibels.

6.3 Important Operational Note

The back panel power connector to the amplifier/power supply includes two ground connections

to make sure that a solid ground is always maintained between the preamplifier and the power

amplifier. Maintenance of the ground is essential to safe operation of the preamplifier. As a

measure of additional safety, a separate ground wire, connected to the ground posts on the back

panel of the amplifier and the preamplifier, must always be in place and secured. Do not operate

the preamplifier without all covers in place and the separate ground connection installed.

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7 Performance Measurement

While not strictly necessary, measurements should be taken on each channel of the stereo

preamplifier upon completion. It is acknowledged that the only performance measurements for

audio gear that really matter are how it sounds. Still, if the test equipment is available,

measurements are useful in confirming proper construction and serve as a baseline for any future

maintenance tasks.

Performance measurements should be taken at an output level of 1 V rms into a load of 1 MΩ.

Typical readings are given in Table 7.1. Measurements should be taken at the following input

positions:

• Phonograph.

• Microphone.

• Tuner/Auxiliary/Front Aux. The performance of these three inputs should be identical.

Set the tone-control potentiometers for their center (flat) response. The flat setting points for

the Bass and Treble controls are best found by using a sweep generator while monitoring the

output of the preamplifier with an ac voltmeter. Small variations from the optimal flat response

are not unusual between channels. Any deviation of more than about 1 dB may be cause for

further investigation. The following procedure is recommended.

1) Connect a sweep generator to the right channel Auxiliary input. Switch the Input to

Auxiliary. Connect an audio voltmeter to the right channel Line Output jack.

2) Set the sweep generator to produce a 1 kHz signal at 0.3 v rms. Adjust the Volume control to

produce 1 V rms at the Line Output jack. Adjust the audio voltmeter to read 0 dB.

3) Set the generator begin the sweep at about 15 Hz and end the sweep at about 1 kHz. Begin a

repetitive linear sweep of about 1 second duration.

4) Slowly adjust the Bass control for the flattest response, which should be in the range of +/–1

dB.

5) Move the generator input to the left channel Auxiliary input and move the audio voltmeter

input to the left channel Line Output. The left channel response should closely match the

right channel response.

6) Loosen the Bass control knob set screw and reposition the knob so the white line faces

straight up. Tighten the set screw.

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7) Set the generator begin a sweep at about 1 kHz and end the sweep at about 20 kHz. Begin a

repetitive linear sweep of about 1 second duration.

8) Slowly adjust the Treble control for the flattest response, which should be in the range of +/

–1 dB.

9) Move the generator input to the right channel Auxiliary input and move the audio voltmeter

input to the right channel Line Output. The right channel response should closely match the

left channel response.

6) Loosen the Treble control knob set screw and reposition the knob so the white line faces

straight up. Tighten the set screw.

Measurements should next be taken on the Tape Out buffer amplifier circuit. The following

procedure may be used.

7) Set the front panel Input switch to Auxiliary.

8) Connect the signal generator to the Tuner RCA jack on the back panel.

9 Connect the audio analyzer to the appropriate Tape Out RCA jack on the back panel.

10) Performance of the Tape Out circuit should meet or exceed the targets listed in Table 7.1 for

the Auxiliary input. With an input of 0.3 V rms, the output level at the Tape Out jack should

be approximately 1 V rms into 1 MΩ.

For the measurements listed in Table 7.1, with the input signal levels as given, a 1 V rms

output at the Line Out jacks should be observed with the front panel Volume control in the 8 to 11

o’clock position. For measurements of the phonograph and microphone circuits, use the Tape

Output jack since that port bypasses the tone-control stage and thus provides for more accurate

measurements.

Table 7.1 Performance Targets for the Stereo PreamplifierTest Parameter Target Value Notes

Tuner/Auxiliary/Front Aux Inputs

Input level 0.3 V rms1.0 V rms

AuxTuner

Frequency response ±2 dB, 15 Hz to 20 kHz

THD 0.6% Note 1

IMD 0.5% 4:1 mix ratio

Noise, unweighted, (shorted input terminals)

–70 dB

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Measurement of the frequency response of the phonograph circuit may indicate performance

that is not within the specified tolerance of the RIAA curve at the high end of the frequency range.

