The Dynaco Tube Audio Forum

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


    SDS labs ST-70 capacitor board advice

    Share

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    SDS labs ST-70 capacitor board advice

    Post by richtp on Sun Nov 30, 2014 12:29 pm

    I acquired my first ST-70 several years ago. It was a basket case! It came with an SDS labs driver board and SDS labs capacitor board. I had to pull all the resistors off the driver board and replace them. He had half of them in the wrong place. Everything on the cap board was ok. I installed both boards and ran the amp daily for about 3 years.

    I swapped all the tubes one day with some Sylvania EL-34's I acquired with another amp as well as the rectifier tube. I don't remember if I checked the bias like I should have (was inpatient to try the tubes) after running it at moderate volume levels for about an hour one channel cut out.

    I shut it down right away. And found the Cap board had got hot. The wrap on several caps swelled and 2 resistors that are tied to pin 8 of the rectifier tube look burnt. I am thinking the rectifier tube shorted??

    Here's my dilemma.... looking through this board it seems the SDS cap board may be problematic. I think I remember reading other posts on fried SDS boards. I am debating on re-populating the board or putting a upgraded quad cap and bias circuit. I was pretty happy with the way the amp sounded but not sure about rebuilding the cap board every few years?

    Also, if it was a shorted rectifier tube that caused this. Would the rectifier modification (adding 2 1n4007 diodes in series with the rectifier tube) possibly prevented the fried cap board?

    Any advice or thoughts would be greatly appreciated.

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Sun Nov 30, 2014 12:49 pm

    The burnt resistors are R1 and R2 one of them reads 330k and the other in megaohms.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: SDS labs ST-70 capacitor board advice

    Post by Bob Latino on Sun Nov 30, 2014 2:08 pm

    The problem with the SDS cap board for the ST-70 is that > Although they use caps in series for the FIRST section which gave an 800 volt rating - the last 3 sections (C3 through C8) are only rated at 500 volts. (3 sets of TWO 250 volt caps in series... caps in series double the voltage rating hence the 500 volt rating of two 250 volt caps in series) When you turn any ST-70 amp on from a cold start the initial power surge can go over 500 volts. As a side note > Dynaco wisely used a 525 volt cap here. On ALL the VTA kit amps we use a 550 volt rated quad cap which takes any initial power surge in stride.

    What I would do is try to replace the caps that are bulged and the two burned resistors and see if that solves the problem. Another thing > When you put in a new set of tubes in a tube amp you should turn DOWN THE BIAS before you even turn the amp on. It is possible that the Sylvania EL34's that you put in the amp had a noticeably different bias point and drew much more current than the output tubes you had in there at that bias setting on the bias pots? This could have sent the SDS cap board over the edge of its ability to handle voltage/current ..

    Bob

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Sun Nov 30, 2014 3:36 pm

    "The wrap on several caps swelled..."

    I'm not sure that several of the caps swelling at once is necessarily an indicator of a general reliability problem with the board.  I wonder if something else is amiss that might have caused a catastrophic failure?

    I think there are a few considerations here.  Going with the quad section cap is, of course, a fine option.  

    However, if you go with the newer kind of quad cap with the even higher voltage rating, be aware that it has discrete electrolytics inside, but does anyone recall how they are arranged?  In other words, in order to achieve the higher voltage ratings, does it have electrolytics in series inside the quad can?  If so, then you’re really no better off reliability-wise than using the cap board.  At least with the cap board you know you can have resistors across the electrolytics in an attempt to equalize the voltage – does anyone know if the electrolytics in the quad can have voltage equalizing resistors?  OTOH, if there are only four discrete electrolytics inside this newer style can, this would not be a concern, but I’d sure want to find out.

