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    A few ST-70 Questions

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    Kinski

    Posts : 26
    Join date : 2017-09-20

    A few ST-70 Questions

    Post by Kinski on Thu Oct 26, 2017 5:23 pm

    I have a few questions.

    1. I've read countless times that people increase the .1 caps to .22 and the .05 caps to .1. And that this generally "improves" the sound. But I can't find any detailed description of what changes. Could someone please let me know the sonic changes would occur if I were to up the capacitor values? More/less bass? More/less treble? What about he midrange? Etc...

    2. I really like the sound of my stock ST-70. However I haven't replaced a single capacitor or resistor on the driver board. And i'm sure most if not all the caps and resistors are out of spec. Most people seem to replace the WORN OUT driver board with a VTA or something similar. And almost always say its better. I'm curious though, can anyone here describe the sonic differences between a newly rebuilt STOCK ST-70 and a VTA?

               I'm curious about this as I'm kind of considering replacing the caps and maybe the resistors. At which point, perhaps a whole new board is in order. May I would build a new Stock version, or maybe something different like the VTA.

    (FYI, I have replaced the selenium and the bias caps.)

    I love this forum. Can't tell you all how many hours i've spent reading here. Thanks!
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    Bob Latino
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    Re: A few ST-70 Questions

    Post by Bob Latino on Thu Oct 26, 2017 7:33 pm

    Kinski,

    1. You can increase the size of the caps one size if you want. The key thing is that you replace the caps on the stock driver board if you want to use the original board. (but see # 2 below) Some of the original Dynaco ST-70's are now approaching 60 years old and parts don't last forever. (The Dynaco ST-70 first came out in 1959)

    2. It makes more sense to REPLACE the driver board than to work on a 50+ year old board. Reasons below ..

       A. The original driver board had open solder traces on the bottom of the board. Sometimes these traces will lift and/or develop a crack. Usually this happens after the amp warms up and parts expand. Common symptom of this is noise or lowered volume on one channel or complete loss of one channel's output.

       B. The original Dynaco ST-70 driver board had carbon COMPOSITION resistors. Carbon composition resistors will stray from their normal value over the years (usually they go UP in value). It is rare to see a Dynaco ST-70 with the original board and have the resistors match well from channel to channel. Soundstaging goes downhill because now the two channels do not match well anymore.

       C. The 7199 driver tubes are not made anymore. The existing stock gets lower each year so these tubes are getting expensive. Sure you can also use 6GH8 tubes in their place with an adapter but then you have to buy the adapters. The original Dynaco ST-70 driver circuit was designed to "save a tube" not for "best sound". Dynaco saved maybe $1.50 on each amp by omitting a 3rd tube socket and tube. They jury rigged this circuit with the pentode/triode 7199. You really need 3 dual triode tubes to design a good driver circuit for two channels.

       D. The board was made of a brown phenolic material that was not very heat resistant. If you open the amp up on an amp with the original brown board, you will always see that the brown material under the 7199 tube socket has turned black from the many hours of heat on the board.

    The VTA driver board corrects all of the above .. It has no open solder traces. It uses metal film, metal oxide or carbon film resistors that will not stray from their value over the years. It uses easy to source 12AU7 type tubes. It is also a BETTER CIRCUIT which was designed by Roy Mottram in the 1980's, has been tweaked slightly over the years and is now in its 16th iteration.

    Bob

    Kinski

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    Join date : 2017-09-20

    Re: A few ST-70 Questions

    Post by Kinski on Thu Oct 26, 2017 9:48 pm

    Thanks so much for your response! I really appreciate it.

    However, I'd still like to know what difference I would HEAR upping the cap values .1 to .22 and .05 to .1.

    And I get the technical differences of the VTA vs Stock. But I still don't know what the difference in sound is.

    I think I'm leaning towards getting a new board from dynakitparts.com and doing a full stock replacement. But not sure if i want to up the 6 cap values. Thats why I'm wondering the the difference in SOUND will be.
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    corndog71

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    Re: A few ST-70 Questions

    Post by corndog71 on Thu Oct 26, 2017 10:49 pm

    The only way to know for sure is to swap them and listen for yourself.

    Kinski

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    Join date : 2017-09-20

    Re: A few ST-70 Questions

    Post by Kinski on Thu Oct 26, 2017 10:55 pm

    Most certainly. I just figured since it seems to be quite common for people to up the values of those caps, that some people could chime in here and share their experience. I'll probably just stick with stock values and go from there.

    Kinski

    Posts : 26
    Join date : 2017-09-20

    Re: A few ST-70 Questions

    Post by Kinski on Fri Oct 27, 2017 12:21 am

    Sorry, one more question.

    I'd like to replace the 15.6 ohm resistors with something more easily available. I've read that 10 ohm 1/4 1% should work well. And that the low rating would allow them to act as a fuse. Is this correct?

    Also, I currently bias the amp at 1.25v using the stock 15.6 resistors. Where would I bias the amp when using 10 ohm resistors?

    Thank you!
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    j beede

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    Re: A few ST-70 Questions

    Post by j beede on Fri Oct 27, 2017 12:59 am

    I=V/R

    With 1.25V across your 15.6 Ohm resistor your output tubes are biased for about 80mA (40mA each if your output tubes are matched for plate current).

    If you wanted to run the same bias current after replacing the 15.6 Ohm resistor with 10 Ohms you would want to set Vbias=I*R=0.08*10=0.8V.

    Kinski

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    Join date : 2017-09-20

    Re: A few ST-70 Questions

    Post by Kinski on Fri Oct 27, 2017 1:03 am

    Thank you!

    I just ordered a new stock board and upgraded quad cap, along with all new caps and resistors from dynakitparts.