If, upon test, excessive falloff at high frequencies is observed, add capacitor CP (20 pf) to the

phono preamp circuit. This component can be soldered in place from the foil side of the board. It

is not necessary to remove the PWBs to add this part. Remove all power from the unit before

removing the bottom chassis plate. Using a voltmeter, check to be certain that all high voltage

capacitors have fully discharged. Replace the chassis bottom plate before restoring power to the

unit. Repeat the phono circuit frequency response tests after capacitor CP has been added.

It is important to remember that the performance of any circuit is determined in large measure

by the performance of the active devices. In the case of tubes used for low-level audio circuits,

some suppliers offer specialized services for certain types as a way of optimizing the application.

Tone Control Treble control maximum boost +16 dB At 20 kHz

Treble control, maximum cut –16 dB At 20 kHz

Bass control maximum boost +16 dB At 20 Hz

Bass control maximum cut –16 dB At 20 Hz

Phonograph Input Input level 0.0072 V rms


Note 2

THD 0.6% Note 1

IMD – Note 3


–70 dB

Microphone Input Input level 0.0145 V rms


Note 4

THD 0.5% Note 1

IMD 0.8% 4:1 mix ratio


–70 dB

Notes:

1 THD measurements taken at 20 Hz, 100 Hz, 1 kHz, 10 kHz, ands 20 kHz. The highest reading is recorded.

2 The frequency response of the phono preamp is relative to the RIAA equalization curve.

3 No IMD measurement is taken on the phono preamp because of the equalization curve of the circuit.

4 If an external impedance matching transformer (low-to-high) is used, performance of the microphone preamp will be largely determined by the characteristics of the transformer.

Table 7.1 Performance Targets for the Stereo Preamplifier

86


For example, the 7025 (12AX7) can be ordered from at least one online vendor4 as: 1) standard

functional test, 2) high gain, 3) low noise and microphonics, or 4) matched set. For the

preamplifier, it would probably be a good idea to consider the low noise selection service, since

any reduction in noise—particularly for the phono and mic stages—will be beneficial to overall

performance.

7.1 RIAA Equalization

The purpose of pre-emphasis in vinyl disk recording and the complementary de-emphasis in

playback is to boost the amplitude of the recorded waveform at higher frequencies to improve the

signal-to-noise ratio (S/N). The emphasis, adopted as a standard by the Recording Industry

Association of America (RIAA) is known as the RIAA characteristic. The complementary

recording and reproduction curves are illustrated in Figure 7.1. Note the response is not a simple 6

dB per octave filter, but rather a shelved response, which was intended to compensate for certain

constraints relating to fine-groove disk record production and the legacy practices that existed at

the time the curve was adopted by the RIAA.

4. www.tubedeopt.com; there may be other vendors offering this service as well. The cost difference for the various

selection options is typically small.

Frequency, Hz20 50 100 200 500 1000 2000 5000 10,000 20,000

20

15

10

5

0

–5

–10

–15

–20

Rel

ativ

e ve

loci

ty o

f vol

tage

leve

l, dB

Reproducing characteristics

Recording characteristics

75 μs318 μs3180 μs

Figure 7.1 Characteristics for fine-groove records. (RIAA Dimensional Characteristics for 33-1/3 rpm Records.)

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WhitakerAudio

Table 7.2 provides tabular values for the characteristic curve., referenced to 1 kHz. The data

range from 20 Hz on the low end to 20 kHz on the high end.

7.2 Final Touches

The stereo preamplifier has now been completed. Remove power from the unit and carefully

place the Plexiglas cover over the top. Secure with the four #6 screws and flat washers provided.

Use of the cover is recommended as it serves to protect the tubes from users (and users from the

tubes). It also aids cooling somewhat by creating a “chimney effect” that collects cool air from the

edges of the chassis and exhausts heated air from around the tubes and transformers. Do not

overtighten the mounting screws.

If the bottom chassis plate has not already been screwed on, do so now.