    Alternatively, if you use the traditional style quad can that’s built like a Swiss roll on the inside, then you’re going to have higher ESR and also a lower temperature rating (55C, IIRC) that seems a bit on the low side for this application.  Not that the Swiss roll style wouldn’t work well enough, but I think the higher temperature ratings of the low ESR electrolytics and related features of the cap board provide an overall performance advantage.  The increased capacitance alone of the cap board is likely an advantage over both kinds of the quad cans.

    There was more than one cap board that Triode USA was selling.  One was the original Sheldon Stokes board while the other is somewhat different, which Triode calls their “Upgrade Capacitor Board.”  This second board is the one they appear to be selling now.   Which one do you have, the original board by Sheldon Stokes or the Triode USA Upgrade Capacitor Board?

    I am not very familiar with the original SDS labs board, but I have the Triode USA Upgrade cap board in my Stereo 70.  One thing I did do was to calculate different resistor values for the series resistor pairs in parallel with the electrolytic pairs.  The original values are okay for bleeding the supply when the amp is shut down but do not do much for protecting the electrolytic pairs in operation.  I reduced the values of these resistors so that they draw roughly double the maximum leakage current of the electrolytics.  This could add some protection as the capacitors age.  

    But, in your case, it does seem kind of suspicious that you had your meltdown coinciding with the tube changes.  You might not want to write-off the cap board too quickly, especially since you like the way the amp was performing before the meltdown.  As far as the voltage ratings of the last three sections on the Triode Upgrade cap board, each one of those electrolytics is rated for 250 working volts but typically have a 300V surge rating, which means each of the last three sections actually have a surge rating of 600VDC.

    So, IMO the quad can is a fine choice and also the easier choice.  Although the cap board is more of an involved affair, I think it can offer a performance advantage while I would not think it should have to be serviced regularly under normal circumstances, though I would want to reduce the value of the resistors in parallel with the electrolytics.

    In the meanwhile, I’d consider other possible reasons for your cap board’s failure.  Did your AC line voltage spike?  Maybe there is an issue with the tubes...?

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Sun Nov 30, 2014 6:02 pm

    Thanks for the advice! It is the Triode update board. From what I read the original one SDS? Had some surface mount parts on it. This one does not. After looking more closely at the caps it appears they just got hot and the plastic bulged a bit. The cans did not. And six of the series resistors are discolored.Checking them quickly with an ohm meter (the caps) they seem to be charging ok. I think I may replace the questionable caps and all the series resistors and see what happens with the original tubes.

    What value resistors did you use for the series resistors?

    I sure wish I had not gave my tube tester away! Didn't use it much and had a shop I would occasionally have tubes tested at. But it closed a while back.

    I searched for some higher voltage caps that would squeeze in their without to much trouble and that doesn't look like an easy alternative.

    I really appreciate the advice. I know enough to be dangerous but have a lot to learn. Thanks for your time.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Sun Nov 30, 2014 7:41 pm

    It's sounding more and more like some kind of near catastrophic event involving more than just one isolated portion of the cap board.  I suggest replacing any parts that look a little toasty, even if they still measure/work to some degree.  

    Remember: a quad cap will be easier.  

    I'll have to search through my notes to see what I used on my cap board - might take me some time.

    If you can do so safely, I suggest taking some readings of your AC line voltage throughout the day/night over several days, maybe even weeks, to get a sense for how it runs. This could also be a factor.

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Sun Nov 30, 2014 8:19 pm

    I can monitor the a.c. line voltage for a while. I don't believe I have ever seen it go above 122 volts in the past. I think I will be working out of town for a few days starting tomorrow (with my meter) but when I get back I can set my meter to record max voltage and leave it and see what happens.

    I just ordered some of the caps. I will order the rest later (probably all of them)

    The board was not all that hard to install. So I won't have any problems. The instructions I found for it were not the greatest though. Wish they would have put a schematic in them. I tend to work better with schematics and wiring diagrams that instructions (:

    I am pretty good with schematics, it's the electronics theory that I don't always get.

