Dynaco ST-35 Build – DG Style

Discussion in 'Tube Audio' started by njcanuck, Jan 28, 2017.

  1. TravAndAlex

    TravAndAlex Active Member

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  2. nj pheonix

    nj pheonix AK Subscriber Subscriber

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  3. njcanuck

    njcanuck aka: MacGyver Subscriber

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    I played around with the input filter today. The biggest effects came from varying the 47K resistor R1. The lower the resistance, the steeper the transients however stability suffered somewhat (as expected). Changing the value of the 33pF capacitor C2 yielded only marginal differences. I eventually settled on 22K for R1 while keeping C2 at 33pF. To my eyes, this yielded slightly better transients but introduced some slight ringing at the peaks of the square wave. The new 10KHz square wave is below.

    The amplifier sounds extremely good - very dynamic. Interestingly, my 4 ohm Altec Segovia speakers sound significantly better on the 8 ohm taps than they do on the 4s. Bass is much tighter and the highs seem more detailed. They are a great combination! :thumbsup:

    Dyna 23.jpg
     
  4. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Rich,
    Did that effect the 1db drop off at 20k at all?
    Also, (i understand this is completely subjective)
    Do you perceive any difference in presentation now from stock values
     
  5. njcanuck

    njcanuck aka: MacGyver Subscriber

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    Andy, I confess I didn't remeasure. I would be surprised if it had any effect but I'll check it soon.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Hummm. There's a better than average chance that you'll find that response is now too good at 20 kHz. If so, adjust the value of the resistor to give the flattest response up to 20 kHz.

    Dave
     

     

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  7. njcanuck

    njcanuck aka: MacGyver Subscriber

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    Well, you guys were right. There was a pretty significant impact on frequency response - it's now perfectly flat out to 30KHz! :banana: There is a 0.5dB drop at 40KHz going down to 1.4dB at 50KHz on the worst channel. Sometimes you get lucky!!! :p :beerchug:
     
  8. njcanuck

    njcanuck aka: MacGyver Subscriber

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    I think I'll call this one done. Thanks to all who helped - especially Dave G. (I highly recommend his boards :thumbsup:).

    This has been a very satisfying project. Here are a few final beauty shots of the finished amp:

    Dyna 24.jpg Dyna 25.jpg Dyna 26.jpg Dyna 27.jpg
     
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  9. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    :beerchug: Pretty!:thumbsup:
    If it sounds as good as it looks its a winner. Congrats!
     
  10. Lavane

    Lavane AK Subscriber Subscriber

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    Very nice! I really want to build one now.
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    I swear its like they took 3 transformers, made a chassis to fit them, then crammed the tubes in wherever they could fit. Not an inch of wasted space on those things.
     
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  12. njcanuck

    njcanuck aka: MacGyver Subscriber

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    Well crap! Thought I was done but it has started kicking out some static on the right channel. Grrr!

    I changed out the the input jack and several of the input components but no joy so far. It only seems to show up after it warms up for an hour or more. I'm starting to think it may be a coupling capacitor as they seem most vulnerable to heat from the output tubes. Following Dave G's suggestion in another thread, I connected a power cap between pin 6 of the 7247 and ground. This immediately silences the static and so should rule out the inverter section. Guess I'll go after the coupling caps next.

    Anyone have tips on picking up static on a scope? So far I have not been able to see it. That would sure make it easier to track down.
     
  13. TravAndAlex

    TravAndAlex Active Member

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    Well, between mine with static on the left channel and yours with static on the right... between us we've got one working one. ;P

    Good luck! I'm praying I'll have a silent one once done building Dave's boards.
     
  14. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    I'm mixing apples with aardvarks here. I was reading a cohibajoe thread . After a restore (all new everything) noise came from new resistor. I've never seen it happen, but don't rule it out (I think they were fancy ones from parts connexion). I was hunting static (though it was immediate and constant) after rebuilding a board for something. Probably 50 components on the board of which all the resistors and caps were replaced. I was down to a dozen semi conductors. Changed all the transistors (in pairs and sections at a time) no help. Down to 3 diodes (3c items) I just changed them all and it finally solved my issue. Sorry Rich, i know that isn't much help. I think you have good access which in my project i didn't you scope is your friend here. It shouldn't be too bad to hunt down.
     
  15. njcanuck

    njcanuck aka: MacGyver Subscriber

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    Hi Andy, My first thought was a bad resistor, too. I've changed out a couple but not all, yet. I thought my scope would help find it fast but so far I have not been able to pick up the static on it.
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    NJ -- If AC grounding pin 6 of the 7247 via a large cap to ground stops the noise, then your first suspects will be the tube itself, or the plate and cathode resistors associated with pin 6 and pin 8 of the 7247. I too have had my fair share of noisy, new resistors! Shouldn't be coupling caps in this case. A noisy grid return resistor at pin 7 of the 7247 could make the noise as well.....

    Dave
     
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  17. kward

    kward AK Subscriber Subscriber

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    Never seen one of these close up. Good looking little compact amp you have there. Nice work.
     
  18. njcanuck

    njcanuck aka: MacGyver Subscriber

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    Dave: Thanks for chiming in. The static does not follow the tubes so it has to be another component. I'll try the resistors that you have suggested next and report back.

    Kward: Thanks. I really like the footprint of this little amp. It was one of the aspects that drew me to it.
     
  19. TravAndAlex

    TravAndAlex Active Member

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    I'll be curious if it turns out to be R5. That's the pesky one that seems to go on my board when static shows up again.
     
  20. njcanuck

    njcanuck aka: MacGyver Subscriber

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    Based on Trav's experience, I tackled R5 first. So far . . . . completely silent! It's been running successfully all day. We'll see if it lasts. That was a brand new Dale precision resistor too. :rant:
     
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