Table 7.2 Frequency Response as a Function of the RIAA Target ValuesFrequency, Hz RIAA Curve Target

Level, dBFrequency, Hz RIAA Curve Target

Level, dB

20 +19.3 1,000 0

30 +18.6 1,500 –1.4

40 +17.8 2,000 –2.6

50 +17.0 3,000 –4.7

60 + 16.1 4,000 –6.6

70 + 15.3 5,000 –8.2

80 +14.5 6,000 –9.6

100 +13.1 7,000 –10.7

110 + 12.4 8,000 –11.9

115 +11.6 9,000 –12.9

150 +10.2 10,000 –13.7

200 +8.3 12,000 –15.3

250 +6.7 14,000 –16.6

300 + 5.5 15,000 –17.2

400 +3.8 16,000 –17.7

500 + 2.6 18,000 –18.7

600 + 1.9 20,000 –19.6

700 + 1.2

800 + 0.7

88


It is recommended that the performance of each channel be recorded for future reference.

Table 7.3 is provided for that purpose.

Figure 7.2 shows the completed stereo preamplifier with the Plexiglas cover installed.

Since the shelf life of a receiving tube is essentially unlimited, it is good practice to purchase

at least one spare tube for each type used.

Important: Never operate the stereo preamplifier unless all tubes are installed. The voltage of

the dc filament supply is not regulated and if multiple tubes are removed with power applied the

filament voltage can rise to the point that it could damage the remaining tubes.

Table 7.3 Measured Performance of the Stereo PreamplifierTest Parameter Right Channel Left Channel

Tuner/Auxiliary/Front Aux Inputs

Input level, AuxInput level, Tuner

Frequency response

THD

IMD


Tone Control Treble control maximum boost

Treble control, maximum cut

Bass control maximum boost

Bass control maximum cut

Phonograph Input Input level

Frequency response

THD

IMD


Microphone Input Input level

Frequency response

THD

IMD


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WhitakerAudio

Figure 7.2 Stereo preamplifier with the decorative cover in place.

90


8 Troubleshooting Guidelines

Even an audio device that has been carefully built can experience problems. When presented with

a problem, proceed in an orderly manner to trace it down. Many failures are simple to repair if

you stop and think about what’s happening. Looking over the schematic diagram for 10 to 15

minutes to consider the possible causes can save hours of trial-and-error troubleshooting. Never

rush through a troubleshooting job.

Stereo audio products offer the technician an added benefit in that the same problem rarely

manifests itself in both channels. As such, the technician has a “known good” circuit as a

reference.

When checking inside the unit, look for changes in the physical appearance of components.

An overheated resistor or leaky electrolytic capacitor may be the cause of the problem, or point to

the cause. Devices never fail without a reason. Try and piece together the sequence of events that

led to the problem. Then, the cause of the failure—not just the more obvious symptoms—will be

corrected. In high voltage power supplies, look for signs of loose connections and overheating.

Do not overlook the possibility of tube failure. Tubes can fail in unpredictable ways, and

substitution may be the only practical test.

Troubleshooting through the process of elimination can also be useful. This technique

involves isolating various portions of the circuit—one section at a time—until the defective

component is found.

Never touch anything inside the unit without first removing all ac power and discharging all

filter capacitors with a test lead to ground. Most plate power supplies include bleeder resistors to

drain-off the charge on the capacitor(s) in the circuit. However, some designs do not include this

safety feature. Be careful.

Analyze each planned test before it is conducted. Every test in the troubleshooting process

requires time, and so steps should be arranged to provide the greatest amount of information about

the problem.

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9 Safety Considerations

Electrical safety is important when working with any type of electronic hardware. Because

vacuum tubes operate at high voltages and currents, safety is doubly important. The primary areas

of concern, from a safety standpoint, include electric shock and hot surfaces of vacuum tube

devices. Table 9.1 lists required precautions for persons working around high voltages.

All persons working around high-voltage equipment should be familiar with first aid

treatment for electric shock and burns. Always keep a first aid kit on hand. Obtain detailed

information from the local heart association or Red Cross chapter. Personalized instruction on

first aid usually is available locally.

It is advisable to wear glasses or other protective eye wear when building or working on

electronic equipment. Injuries can result from solder splashes, cleaning chemicals, and other

unexpected events. When trimming the leads of a component, be careful to hold the excess length

so that when cut the lead does not fly towards the face.

Table 9.1 Safety Practices for Working Around High Voltage Equipment Remove all ac power from the equipment. Do not rely on internal switches to remove dangerous ac.

Discharge all capacitors using a grounded probe.