    I have one of those kill A Watt meters, maybe ill just leave that plugged in for a while.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Mon Dec 01, 2014 10:52 pm

    richtp,

    I wanted to follow-up on our posts regarding your cap board. I looked up some of the resistor values I used, but I will have to find a time to open up my amp to find others.

    I wanted to mention that even if you had lower value resistors as I suggested, it might not have prevented the problem you have experienced, which seemed like some kind of catastrophic event perhaps related to the tubes. Plus, changing the resistor values as I have done in my amp might possibly have other effects that I am still in the process of researching.

    If you want me to, I will continue to work on finding the resistor values I used and then post them. I leave it up to you.

    Peter

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: SDS labs ST-70 capacitor board advice

    Post by Bob Latino on Mon Dec 01, 2014 10:58 pm

    The info on the Triode Electronics (SDS) cap board with the values for all the resistors and capacitors for the ST-70 is at the link below.

    Triode Electronics ST-70 cap board manual

    Bob

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Tue Dec 02, 2014 7:09 am

    Thanks Bob and Peter! I am going to stick with the 330k 1/2 watts that triode used. I have some but ordered more. I think you are correct that it was a tube issue. The amp worked fine until I started playing with other tubes and I honestly can't remember if I checked the bias. That'll teach me to be more patient.
    I really wanted these tubes to be good, Sylvania big bottle. Guess ill have to start looking for a tube tester soon. I really regret giving mine away now.

    Ill get it all back together and see what happens with a set of known good tubes and see what happens. I checked out the power transformer with an ohm meter and it appears to be ok.
    Thanks again!
    Rich

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Sat Dec 06, 2014 7:49 pm

    I finally got some time to look at the cap board more closely and I did find an issue with it.Not sure if it was the cause of the problem but I would guess it's not good.
    The C1 and C2 caps are supposed to be 82uf 400v and the caps that are in those slots are 270uf  400v. I believe that would be 135uf in the first section of the power supply. I believe I read some were that the first section should not be more than 40uf.All the other caps match the triode layout.
    When I got this amp it was bits and pieces in a box and I spent a great deal of time going through the components on the driver board, apparently I didn't look over the capacitor board as well as I thought.

    Hopefully ill get some parts for it soon.

    I monitored my a.c. power in the house all week and most of the time it's 120 on my Fluke and the max was 124.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: SDS labs ST-70 capacitor board advice

    Post by tubes4hifi on Sat Dec 06, 2014 9:09 pm

    If you had a tube rectifier (5AR4-GZ34) connected to that board with those large capacitors, most likely the rectifier is toast, max capacitance should be not more than 40uF,
    which you would have by using two 82uF capacitors in series on the SDS board.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Sat Dec 06, 2014 10:51 pm

    I'm surprised it lasted as long as it did.

    If you have the original Dynaco power transformer, I suggest installing a thermistor in series with the primary to drop the AC line a bit - I'd suggest an Ametherm SL22 12103, Mouser Part #: 995-SL22-12103.

    BTW, I exchanged e-mail with Sheldon Stokes, and it's not his board.

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Sun Dec 07, 2014 8:02 am


    Well that's strange because when I first fired this amp up it almost immediately blew the GZ34. I went through the wiring and found no problems. I ordered another GZ34 and decided to try it and it worked for better than 2 years. Thought I just got a bad rectifier.
    I pulled the board and removed all the caps and resistors last night and I was wrong, the 82uf caps were in there, but in the wrong place.

    I ordered all new caps and resistors last night.

    A thermister? I've used them for temp circuits. How much will a thermister drop the voltage? I understand that the old transformers were wound for 117 volts so dropping voltage makes sense, I guess that 4 or 5 volts on the secondary end could be about 15 volts higher.
    I have another amp that I built (solid state) that I used a salvaged power transformer on that is a bit to hot after rectification. If a thermistor will drop the input voltage a few volts that may help in that amp as well.


    Thanks
    Rich

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Sun Dec 07, 2014 12:41 pm

    When you’re operating outside of specified parameters, there’s no guarantee of what might happen when.