    Is the upgraded quad-cap wired the same as the original cap? And do I put 22,000 and 6800 resistors on as well?
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    PeterCapo

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    Re: A few ST-70 Questions

    Post by PeterCapo on Fri Oct 27, 2017 9:02 am

    If you have the quad cap with the larger values, it should be wired in this order: 40/80/30/20.  Yes, keep original resistor values associated with the quad cap.  The original 15.6 ohm Biaset resistors are likely okay, unless they were subjected to unusual stress – you might try measuring them with a meter.

    Increasing the value of the coupling caps on the PC board might get tricky.  IIRC, increasing the values might destabilize the amp while others say just going up by one value should be okay.  Not sure how much difference going up one value makes, but if there is any change it might tighten up the bass a bit, but this would remain to be seen (heard).  But, again, there might be a risk of destabilizing the amp.  What would destabilizing the amp mean?  Maybe some kind of oscillation under some circumstances, or other possible problems I guess?  Here are a couple of resources that might be of interest: http://audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/page-14#post-8400580
    and
    http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

    I suggest also rebuilding the bias supply.  Cleaning and retensioning the tube socket contacts would be a good idea.

    Kinski

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    Join date : 2017-09-20

    Re: A few ST-70 Questions

    Post by Kinski on Fri Oct 27, 2017 10:00 am

    Thanks!

    Yeah the bias resistors were are both off by a few ohms. So they certainly need to be replaced. I have ordered replacements for every capacitor and resistor in the entire amp.

    Kinski

    Posts : 26
    Join date : 2017-09-20

    Re: A few ST-70 Questions

    Post by Kinski on Thu Nov 02, 2017 12:53 pm

    Well, i got the new STOCK ST-70 board up and running. Along with the new 40/80/30/20 cap. Sounds great! Also replaced the bias circuit and a few other resistors on the underside.

    Also, I finally took a listen to my PAS-2 with the tone controls removed. Sounds so smooth and natural now! I'll keep listening this way, but I wanted to keep the options of using the tone controls, so I put them on a switch to turn them on or off. PERFECT.

    The whole system is sounding awesome with my early KLH Model 6 pair!

    Thank you all for the help!
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    Dynalover

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    Re: A few ST-70 Questions

    Post by Dynalover on Thu Nov 16, 2017 9:01 pm

    I never had much luck with original sockets. YMMV.
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    PeterCapo

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    Re: A few ST-70 Questions

    Post by PeterCapo on Thu Nov 16, 2017 9:38 pm

    Dynalover wrote:I never had much luck with original sockets. YMMV.

    They can be a problem.  But, if painstakingly retensioned and/or cleaned, they can still work.

    davidpg

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    Join date : 2013-11-23

    Re: A few ST-70 Questions

    Post by davidpg on Thu Nov 16, 2017 11:33 pm

    PeterCapo wrote:If you have the quad cap with the larger values, it should be wired in this order: 40/80/30/20.

    Peter,
       Could you explain the rationale for this recommendation?

    Thanks,
    DAvid
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    PeterCapo

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    Re: A few ST-70 Questions

    Post by PeterCapo on Fri Nov 17, 2017 12:29 am

    My understanding has been as follows... please feel free to add additional rationale.  

    First section, 40µF, node "D"
    The 5AR4 rectifier has a datasheet maximum of 60µF for an input filter, so the 40µF section typically goes right after the 5AR4.  For a time, it was not unusual to see comments to the effect that current production 5AR4s were not very robust, and old stock are expensive, so 60µF or higher should not be risked immediately after the 5AR4.  However, there are now robust, less expensive 5AR4 current production, and with the use of an NTC thermistor the inrush concern even for an indirectly heated 5AR4 should be mitigated, or so I would think.  Other tube gear exceeds the capacitance rating for their respective rectifiers, so it should work okay here, too.  Nevertheless, the conventional wisdom is to put the 40µF right after the 5AR4.

    Second section, 80µF, node "C"
    This is to provide the largest amount of energy for the power tubes.

    Third and fourth sections, 30µF, 20µF, nodes "B" and "A"
    These provide energy for the phase splitter and the pentode voltage gain stage of the 7199s on the PC board.  Since they do not put out the kind of power the output stage does (the EL34s), these two get the lesser portions of the quad cap.

    So, this is typically how they are apportioned.  But, it’s not written in stone.  I have been thinking lately that it might actually be slightly better to arrange it as 40/80/20/30 so that the voltage gain stage of the 7199 gets 10 more µF than the phase splitter, but it might be splitting hairs as much as the phase.
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    Peter W.

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    Re: A few ST-70 Questions

    Post by Peter W. on Fri Nov 17, 2017 9:34 am

    Kinski wrote:Thanks!

    Yeah the bias resistors were are both off by a few ohms. So they certainly need to be replaced. I have ordered replacements for every capacitor and resistor in the entire amp.

    Coming in late to this, however:

    Before I trashed these resistors, I would triple-check my VOM to make sure that what I am seeing is the actual reality. I keep a range of test-resistors - precision resistors purchased specifically for the purpose. Far too often, I have seen weak batteries or other problems with the VOM causing misreads.

    Same reason, I keep calibration tubes for the tube-tester.

    davidpg

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    Join date : 2013-11-23

    Re: A few ST-70 Questions

    Post by davidpg on Fri Nov 17, 2017 11:10 pm

    PeterCapo wrote:My understanding has been as follows... please feel free to add additional rationale.  

    First section, 40µF, node "D"
    The 5AR4 rectifier has a datasheet maximum of 60µF for an input filter, so the 40µF section typically goes right after the 5AR4.  . . .

    Thanks very much. This helps clarify things, including how the order you specified relates to the circuit.

    David

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