Do not remove, short-circuit, or tamper with interlock switches on access covers, doors, or enclosures.

Keep away from live circuits.

Allow any component to completely cool-down before attempting to replace it.

If a leak or bulge is found on the case of an electrolytic capacitor, do not attempt to service the part until it has completely cooled.

Avoid contact with hot surfaces within the system.

Know your equipment and do not take chances

92


10 Notes and Tube Data

The basic design of the stereo preamplifier is adapted from a circuit contained in the RCA

Receiving Tube Manual, dated 1975. Circuit descriptions are based on the text contained in that

publication.

10.1 Tube Characteristics

The data given here have been abstracted from the RCA Receiving Tube Manual (1975) to reflect

usage in this product. The illustrations are adapted from the RCA manual as well.

10.1.1 6EU7

The 6EU7 is a miniature type used in high-gain resistance-coupled low-level audio amplifier

applications where low hum and non-microphonic characteristics are important. A 6EU7 is used

in the tone-control stage of the stereo preamplifier.

The 9-contact pinout is given in Figure 10.1. Device characteristic curves are shown in Figure

10.2.

Selected tube parameters are listed in Table 10.1.

H

H

NC

Kt2Gt2

Pt2Pt1

Gt1

Kt21

2

3

45

6

7

8

9

Legend:H = heaterK = cathode, triode (t) 1 or 2G = grid, triode (t) 1 or 2P = plate, triode (t) 1 or 2NC = not connectedFigure 10.1 Terminal pinout of the 6EU7 tube.

Type 6EU7each unit

Pla

te m

illia

mpe

res

Plate volts0 100 200 300 400

4

3

2

1

5

Figure 10.2 Characteristic curves for the 6EU7 tube.

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WhitakerAudio

10.1.2 5879

The 5879 is a miniature type used as the input stage of a high-quality audio amplifier. A 5879 is

used in the microphone preamplifier circuit.


10.4.

Table 10.1 Key Parameters of the 6EU7.General Characteristics

Heater (ac/dc) 6.3 V, 0.3 A

Heater-cathode voltage

Peak value ±200 V maximum

Average value ±100 V maximum

Direct interelectrode capacitance (approx.), each unit

Grid to plate 0.15 pF

Grid to cathode and heater 1.6 pF

Plate to cathode and heater 0.2 pF

Equivalent noise and hum voltage (referenced to grid, each unit), average value1

1.8 μV rms

Class A1 Amplifier, Maximum Ratings (Design-Maximum Values), Each Unit

Plate voltage 330 V

Plate dissipation 1.2 W

Grid voltage

Negative bias value 55 V

Positive bias value 0 V

Characteristics

Plate voltage 100 V 250 V

Grid voltage –1 V –2 V

Amplification factor 100 100

Plate resistance (approx.) 80 kΩ 62.5 kΩTransconductance 1250 μmhos 1600 μmhos

Plate current 0.5 mA 1.2 mA

Notes:1 Measured in “true rms” units under the following conditions: heater volts (ac) = 6.3 V; center tap of heater trans-

former connected to ground; plate supply = 250 V; plate load resistor = 100 kΩ; cathode resistor = 2700 Ω, cath-ode bypass capacitor = 100 μF; grid resistor = 0 Ω; and amplifier covering frequency range 25 Hz to 10 kHz.

G1

G2

G3

NC

K

HH

NC

P1

2

3

45

6

7

8

9

Legend:H = heaterK = cathodeG = grid #1, 2, P = plateFigure 10.3 Terminal pinout of the 5879 tube

94



Table 10.2 Key Parameters of the 5879.General Characteristics

Heater (ac/dc) 6.3 V, 0.15 A

Peak heater-cathode voltage ±100 V maximum

Direct interelectrode capacitance (approx.)

