Regens international - Newsletter mercato immobiliare in Russia, febbraio 2017
Designing simple low-voltage vacumm-tube regens
Transcript of Designing simple low-voltage vacumm-tube regens
Designing Simple Low-Voltage Vacumm-Tube Regenerative Receivers
Ramon Vargas Patron
INICTEL-UNI
The design and construction of RF and AF electronic circuits using vintage discrete
technologies has been a very rewarding part-time entertainment for decades, and will
certainly continue to be so in the future. You can find everywhere experimenters
acquainted with solid-state technology replicating with great interest old technology
approaches for amplifiers, oscillators and radio receivers, and even mixing them in a
particular design. We are referring to the use of Galena (PbS), Pyrates (FeS), germanium
and silicon diodes, vacumm tubes, germanium and silicon bipolar transistors (BJTs),
junction field effect transistors (JFETs), 555 IC timers, and even logic ICs as signal
amplifiers when biased for linear operation. We can find there are myriads of possible
applications.
This article will show how to build an AM MW BCB regenerative receiver using mixed
technologies, specifically, vacumm tube, JFET and BJT. We would like a vacumm-tube
RF stage followed by a JFET buffer stage and a BJT pre-amplifier, just for headphone
listening.
The direct approach for a regenerative receiver design employing vacumm-tube
technology usually starts with a review of the technical specifications of the devices
selected, the required power supply, parts and components. Amplifying tubes have
generally been designed to work with more than 40Volts DC on plate (anode), imposing
a threat on unaware folks. Portable electronic equipment made use of “B” batteries for
the plate voltages. Common values for these were 45 Volts, 67.5 Volts and 90 Volts. “A”
batteries were used for filament heating, being 1.5 Volts, 2 Volts and 7.5 Volts the most
used types.
A careful study of the plate-current vs plate-voltage characteritics (Ip vs Vp) of some
amplifying tubes led curious experimenters to test the operation of these devices with low
plate voltages. These tests were successful for many of those tubes designed for portable
operation on batteries. The author tested a couple of these, i.e., a 1A5GT, a filament-type
power-amplifier output pentode for use in low-drain battery-operated equipment (see
Annex I), and a 2SH27L, a Russian universal pentode featuring also directly-heated
cathode operation (Annex II). The suppressor grid of the first tube is internally tied to the
filament (cathode), while the second tube has that grid wired for external connection. In
this article we shall dedicate efforts towards the construction of a regen with the 1A5GT
pentode. In a next article we will describe a Transitron negative-resistance type AM MW
BCB receiver using a 2SH27L pentode.
Let see Fig.1. It shows the schematic diagram of an experimental regen for medium wave
frequencies built on a solderless breadboard using the 1A5GT. It uses a 1.5-Volt alkaline
battery for filament heating and a 14-Volt DC supply for the “high tension”of amplifier
stages. Regeneration is adjusted varying the screen grid bias voltage, and is quite smooth
on its action. The “high tension” supply can be reduced to 12 Volts DC and still get
satisfactory operation of the circuit. Exact limits for an acceptable operation will depend
on the tube and the state of the 1.5-Volt battery. The autor has been testing the circuit
with a partially exhausted battery measuring 1.38 Volts with very good results.
The stage following the 1A5GT is a buffer designed around an MPF102 N-channel JFET
for driving the rather low input impedance of the BJT output stage. The JFET stage has a
voltage gain of 1.46. The bipolar stage with a 2N3906 has a voltage gain of 310. Thus,
the overall AF voltage gain is 1.46 x 310 = 452.6, enough for comfortable hearing, as
tested. Current drain from the solid-state audio stages is about 6.8mA. The tube’s plate
(anode) current is less than 100uA DC at pleasant hearing levels.
Due to operating the receiver from two different types of supplies (one is an isolated
battery and the other is a mains-operated DC source), some 60Hz hum is noticed,
however, wrapping the vacumm tube with a coil made up with AWG#22…24 PVC-
insulated stranded copper wire and connecting one end of the coil to ground, the hum is
reduced to very tolerable levels. An aluminum ground plane was used to mount the
solderless breadboard, the air-variable tuning capacitor and the ferrite antenna assembly.
Listening is through a low-cost ceramic piezoelectric earphone or magnetic 2k-ohms
headphones. The screen-voltage control potentiometer acts doubly as a volumen control
also for listening. Sensitivity is very good as well as selectivity. A vernier reduction drive
is recommended for the tuning capacitor because tuning is very sharp due to regeneration.
Figures 2.A, 2.B, 2.C below show the receiver’s experimental layout, a front view and a
top view, respectively.
Fig. 1 Experimental Hybrid Regenerative AM MW BCB Receiver
Fig. 2.A Layout of the receiver
Fig. 2.B Front view of the receiver
Fig. 2.C Top view of the receiver
ANNEX II
Fig. ANX 2 Russian Universal Pentode 2SH27L
2SH27L Universal Pentode with directly heated cathode
Electrical data Limit Data
Heating voltage: 2.2 V Heating Voltage: 2.0 V min - 2.4 V max
Heating current: 57 mA Anode Voltage: 200 V max
Grid 1 voltage: 0 V Voltage Grid 2 [screen grid]: 120 V max
Grid 2 voltage [Screen grid voltage]: 45V Anode Power Dissipation: 1.0 W max
Voltage at Grid 3: 0V Screen Grid Load: 0.3 W
Anode Voltage: 120V Maximum Cathode Current: 5.0 mA
Grid 2 current [Screen grid current]: 0.35mA
Anode Current: 1.9mA Tube’s Base circuit
Slope: 1.25mA /V 1 - cathode [heater]
Capacitance Input / Output: 0.015pF (max) 2 - inner shield
Input Capacity: 5.3 pF 3 - anode
Output Capacity: 4.9 pF 4 - grid 3
Capacitance Anode/Cathode: 0.01pf (max) 5 - grid 2
End of Life Slope: 0.85mA/V 7 - grid 1
8 - cathode [heater]
Ramon Vargas Patron
Lima – Peru /South America / October 6th 2021