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Dynaco ST-70 PC-3 driver board parts layout - photo ...

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Bob Latino
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Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Wed Nov 04, 2009 10:43 am

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



Hirak

Posts: 1
Join date: 2009-11-06

TC-7510 PCB

Post by Hirak on Fri Nov 06, 2009 9:58 am

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!

Bob Latino
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Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Fri Nov 06, 2009 12:25 pm

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 $60 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


Last edited by Bob Latino on Thu Jul 07, 2011 12:20 pm; edited 2 times in total

scsidude2

Posts: 21
Join date: 2010-03-19
Age: 28
Location: Dekalb, IL

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by scsidude2 on Fri Mar 19, 2010 4:11 pm

First of all thank you for the picture, it has been very helpful in my rebuilding project. I was wondering, though, if there are any side effects to moving to .22uF at 600 volts from .1uF at 400 volts? Thank you! -Ryan

Bob Latino
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Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Fri Mar 19, 2010 4:33 pm

scsidude2 wrote:First of all thank you for the picture, it has been very helpful in my rebuilding project. I was wondering, though, if there are any side effects to moving to .22uF at 600 volts from .1uF at 400 volts? Thank you! -Ryan

Hi Ryan,

No side effects that I have seen. The 4 main coupling caps on the ST-70 "should have been" (IMHO) .22 uF @ 600 volts. Dynaco saved a few cents on each amp by using the .10 uF @ 400 volts. I think the .22 uF cap may also help give a stock amp a little better bass response and the 600 volts cap will stand up a little better to the high voltage on one side of the cap.

The only minor disadvantage is that many .22 uF @ 600 volt caps will be a little larger. Nice caps for the money that would work here are the SBE 716P .22 @ 600 volt "orange drop" caps sold by Antique Electronic Supply. They are larger than the stock caps but will fit OK onto a stock board. You just bend the leads to fit .They are $2.36 each. Small photo below of these caps below ...



Bob

scsidude2

Posts: 21
Join date: 2010-03-19
Age: 28
Location: Dekalb, IL

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by scsidude2 on Sat Mar 20, 2010 10:15 am

Thank you for your quick and thorough response Bob, you answered my question completely. I do have one last question though, which is what voltage the 390pF Caps run at? I've seen them for both 1000v and 500v. Thanks! -Ryan

Bob Latino
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Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Sat Mar 20, 2010 10:20 am

scsidude2 wrote:Thank you for your quick and thorough response Bob, you answered my question completely. I do have one last question though, which is what voltage the 390pF Caps run at? I've seen them for both 1000v and 500v. Thanks! -Ryan

Ryan,

For the 390 pF cap a 500 volt rated cap is more than enough. A silver mica cap IMHO is best here.

Bob

scsidude2

Posts: 21
Join date: 2010-03-19
Age: 28
Location: Dekalb, IL

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by scsidude2 on Sat Mar 27, 2010 5:02 pm

Thanks for all the help Bob, i just installed the new caps and it sounds great! I was wondering what your recommendation was for the two 50 MFD caps on the terminal strip under the right channel output transformer as to what would be a good upgrade for them? Thanks again! -Ryan

Bob Latino
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Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Sat Mar 27, 2010 5:27 pm

scsidude2 wrote:Thanks for all the help Bob, i just installed the new caps and it sounds great! I was wondering what your recommendation was for the two 50 MFD caps on the terminal strip under the right channel output transformer as to what would be a good upgrade for them? Thanks again! -Ryan

Hi Ryan,

I think you mean the 7 lug terminal strip under the LEFT output transformer.

Those 50 uF caps on the 7 lug terminal strip are only looking at maybe 70 VDC. Any two 47 to 50 uF electrolytic caps at 100 volts or higher would work fine here. Just make sure that you have the polarity correct on these two caps. The "+" end of each cap goes to lug 1 and on the two "-" ends, one goes to lug 3 and one to lug 4. See pictorial below ...
Bob



sventube

Posts: 1
Join date: 2010-02-22

VTA with Quad cap

Post by sventube on Mon Mar 29, 2010 4:44 am

Bob : I just purchased the VTA board from Roy . Can I use the F+T quad cap you recommend 80,40,30,20 ?

Thanks, Sventube

Bob Latino
Admin

Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Mon Mar 29, 2010 4:49 am

sventube wrote:Bob : I just purchased the VTA board from Roy . Can I use the F+T quad cap you recommend 80,40,30,20 ?

Thanks, Sventube

Hi Sventube,

Yes - You can ... When you put the F&T cap in there to replace the original quad cap just make sure that the "80" section faces the back of the amp. If you do that then all the other sections will be properly oriented.

Bob

Kuma
Guest

Dynaco cap installation

Post by Kuma on Mon Mar 29, 2010 6:59 pm

I plan on replacing the entire board when funds become available - but in the meantime, it's nice to hear your expert commentary on upgrading the original. I just ordered the 716p Orange Drops and Silver Mica caps at your recommended values.

Thank you very much for providing this information.

Bob Latino
Admin

Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Mon Mar 29, 2010 7:19 pm

Kuma wrote:I plan on replacing the entire board when funds become available - but in the meantime, it's nice to hear your expert commentary on upgrading the original. I just ordered the 716p Orange Drops and Silver Mica caps at your recommended values.

Thank you very much for providing this information.

Hi Kuma,

The 716P "orange drop" caps are large and will stick up higher than the stock caps. Just bend the leads to fit the holes in the driver board.

If you do ever replace the stock board IMHO it is best not to use the stock circuit. The stock circuit with 7199 tubes is not linear at all drive levels. 7199 tubes are also extinct. No one makes them anymore. The only source of these tubes is what is in circulation right now. IMHO if you do replace your driver board at some time in the future, you would be better off to use one of the VTA upgraded driver boards. It has a number of advantages over the stock driver board like ..

1. Individual bias controls for each output tube and an "on board" bias system
2. No open solder traces and all eyelets have plated through holes so that components may be placed on the top or the bottom of the board.
3. An "on board" power supply so that the board components don't have to pull B+ high voltage from the quad cap (or underchassis power board)
4. The VTA driver board uses easy to source 12AT7 tubes (standard gain board) or 12AU7 tubes (lower gain board)

Bob

vleena

Posts: 9
Join date: 2010-01-22

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by vleena on Mon May 31, 2010 8:10 pm

Hi Bob,
Beside my ST-35, I just purchased a ST-70 with original PC board. (latest version as you described above), I am thinking of replacing the original BalckCat coupling caps with PIO caps, namely Jensen. As I expect sweeter, mellow and warmer sound from them. Any other options. Shall I go for.22 (4 units) amd .1 (2 units)? How Jensen compared to Orange Drop 716P ?

thanks

Bob Latino
Admin

Posts: 1954
Join date: 2008-11-26
Location: Massachusetts

Re: Dynaco ST-70 PC-3 driver board parts layout - photo ...

Post by Bob Latino on Mon May 31, 2010 8:19 pm

Hi,

Jensen makes a few different types of audio capacitors. I have never personally heard any of them so cannot comment on their sound. The 716P orange drop caps are very good caps and are inexpensive at $1.50 - $2.00 per cap. I would go one size up as you mentioned with four .22 uF caps and two .10 uF caps.

Bob
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