Grid #1 to plate 0.11 pF maximum

Grid #1 to cathode, heater, grid #2, and grid #3

2.7 pF

Plate to cathode, heater, grid #2, and grid #3

2.4 pF

Class A1 Amplifier, Maximum Ratings (Design-Maximum Values)

Plate voltage 330 V


Grid #1 (control grid) voltage



Grid #2 (screen grid) voltage See Figure 10.5

Grid #2 (screen grid) supply voltage 330 V

Grid #2 (screen grid) input

For grid #2 voltages up to 165 V 0.24 W

For grid #2 voltages between 165 V and 300 V

See Figure 10.5

Characteristics

Plate voltage 250 V

Grid #1 (control grid) voltage –3 V

Grid #2 (screen grid) voltage 100 V

0 100 299 300 400

Type 5879 triode sectionType 5879pentode connection

Grid #2 = 100 Vgrid #3 = 0 V

Plate volts Plate volts0 100 200 300 400

Pla

te m

illia

mpe

res

Pla

te (I

) o

r grid

#2

(I )

mill

iam

pere

sb

c2

14

12

10

8

6

4

2

6

5

4

3

2

1

0 100 200 300 400

Grids #2 and #3connected to plate

(a) (b)

Figure 10.4 Characteristic curves for the 5879 tube: (a) triode section, (b) pentode section.

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WhitakerAudio

For certain voltage amplifier types, this device included, the maximum permissible screen

grid input varies with the screen grid voltage, as shown in Figure 10.5. Full rated screen grid input

is permissible at screen grid voltages up to 50 percent of the maximum rated screen grid supply

voltage. From the 50 percent point to the full rated value of supply voltage, the screen grid input

must be decreased. The decrease in allowable screen grid input follows a curve of the parabolic

form. The rating chart is useful for applications utilizing either a fixed screen grid voltage or a

series screen grid voltage-dropping resistor; specifically:• When a fixed voltage is used, it is necessary only to determine that the screen grid input is

within the boundary of the operating area on the chart at the selected value of screen grid volt-

age to be used.

· When a voltage dropping resistor is used, the minimum value of resistor that will assure tube

operation within the boundary of the curve can be determined by the relation

Where:

Rg2 = minimum value for the voltage-dropping resistor in ohms

Ec2 = selected screen grid voltage in volts

Ecc2 = screen grid supply voltage in volts

Pc3 = screen grid input in watts corresponding to Ec2

Grid #3 (suppressor grid) Connected to cathode at socket

Plate resistance (approx.)

2 mΩ

Transconductance 1000 μmhos

Plate current 1.8 mA

Grid #2 (screen grid) current 0.4 mA

Grid #1 (control grid) voltage (approx.) for plate current of 10 μA

–8 V

Maximum Circuit Value

Grid #1 (control grid) circuit resistance 2.2 mW

Table 10.2 Key Parameters of the 5879.

( )2 2 22

2

c cc cg

c

E E ER

P−

≥

96


10.1.3 7025 (12AX7)

The 7025 is a miniature type used as a phase inverter or resistance-coupled amplifier in high-

fidelity audio amplifiers. The 7025 is used in the phonograph preamp and line-out buffer

amplifiers in the stereo preamplifier. This type is identical with the 12AX7A (ECC83) tube except

that it has a controlled equivalent noise and hum characteristic. Each triode is independent of the

other except for the common heater.


10.7.

This curve also applies to typesin which grids #2 and #4 are connected

together within the tube

Grid

#2

inpu

t exp

ress

ed a

s a

perc

enta

ge o

f max

imum

grid

#2

inpu

t rat

ing

100

80

50

40

20

0 20 40 60 80 100 120Grid #2 voltage expressed as a percentage of maximum

grid #2 supply voltage rating

Area of permissible operation

Figure 10.5 Grid #2 (screen grid) input rating curve.

Pt2

Pt1Kt2

Ht2

Gt2

Ht1

Gt1

Kt1H

1

2

3

45

6

7

8

9

Legend:H = heaterK = cathode, triode (t) 1 or 2G = grid, triode (t) 1 or 2P = plate, triode (t) 1 or 2Figure 10.6 Terminal pinout of the 7025 tube.

Plate volts0 100 200 300 400

Type 7025

Pla

te m

illia

mpe

res

3

2

1

Figure 10.7 Characteristic curves for the 7025 tube.

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WhitakerAudio


Table 10.3 Key Parameters of the 7025.General Characteristics

Heater (ac/dc)

Parallel connection 6.3 V, 0.45 A

Series connection 12.6 V, 0.15 A

Heater-cathode voltage

Peak value ±200 V max.

Average value 100 V max.

