Power Supply Boardfor the

Dynaco ST-70with “failsafe” bias and balance

Classic Valve Design

!! CAUTION !!

The voltages this product is designed to operate at can kill you.

It is assumed that you are reasonably competent in the field of high voltage electronics, DIY and/or repair.

Improper handling or operation of this board is not only hazardous to you, but anyone coming in contact with the amplifier, including children and

pets.

If you are not capable of installation and use of this board, please refer to a qualified and experienced individual who is.

There is no shame in knowing your limitations!

!! CAUTION !!

The Dynaco ST-70 is one of the more commonly found vintage stereo amplifiers on the used market today. They are even manufactured still as kits by at least two companies at the time of this writing.

Like many vintage amplifiers found, they are often in need of repair and/or restoration. Updating of the circuitry also goes a long way to improving their performance. This versatile power supply board will help you along in your restoration project, or scratch build.

This power supply board allows you the opportunity to add a bias balance control to your Dynaco, making output tube biasing a breeze and frees you from the need to buy tubes in two matched pairs, or in matched quads.

Notice we have used the term failsafe bias, this is referring to the way the bias and balance pots are wired to deliver the required bias voltages. On the original Dynaco circuit, if the bias pot wiper “lifts” (the most common failure mode of a potentiometer), the output tubes lose bias, go into full conduction and are often destroyed (sometimes taking the output and/or power transformers with them). In this power supply, if any potentiometer wiper lifts, the output tubes will receive full negative bias voltage to their grids and simply cut off.

Schematic and Parts List

Resistors:

R1 - R8: 220K, 1W resistor. Digikey part number 220KW-1-ND

R9: 22K, 2W resistor. Digikey PN 22KW-2-ND (for original ST-70 driver board. See documentation for your driver board if other.)

R10: 6.8K, 2W resistor. Digikey PN 6.8KW-2-ND (for original ST-70 driver board. See documentation for your driver board if other.)

R11, R12: 12K, ½ W resistor. Digikey PN 12KH-ND

R13, R14, R15, R16, R18, R19: 10K, 1/2W resistor. Digikey PN 10KH-ND

R17, R20: 56K, ½ W resistor. Digikey PN 56KH-ND

RV1, RV2: 25K, ½ W trimming resistor. Single or multiple turn, like Digikey PN 3386P-253LF-ND or 3296W-253LF-ND

RV3, RV4: 10K, ½ W trimming resistor. Single or multiple turn, like Digikey PN 3386P-103LF-ND or 3296W-103LF-ND

Capacitors:

C1 - C10: 47uF, 350V electrolytic. Digikey PN 565-1453-ND

C11 - C13: 47uF, 100V electrolytic. Digikey PN 493-1973-ND

C14, C15: 0.022uF, 400V MKP film. Digikey PN 495-1305-ND

C16, C17: 0.22uF, 250V film. Digikey PN P10971-ND

Semiconductor:

D1: 1N4007 diode. Digikey PN 1N4007DICT-ND

The preceding is a list of parts we used in our prototype and function perfectly for our listening. They are also economical choices.

Feel free to upgrade to “audiophile” grade parts, just mind the heights of C1-C10 (don't go over 25mm hight) and thicknesses of the bottom mounted resistors (¼ “ or less for the following mounting instructions).

Circuit Notes

A safety note about the capacitors: The exposed tops of C2, C4, C6, C8, C10 are HOT at ½ the rectified and filtered high voltage power supply voltage (210 to 265V) and should not be touched. The application of “circle stickers” found in stationary stores can be used to insulate these capacitor tops.

Likewise, because of the -V bias supply, the exposed tops of bias capacitors C11-C13 are at full negative bias voltage (up to -80V). Again application of “circle stickers” found in stationary stores can be used to insulate these capacitor tops.

Some parts marked in the schematic with an asterisk need to be installed first. Some parts are mounted on the bottom, others the top. Installing the marked parts first will save you some time desoldering.

Please take care to make sure the capacitor's polarity is respected as outlined on the PCB and as per above schematic – reversed polarized electrolytics have a tendency to EXPLODE, spewing hot electrolyte all over the place!

Installation

Once you have assembled the parts on your circuit board, it will install underneath the left output transformer.

Because this board contains the bias power supply, you can remove the terminal strip the original bias components were mounted on. This will leave you room to mount the board.

The screws that hold the output transformer on to the chassis are not long enough to mount the board as well. You will need four #8-32 x ¾ “ long screws to complete your task, as well as associated hardware, depending on the method you chose below:

The “B” method uses less hardware, but more skill to install and a 3/8” long #8 standoff isn't something you can get at most neighborhood hardware stores.

Hooking Up to the Circuitry

Once the board is installed, you can begin hooking up the wiring from the top. Since the holes are plated through, soldering from the bottom is not necessary.

● The output from the GZ34/5AR4 rectifier goes to the HT-in.● Attach the bias tap to the Bias input.● Attach the power transformer's HV centre tap to the TR-CT and run a wire

from the second TR-CT hole to the chassis star ground. If you (or someone else) have modified your ST-70 to use a floating or other isolated ground system, attach the power transformer's HV centre tap to the TR-CT and ground to your circuit as per your modifications.

● Attach the choke leads to choke. You may need to extend the choke leads to accomplish this.

● Attach the transformer filament centre taps to the Fil-CT. Order is not important here.

● Connect the centre taps of the output transformers to the EL34.● Connect wires from the Phase to hole #20 on the driver board if yours is an

original Dynaco circuit. If yours is another driver, this should go to the phase inverter B+ supply.

● Connect wires from the Preamp to hole #19 on the driver board if yours is an original Dynaco circuit. If yours is another driver, this should go to the preamp B+ supply.

