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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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

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    • Post n°1

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

    Post by Bob Latino Wed Nov 04, 2009 12:43 pm

    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

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

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    Dynaco ST-70 PC-3 driver board parts layout - photo ... Empty TC-7510 PCB

    Post by Hirak Fri Nov 06, 2009 11:58 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!
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    Post by Bob Latino Fri Nov 06, 2009 2:25 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 $75 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.

    Dynakitparts 550 volt 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 Fri Aug 05, 2022 7:40 pm; edited 3 times in total
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    Post by scsidude2 Fri Mar 19, 2010 5:11 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
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    Post by Bob Latino Fri Mar 19, 2010 5:33 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 .

    Bob


    Last edited by Bob Latino on Fri Jul 17, 2015 11:46 am; edited 1 time in total
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    Post by scsidude2 Sat Mar 20, 2010 11:15 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
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    Post by Bob Latino Sat Mar 20, 2010 11:20 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
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    Post by scsidude2 Sat Mar 27, 2010 6:02 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
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    Post by Bob Latino Sat Mar 27, 2010 6:27 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

    Dynaco ST-70 PC-3 driver board parts layout - photo ... ST-70pictorialHI-REZwithcolor_10-1
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    Dynaco ST-70 PC-3 driver board parts layout - photo ... Empty VTA with Quad cap

    Post by sventube Mon Mar 29, 2010 5:44 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
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    Post by Bob Latino Mon Mar 29, 2010 5:49 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
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    Dynaco ST-70 PC-3 driver board parts layout - photo ... Empty Dynaco cap installation

    Post by Kuma Mon Mar 29, 2010 7:59 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.
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    Post by Bob Latino Mon Mar 29, 2010 8:19 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)

    NOTE (on 4/20/18) - All VTA amps since April of 2012 now use just the LOW GAIN board which uses 12AU7 type driver tubes.

    Bob


    Last edited by Bob Latino on Fri Apr 20, 2018 8:51 pm; edited 1 time in total
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    Post by vleena Mon May 31, 2010 9:10 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
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    Post by Bob Latino Mon May 31, 2010 9:19 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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    Post by Bob Latino Thu Jul 08, 2010 2:52 pm

    Hi,

    The Mark II uses the same board layout as the Mark III which is here > Board layout Mark II/III

    Check with Kevin at Dynakitparts. He should have the board layout for the Mark IV.

    Bob
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    Dynaco ST-70 PC-3 driver board parts layout - photo ... Empty Resistor Ratings?

    Post by Aljaheejus Wed Aug 25, 2010 8:44 pm

    Bob Latino wrote: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. ....

    Bob,
    Great info and photo for replacing parts on my original board. I don't know from this, however, what the power ratings on the resistors should be. Can you point me in the right direction for either original specs, or your recommendations?
    Thanks,
    David
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    Post by Bob Latino Wed Aug 25, 2010 9:30 pm

    Hi,

    The 270K, smaller 18K, 620 ohm, 47 ohm, 1.5 meg and 10 ohm are all 1/2 watt resistors. All the other resistors on the board are 1 watt. I recommend metal film resistors since they are quiet, don't change their values over time and can be had in 1% values so that the two channels can be closely matched.

    Bob
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    Post by Mr.Miele Sat Feb 05, 2011 8:04 pm

    Bob, can you let me know dimensions of st70 without cage please. I have about 6 1/4" opening in my shelf. Hope this unit can fit.
    Thanks!
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    Post by Bob Latino Sat Feb 05, 2011 9:44 pm

    Hi,

    The amp is just about 6 1/4 inches high but I would not place the amp in any small space where there would be restricted air flow. These amps should "look at the ceiling" IMHO. Placing the amp in a confined space can cause the amp to run hot and could damage the power transformer.

    Bob
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    Post by Mr.Miele Wed Apr 06, 2011 11:23 pm

    This is how my project looks now..

    Dynaco ST-70 PC-3 driver board parts layout - photo ... IMG_1354

    Dynaco ST-70 PC-3 driver board parts layout - photo ... IMG_1344

    Dynaco ST-70 PC-3 driver board parts layout - photo ... IMG_1343
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    Post by Bob Latino Tue Apr 12, 2011 2:18 pm

    The original Dynaco ST-70 PC-3 board was 6 1/2 inches long by 3 1/4 inches wide.

    Bob
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    Post by metlmrk Thu Sep 29, 2011 8:19 am

    Hi, I realise this is an old thread. I am considering updating the coupling caps as noted (.22 & .1) I was hoping that I could get some opinions from people who have done this! Please let me know the pros and cons. The bottom line is it worth doing? Does the sound improve. Can anyone define "improved bass responce" = deeper or tighter bass or...?

    thanks in advance, Mark

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    Post by Roy Mottram Thu Sep 29, 2011 5:43 pm

    is it worth doing? That's a totally subjective topic. Each person hears things differently.
    I recommend you start with a reasonably priced upgrade, like 716P capacitors, under $2 each.
    Some people like Solens for about the same price (brighter sound, not as warm).
    A high percentage of people really like the russian PIO caps we have on our website, for around $6 each.
    I'm a big believer in value for the $$.
    Other people can't live with anything less than $75 (each piece) Mundorf caps.
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    Post by metlmrk Thu Sep 29, 2011 8:23 pm

    Hi, thanks for your input. Can you give me a link to this site that sell the PIO Caps?

    Regards, Mark
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