Direct interelectrode capacitance (approx.) Unit #1 Unit #2

Grid to plate 1.7 pF 1.7 pF

Grid to cathode and heater 1.6 pF 1.6 pF

Plate to cathode and heater 0.46 pF 0.34 pF

Class A1 Amplifier, Each Unit, Maximum Ratings (Design-Maximum Values)

Plate voltage 330 V


Grid voltage




Equivalent Noise and Hum Voltage Referenced to Each Grid (each unit)

Average value (rms)1 1.8 μV

Maximum value (rms)2 7 μV

Notes:

1 Measured in “true rms” units under the following conditions: heater volts (ac) = 6.3 V (parallel connection); center tap of heater transformer connected to ground; plate supply = 250 V; plate load resistor = 2700 Ω; cathode bypass capacitor = 100 μF; grid resistor = 0 Ω; and amplifier covering frequency range 25 Hz to 10 kHz.

2 Same conditions as for “average value” except the cathode resistor is unbypassed and the grid resistor = 50 kΩ.

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WhitakerAudio


Stereo Preamplifier

Addendum

The Stereo Preamplifier User and Assembly Manual is written with the assumption that the user

has purchased the product from WhitakerAudio as a ready-to-assemble package. For builders that

have acquired the components on their own and are using the manual as an assembly guide, it is

recommended that the parts be divided into the packaging referred to in the manual.

In the process of acquiring parts for this amplifier, a number of vendors will be used. As such,

parts will arrive lumped together and will also arrive at various points in time. The process used

by WhitakerAudio involves organizing the components into three major groups:

• Right channel printed wiring board (PWB) components

• Left channel PWB components

• Chassis components

Parts are organized by component number and device type. For example, all 1 kΩ 1/2-watt

resistors for the right channel PWB are packaged together. Each group of parts is placed in a

resealable (clasp-type) envelope with a self-adhesive label applied. Labels such as Avery #5260

are useful for this purpose. Recommended envelopes include the following:

• 2.25-inch by 3.5-inch envelope (Staples no. 1 coin envelope)

• 3-3/8-inch by 6-inch envelope (Staples no. 6 coin envelope)

• 6-inch by 9-inch clasp envelope (Staples)

While this sorting requires some up-front organizational time, it serves as a check that all parts

are accounted for and speeds assembly of the PWBs and chassis.

The following tables list the labels used for the three component groups.

Table 1 Right Channel PWB Component Package LabelsC1, C4, C11, C24, C3125 μF, 50 V, electrolytic,

