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Dynaco St-70 Voltage Question

Discussion in 'Tube Audio' started by BrianC7927, Dec 7, 2018.

  1. BrianC7927

    BrianC7927 New Member

    Messages:
    4
    Hi All,

    First post and kinda new to tube audio so forgive any ignorance.

    I purchased about 6 months ago an original Dynaco St-70 amp, not the new ones but an early 60s model.

    Been enjoying it and it's all be working great, i have a Scott Type 130 pre-amp that it is plugged into.

    Just moved a few months ago, and as I was resetting it up, i had the cage off, and had a screwdriver in my hand. I was out of it, and saw the two screws on the top of unit (the Bias Potentiometers), and was like, i'll tighten those screws so they don't fall out, not knowing what they were. So I turned them both fully to the right, and have played about 10+ albums or so like that.

    Well, as may be expected the amp didn't sound right, and in a few weeks one of my EL 34 tubes went bad, so I figured I just had a bad tube. I replaced, and the replaced tube is much louder than the other side now (right louder than left channel).

    As I finally read and understood the manual, i realized I turned both potentiometers to full voltage and burned the tube out, and not sure what else damage I did.

    I got a volt meter, and following the manual instructions, i set the potentiometers to the 1.56 v recommended in the manual. But, I know I burned a tube out already, I am concerned about what other potential damage i have done to the system or it's components.

    Can anyone tell me what potential damage I did to the amp and it's components with setting potentiometers to full for 10 albums? Maybe 8-10 hours of listening. Will I need to replace all the tubes? It burned out one EL 34 which was new so not sure what else I did.

    Appreciate any guidance or input for a noob tube guy.

    Thanks and appreciate it
    Brian
     

     

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  2. century tek

    century tek Super Member

    Messages:
    3,441
    Location:
    The Evergreen State
    Welcome!

    If your bias is now set okay and you have a new or different set of tubes, then there should be no issues. If a tube red plated before, just watch it for any red plating now and keep your eye on it for the first half hour or so. If none of the EL34 tubes red plate, you are fine but if the sound still sounds a little off, you probably permanently damaged a tube or two.
     
  3. BrianC7927

    BrianC7927 New Member

    Messages:
    4
    Thanks so much for the reply, very much appreciated. I didn't see any tubes red plating, but one EL was very dim compared to all the others. I'll keep listening and see if anymore need to be replaced. Thanks again!
     
  4. Shadowdog

    Shadowdog Super Member

    Messages:
    2,828
    Location:
    Flin Flon, Manitoba
    Yes welcome to AK!

    Adjusting the bias. settings only affect the 4 big (EL34) output tubes, so the 2 small input tubes and the big GZ34 rectifier tube weren't affected.

    Don't short cycle a tube amp (quick off & on) as the tubes don't like that and always have a load (speakers or big 16 ohm 25W resistors) hooked up even when testing as the output transformers could be ruined.
     
  5. BrianC7927

    BrianC7927 New Member

    Messages:
    4
    Thanks so much for the additional info Shadowdog. I appreciate it.
     
  6. heyraz

    heyraz AK Subscriber Subscriber

    Messages:
    1,932
    Location:
    Long Branch, NJ
    The two tubes in each channel should be replaced as a matched set.

    The bias voltage you are reading for each channel represents the average current drawn by the two output tubes from that channel's pair.
    So if you burn out one tube on a channel and simply replace it, you can't be sure if it draws the same current as the surviving tube.. Right now the newer tube is presumably stronger than the older tube, the tubes are unbalanced and the channel won't sound right.

    The original circuit used two 1.56 ohm bias resistors and that value has been unavailable for years. If the amp was worked on and either of those resistor's were replaced with another value, you can't go by 1.56 Volts recommended to set the bias.
    You need to open the bottom and have a look.

    Nothing about that amp is irreparable as long as the transformer's survive. What you did is normal and part of the learning curve.
     

     

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  7. BrianC7927

    BrianC7927 New Member

    Messages:
    4
    Thanks heyraz. I found the original sale sheet for the Dynaco, it was purchased at a non-profit vintage audio shop in CT.

    It says it has new BIAS electrolytic caps (47/100 volt) and silicon diode replacing selenium rectifier.

    Do you know what the value woulds be based on that info to set the bias by? Or do i need more info from the bottom?

    thanks!
    Brian
     
  8. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    931
    Actually, along with recreating the original line of Dynaco tube amps, dynakitparts.com has offered the original value bias resistor for a while now, as can be seen here: https://www.dynakitparts.com/shop/st-70-bias-resistor/

    May I suggest that Brian post a series of images of his stereo 70, top and bottom?
     
  9. BinaryMike

    BinaryMike Pelagic EE Subscriber

    The original 15.6 ohm cathode resistor can be replaced with a standard (and cheap) 15.8 ohm 1% resistor parallelled with a 1.2K 5% resistor. The resultant value calculates far closer than 1% of ideal, so it's essentially a perfect replacement. Typical voltmeter tolerance was a lot worse than 1% back in the day, so the 15.8 ohm resistor by itself may be good enough at only +1.3% nominal error.
     
  10. heyraz

    heyraz AK Subscriber Subscriber

    Messages:
    1,932
    Location:
    Long Branch, NJ
    The work done by the non profit was to upgrade or repair the bias supply. It didn't address what level to set the bias.
    Since both channels are playing, it's unlikely the resistor used to measure bias from that channel was damaged.
    Pictures from the bottom around the output tube sockets would be good, once we know the value of the resistors, we'll know what voltage you need to set the bias correctly.

    I still think you should use a matched set of output tubes for each channel. There's no way to properly bias a channel on that amp without a matched pair of tubes.
     
    petercapo likes this.

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