    IIRC, the original Dynaco manual specified 125 VAC as the maximum allowable wall voltage for the Stereo 70.  There might be some additional tolerance above this, but Dynaco would not warranty the Stereo 70 above 125 VAC.

    The Ametherm SL22 12103 drops the AC line by roughly 1.5 or 2 VAC.  If you want more of a drop, then you’d have to look through the data sheets of different lines of thermistors and see if you can find one with substantial resistance and rated for 3A or higher current (perhaps the Ametherm MS22 22103).  You’d have to then study the specs and interpret the resistances at different percentages of their rated current.  Or, you could just use two Ametherm SL22 12103 or MS22 22103 thermistors in series. Some experimentation might be in order.

    Of course, there’s no substitute for having the right parts in the right places.  Since there seems to have been at least some ambiguity as far as which parts are where on your board, it’s probably a good idea to start from scratch as you plan to do.  When you’re done, I’d suggest taking some readings and comparing with the voltage chart in the original manual.


    Last edited by PeterCapo on Sun Dec 07, 2014 2:51 pm; edited 1 time in total

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: SDS labs ST-70 capacitor board advice

    Post by Maintarget on Sun Dec 07, 2014 2:50 pm

    PeterCapo wrote:When you’re operating outside of specified parameters, there’s no guarantee of what might happen when.

    IIRC, the original Dynaco manual specified 125 VAC as the maximum allowable wall voltage for the Stereo 70.  There might be some additional tolerance above this, but Dynaco would not warranty the Stereo 70 above 125 VAC.

    The Ametherm SL22 12103 drops the AC line by roughly 1.5 or 2 VAC.  If you want more of a drop, then you’d have to look through the data sheets of different lines of thermistors and see if you can find one with substantial resistance and rated for higher current.  You’d have to then study the specs and interpret the resistances at different percentages of their rated current.  Or, you could just use two Ametherm SL22 12103 thermistors in series.

    Of course, there’s no substitute for having the right parts in the right places.  Since there seems to have been at least some ambiguity as far as which parts are where on your board, it’s probably a good idea to start from scratch as you plan to do.  When you’re done, I’d suggest taking some readings and comparing with the voltage chart in the original manual.


    If two Thermistors are wired in series would it double the results (3 or 4 VAC drop) or is there a formula to calculate?
    Thanks

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Sun Dec 07, 2014 3:03 pm

    Two thermistors in series should double the voltage drop of one, within their resistance tolerance and assuming it's two of the same thermistor.  Thermistors have resistance tolerance like other resistors do.

    Remember to watch the current rating, too.  For instance, the CL-90 thermistor is popular but it is rated for 2A, where the original Stereo 70 has a 3A fuse.

    Maintarget

    Posts : 209
    Join date : 2013-02-10

    Re: SDS labs ST-70 capacitor board advice

    Post by Maintarget on Sun Dec 07, 2014 3:17 pm

    I'm learning and was curious I have no need for this modification from what I understand Thermistors get quite hot.
    Thanks Peter

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: SDS labs ST-70 capacitor board advice

    Post by PeterCapo on Sun Dec 07, 2014 3:23 pm

    Yeah, they get hot, so anyone using them should probably ensure other things in the amp are not in contact with them.  I've had them in my Stereo 70 and PAS in confined areas but not in contact with anything nearby, and no problems so far.

    richtp

    Posts : 31
    Join date : 2011-11-18
    Location : Sterling MI

    Re: SDS labs ST-70 capacitor board advice

    Post by richtp on Sun Dec 07, 2014 5:17 pm

    I plan on checking the voltages before running it this time.
    Heck I could run it on a variac set to about 119.
    Thanks for the input!
    Rich

    Sponsored content

    Re: SDS labs ST-70 capacitor board advice

    Post by Sponsored content Today at 4:37 pm


      Current date/time is Mon Dec 05, 2016 4:36 pm