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The Dynaco Tube Audio Forum » Basket » Dynaco ST-70 PC-3 driver board parts layout - photo ...

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Bob Latino


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Dynaco never gave a parts layout on their PC-3 driver board for the ST-70. They also never listed the parts that were on the driver board in the main parts list for the amp on page 11 of the ST-70 assembly manual. All they said in the parts list on page 11 was that you got a "Printed Circuit Assembly - PC-3 part #557003". When you got your ST-70 kit the driver board was already preassembled. In the photo below you can see the value of every resistor and capacitor on the PC-3. The photo is of one of the fiberglass PC-3 boards that Dynaco started using in the last few years of production. If you have one of the older "brown" PC-3 driver boards, the parts layout is identical.

Bob

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2 TC-7510 PCB on Fri Nov 06, 2009 11:58 am

Hirak


Thanks very much for the picture. I just obtained a ST-70A sans EL-34s (the seller would not include these!) and I am looking to replace the quad can and the caps on this board. I assume I would replace the same values but opt for higher voltages? Any recommendations on capacitor quality (brand) and voltages?
Thanks!

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Bob Latino


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Hirak wrote:Thanks very much for the picture. I just obtained a ST-70A sans EL-34s (the seller would not include these!) and I am looking to replace the quad can and the caps on this board. I assume I would replace the same values but opt for higher voltages? Any recommendations on capacitor quality (brand) and voltages?
Thanks!


Hirak,

The BEST can capacitor you can put in there as a replacement quad cap for the ST-70 is the Dynakitparts 80, 40, 30, 20 uF 550 rated volt quad cap. Kevin at Dynakitparts has them made by F&T capacitors in Germany. I use one in my own ST-70. They are $50 each but worth it in the long run. When you install this cap just have the "80" section face the back of the amp and all the other sections will be in the correct position. This cap has almost double the DC power storage of the original Dynaco quad cap.

Dynakiparts 550 volt rated quad cap

As for cap sizes > With the four main coupling caps on this board (which are .10 uF @ 400 volts), you can go up to .22 @ 400 volts or .22 @ 600 volts. On the two .05 @ 400 volt caps that are beside the two tube sockets you can go up to .10 @ 400 volts. Nice caps for the money in both positions (and caps that will fit the two positions very well) are the SBE 716P "orange drop" caps. Antique Electronic Supply sells these. If you plan to replace EVERYTHING on an original driver board IMHO you are probably better off just getting a NEW DRIVER BOARD AND PARTS SET from Dynakitparts with metal film resistors.($50) Metal film resistors hold their tolerance a lot better than the old carbon composition resistors as found on the original ST-70 board and are nearly always quieter. The parts set with this board includes 600 volt rated capacitors.

Bob

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4 TC-7510 PCB on Mon Nov 09, 2009 10:48 am

Hirak


Thank you!

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