● Finally, connect the bias outputs L-Bal-R to the appropriate bias resistors on your driver board. If you are using the original Dynaco driver, see the next section on conversion.

Converting the ST-70 for Bias Balance Controls

This procedure is in two steps, changing the EL34 cathode circuitry and modifying the driver board (this step isn't necessary if you are using a driver with split bias connections, like the one from KTA-HiFi.net).

Step 1 – Modifying the EL34 Cathodes:

In order to monitor the bias in each EL34, we need to have a resistor on each EL34 cathode. We need to remove the existing 15.6 ohm resistor and the bridge between pin 8 on each channel's output pair and then install two new resistors. We will use 10 ohm resistors on each cathode.

● Locate the 15.6 ohm resistor near the EL34 output tube sockets. There is one for the left output pair and one for the right output pair. Remove them.

● Locate the wire that goes between the two output tubes on pin #8. Remove it.● Solder a 10 ohm resistor (Digikey PN PPC10.0XCT-ND is a good one) from

each EL34 pin 8 to chassis lug on the mounting rings (the 15.6 ohm resistor was tied to one in the original circuit).

● If you (or someone else) have modified your ST-70 to use a floating or other isolated ground system, tie the ground side of the 10 ohm resistors to a terminal strip and run a wire to an auxiliary ground pad on the power supply board (near the bottom right of C13).

Step 2 – Modifying the Classic ST-70 Driver Board:

Or if you prefer not to cut traces on your board, you can simply lift the bias resistor pins:

If you have a board where there are no bias resistors on the driver board, like those original designs from us, you will need to add the resistors to the EL34 grids:

First Power Up and Biasing

Before first power up, remove your GZ34/5AR4 rectifier from the circuit. We want to test first that there is negative bias where there should be before applying B+ to the EL34 tubes.

● Connect a voltmeter on a setting to handle up to -60VDC to any left channel EL34 grid (pin 5) and electrical ground.

● Set RV3 and RV4 to midway.● Turn on your ST-70.● A negative voltage should register immediately, as it's a SS rectified circuit. If

it does not, go through your component mounting and wiring thoroughly!● Adjust RV2 for MAXIMUM negative voltage reading. This should be over

-40VDC. Likely towards -50VDC.● Confirm the same voltage reading on the other left channel EL34 grid.● Connect your voltmeter now to any right channel EL34 grid.● Adjust RV1 for MAXIMUM negative voltage reading. This also should be over

-40VDC.● Confirm the same voltage reading on the other right channel EL34 grid.● Turn off your ST-70 and replace the pulled rectifier tube. You are now ready

for live biasing.

Biasing and Balancing

1) Connect a voltmeter on a setting to handle up to 2VDC to any left channel EL34 cathode (pin 8) and electrical ground.

2) Turn on your ST-70.3) As your ST-70 warms up, you will see the voltage come up. If this cathode

voltage creeps above 550mV (0.55V or 55mA for that tube) at this time, immediately turn off your ST-70 and repeat “First Power Up” to look for the problem. If none is found, your tube grids may be on their way out (unable to restrict current flow enough).

4) After about three minutes warm up time, adjust RV2 for a bias reading of 400mV. This corresponds to 40mA current through that tube.

5) Now remove the negative voltmeter lead from electrical ground and place it on the cathode of the other EL34 (Pin 8).

6) Adjust RV4 for a reading of 0VDC (anything within 10mV (0.01V) is just fine).7) Repeat steps 4-6.8) Repeat entire procedure for right channel using RV1 for bias and RV3 for

balance.9) Check left channel bias again (they interact somewhat because the power

supply is not regulated).

The original Dynaco is specified for 50mA per tube. We find this way too hot in our opinion (especially for an old, original PA060 power transformer) and recommend 40 to 43mA per tube. If you wish original spec, please use 500mV cathode voltage as your guide.

Congratulations! Your ST-70 is now ready for service! :-)

Options

As we said earlier, this power supply board is versatile. Here are some options for you:

Q: What if I do not want to use the balance option, just a straight, classic Dynaco circuit?

A: No problem! Just follow these steps:

● Leave out the following components: R15-R20, C16, C17, RV3 and RV4.● Change R11 and R12 to 18K, ½ W (Digikey PN 18KH-ND for example)● Run a wire from the centre pin (wiper) of RV2 to hole #6 on the classic ST-70

driver board and from the centre pin (wiper) of RV1 to hole #21.● Skip the EL34 cathode modifications in this manual.● Bias as you would any ST-70.

Q: I have my own bias circuitry...

A: Leave out everything in the bias section except D1 and C12. There is a pair of -V bias voltage takeoffs for your own use to the immediate right of the C11 pad spaces.

If you want to make a C-R-C filter, use also C11 and R11 and use the takeoff pads to the immediate left of RV1.

Q: Can I use the pots that came with my Dynaco?

A: You can use them as the “Balance” controls as both are 10K value. But old pots get dirty and unreliable. It is recommended by us you just disregard the pots that came with your Dynaco and just keep them to fill the holes.

Q: I don't like the choice of resistors/capacitors you give in your sample parts list. Can I use something different?

A: Use whatever you want! Just keep in mind the hight and diameter restrictions because the ST-70 is not the tallest chassis ever made and the PCB parts are crammed in pretty good. You also want to avoid having a part too close to the chassis for danger of high voltage arc-over.

Pad spacing may be different for your parts too and bending leads to fit will eat up some hight headroom.

Q: Can I use this as a power supply for some other amp or my own design?

A: Go nuts! Please share your results if you wish and let us know how it goes.

Legal Rubbish

Classic Valve Design guarantees this PCB for workmanship quality only. We make no claim of responsibility for damage or harm to you or your amp from its

use or misuse.