right channel

R147 kΩ, 0.5 W,right channel

V1, V4, V57025/12AX7 tube,

right channel

C2, C5, C15, C17, C26, C28 C29

22 μF, 450 V, electrolytic,right channel

R2, R72.7 kΩ, 0.5 W,right channel

V25879 tube,

right channel

C3, C160.1 μF, 400 V,right channel

R3, R5, R12, R25, R28, R31, R35, R37, R39, R45

100 kΩ, 0.5 W,right channel

V36EU7 tube,

right channel

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WhitakerAudio

C60.0033 μF, 600 V,

right channel


D1, D2Zener, 200 V, 5W,

right channel

C70.01 μF, 400 V,

right channel

R6, R14, R20470 kΩ, 0.5 W,right channel

Heater circuit wire6 ft., #22, green, solid,

right channel

C8180 pF, 500 V, mica,

right channel


4-terminal Molex barrier strip,right channel

C9, C12, C20, C25, C320.22 μF, 400 V,

right channel

R9, R15, R29, R38, R4115 kΩ, 2 W,right channel

¾-inch #4 threaded standoff(quantity = 10),right channel

C10, C14, C330.047 μF, 400 V

right channel

R10, R2322 kΩ, 0.5 W,right channel

12 terminal Molex header(quantity = 2),right channel

C1347 μF, 450 V, electrolytic,

right channel

R112.2 mΩ, 0.5 W,right channel

4 terminal Molex header(quantity = 8),right channel

C18, C220.0022 μF, 600 V,

right channel

R13, R24, R36, R461 kΩ, 0.5 W,right channel

9-pin tube sockets(quantity = 5),right channel

C190.022 μF, 400 V,

right channel

R16, R17, R40250 kΩ potentiometer, linear,

right channel

Printed Wiring Board,right channel

C21220 pF, 600 V, ceramic,

right channel

R211.5 kΩ, 0.5 W,right channel

Tube socket hookupbare wire, heat shrink, tie wrap

right channel

C23, C30100 pF, 1000 V disc,

right channel


CP25 pF, 1000 V ceramic,

right channel

C270.01 μF disc, 1 kV,

right channel


R42100 Ω, 1 W, potentiometer,

right channel

R47100 kΩ, 2W,right channel

R33, R34, R43, R44220 kΩ, 0.5 W,right channel

R18, R19100 kΩ potentiometer, linear,

right channel

Table 2 Left Channel PWB Component Package LabelsC1, C4, C11, C24, C3125 μF, 50 V, electrolytic,

left channel

R147 kΩ, 0.5 W,left channel

V1, V4, V57025/12AX7 tube,

left channel

C2, C5, C15, C17, C26, C28 C29

22 μF, 450 V, electrolytic,left channel

R2, R72.7 kΩ, 0.5 W,

left channel

V25879 tube,left channel

Table 1 Right Channel PWB Component Package Labels

102

Stereo Preamplifier

C3, C160.1 μF, 400 V,

left channel

R3, R5, R12, R25, R28, R31, R35, R37, R39, R45

100 kΩ, 0.5 W,left channel

V36EU7 tube,left channel

C60.0033 μF, 600 V,

left channel


D1, D2Zener, 200 V, 5W,

left channel

C70.01 μF, 400 V,

left channel

R6, R14, R20470 kΩ, 0.5 W,

left channel

Heater circuit wire6 ft., #22, green, solid,

left channel

C8180 pF, 500 V, mica,

left channel

R8680 kΩ, 0.5 W,

left channel

4-terminal Molex barrier strip,left channel

C9, C12, C20, C25, C320.22 μF, 400 V,

left channel

R9, R15, R29, R38, R4115 kΩ, 2 W,left channel

¾-inch #4 threaded standoff(quantity = 10),

left channel

C10, C14, C330.047 μF, 400 V

left channel

R10, R2322 kΩ, 0.5 W,left channel

12 terminal Molex header(quantity = 2),left channel

C1347 μF, 450 V, electrolytic,

left channel

R112.2 mΩ, 0.5 W,

left channel

4 terminal Molex header(quantity = 8),left channel

C18, C220.0022 μF, 600 V,

left channel

R13, R24, R36, R461 kΩ, 0.5 W,left channel

9-pin tube sockets(quantity = 5) ,

left channel

C190.022 μF, 400 V,

left channel

R16, R17, R40250 kΩ potentiometer, linear,

left channel

Printed Wiring Board,left channel

C21220 pF, 600 V, ceramic,

left channel

R211.5 kΩ, 0.5 W,

left channel

Tube socket hookupbare wire, heat shrink, tie wrap

left channel

C23, C30100 pF, 1000 V disc,

left channel


CP25 pF, 1000 V ceramic,

left channel

C270.01 μF disc, 1 kV,

left channel


R18, R19100 kΩ potentiometer, linear,

left channel

R47100 kΩ, 2W,left channel

R33, R34, R43, R44220 kΩ, 0.5 W,

left channel

Table 3 Chassis Component Package Labels

Plexiglas trim,chassis

Large knob(quantity = 4),

chassis

Top chassis decal,chassis

Table 2 Left Channel PWB Component Package Labels

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WhitakerAudio

R26, R301 mΩ potentiometer, audio

taper, dual gang,chassis

Handle, side(quantity = 2),

chassis

Braided sleeving andheat-shrink tubing

chassis

SW1Rotary switch,

chassis

RCA connector(quantity = 10),

chassis

Chassis hookup wirechassis

SW2Toggle switch,

chassis

Mic connector(quantity = 2),

chassis

Ground lug hardware,chassis

Main chassis, 10-in x 17-in x 3-in, steel

chassis

3.5 mm connectorchassis

Cable management hardwarechassis

Main chassis baseplate, 10-in x 17 in, steel,

chassis

Power connector hardwarechassis

Molex crimp terminal(quantity = 128),

left channel

Front panel, 19-in x 3.5-in,chassis

12 terminal Molex housing(quantity = 4)

chassis

Pilot Lamp,chassis

Rear panel overlay(quantity = 3),

chassis

4 terminal Molex housing(quantity = 16)

chassis

Chassis feet(quantity = 4),

chassis

Front panel handles(quantity = 2),

chassis

Molex crimp terminal(quantity = 64),

chassis

R32250 kΩ potentiometer, audio

taper, dual gang,chassis

C284700 μF, 50 V, electrolytic

chassis

CR-1bridge rectifier, 25 W, 50 V

chassis

9-pin tube socket shield(quantity = 4)

chassis

Table 3 Chassis Component Package Labels

104

Cable Assembly Procedure


The stereo preamplifier relies on the power supply contained in the audio amplifiers of the

WhitakerAudio line to function. Power from the amplifier is coupled to the preamp and/or tuner

through a connecting cable. This document describes the procedures for assembling the necessary

cable.

The pinout for the power connector is given in Table 1.

1 Procedure

Expandable flexible sleeving is used to bundle the wires of the cable assembly. To properly

perform the following steps, it will be necessary to have a heat gun on hand. (A common hair

dryer may be sufficient.) In addition, the preferred method of cutting the tubing is to use a so-

called “heat knife.” If such a tool is unavailable, scissors may be used.

Examine the connector body. The pin numbers are stamped on both sides of the device. Be

certain to place each pin in the correct position.

The following procedure is recommended for assembling the connecting cable.

1.1 Step-by-Step Instructions

The standard length for this cable is 3 ft. Users requiring a longer (or shorter) cable should adjust

the following steps accordingly.

– Prepare two 3-ft pieces of #16 black stranded wire. Attach male connectors on one end

and female connectors on the other end.

Table 1 Power Connector PinoutPin No. Function Color Code

1 Ground Black

2 Pilot lamp White

3 Pilot lamp White

4 Auxiliary B+ Red

5 Filament Green

6 Filament Green

7 Not used

8 Not used

9 Ground (Black

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WhitakerAudio

– Prepare a 3-ft piece of #16 red stranded wire. Attach a male connector on one end and a

female connector on the other end.

– Prepare two 3-ft pieces of #16 white stranded wire. Attach male connectors on one end


– Prepare two 3-ft pieces of #16 green stranded wire. Attach male connectors on one end


– Insert the male connectors into a matching housing as follows:

• Black wire to pin #1

• White wire to pin #2


• Red wire to pin #4

• Green wire to pin #5



Note that pin #7 and pin #8 are not used.

– Loosely twist the two white wires together.

– Loosely twist the two green wires together.

– Cut a 3-ft length of expandable sleeving and place over the cable bundle. Cut two 3/4-inch

long pieces of heat-shrink tubing and place on each end of the sleeving. Process (heat) the

tubing closest to the connector socket. Caution: do not use too much heat as it can melt the

sleeving.

– Assemble the connector by screwing the housing onto the connector body. Install and

secure the cable clamp.

– Slide the housing for the female connector on to the cable. Be sure to properly orient the

housing with respect to the connector.

Using an ohmmeter, identify the black wire inserted in the pin #1 position on the male

connector. This wire should be connected to pin #1 on the female connector. For the white wire

pair and the green wire pair, an exact match between the connectors is not necessary.

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– Insert the female connectors into a matching housing as follows:




• Red wire to pin #4




Note that pin #7 and pin #8 are not used.

– Process (heat) the tubing closest to the connector socket. Caution: do not use too much

heat as it can melt the sleeving.

– Assemble the connector by screwing the housing onto the connector body. Install and

secure the cable clamp.

The connecting cable has been completed and is ready for use.

2 Ground Connection Cable

The back panel power connector to the amplifier/power supply includes two ground

connections to make sure that a solid ground is always maintained between the preamplifier and

the power amplifier. Maintenance of the ground is essential to safe operation of the preamplifier.

As a measure of additional safety, a separate ground wire, connected to the ground posts on the

back panel of the amplifier and the preamplifier, must always be in place and secured. Do not

operate the preamplifier without all covers in place and the separate ground connection installed.

The ground connection cable is assembled as described in the following steps:

– Prepare a 3-foot length of green #16 wire.

– Attach a ring terminal to each end of the cable. Crimp.

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WhitakerAudio€¦ · 1 Circuit Description 8 2 Parts List 12 3 PWB Design 17 4 Construction...

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