Bias Drift/runaway

Discussion in 'The Workbench' started by Delete!!, Feb 9, 2019.

  1. Delete!!

    Delete!! Active Member

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    i just finished retubing and biasing my DSL 201. This my was first ever attempt at biasing and I have some questions. I let the amp warm up for several minutes before attempting to bias. I set one side for 550mv, then went to set the other side. When I set the other side I got more fluctuations on the meter. It would sit at 550, then run down as low as 546 and then come up over 550. I kept working both sides to try to get them as close to 550 as possible. When I let the amp sit idle for a few minutes I would see the numbers randomly drift again. What’s weird (to me at least) is that both sides seemed to stay with each other when I would check them. At one point I put the amp on standby for a minute and then turned it back on. The meter then read 574mv.

    Is this behavior normal or do I have a bias drift issue? I wish I could be more descriptive, this was my first attempt at ever doing this. On a side note, the amp seems to work fine and sounds good - although I didn’t want to play it too long and push my luck. My goal was to warm the amp up, set bias, then let the amp sit and recheck 20 minutes later or so and reset again if necessary. But, the numbers seem to move around a lot and not remain static. Any thoughts? It’s going to a tech next week, but I thought I would get a jump on things (and try to learn something).
     
  2. RickyLee

    RickyLee Well-Known Member VIP Member

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    This equates to only a 1.2mA difference on each tube, if I understand that amp. It does have 10 ohm cathode resistors, correct?

    Nothing to worry about really. Could be fluctuations in your wall voltage causing that slight change.

    And, you should be basing that off your plate voltage as well to make sure that is not too hot for EL84's. JJ's can take a lot of punishment if that is what you are running.

    If it climbs up a bit just turn it back down to the lower setting.
     
  3. Delete!!

    Delete!! Active Member

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    Thanks for the reply. Yes I am running JJ’s. I am not 100% sure about the resistors, but I think 10 ohm is correct.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Take out the power tubes.
    Turn the amp power ON, but leave standby switch in "warm up" position. Do not turn the standby to operate position.
    Measure DC voltage on Pin 2 of EL84 (pin 5 of EL34) output tube sockets. This is a negative DC voltage.
    Write down the readings of each socket.
    Is the voltage holding steady? Or is it drifting around?

    Turn the standby to "operate." Measure pin 2 again and record the DC readings again as above.
    Did any output tube socket pin 2 have a large difference in DC voltage between standby "warm-up," and standby "operate?" Compare the DC pin 2 readings and check for a change in voltage.

    If the amp has a large difference, it indicates some problem.
    The tubes should not be installed until that problem is resolved completely.

    The drift is probably / usually caused by:
    Oxidized or defect bias adjust pot.
    Old bias voltage filter capacitors.
    Usually it's the pot. The pot can be cleaned with LPS-1, and if it still won't hold, replace the pot.
    The bias filter caps can go sour with age. This can cause drifting all over the place. Usually replace the filter caps with better ones.

    It's funny that the bias pot is usually some really cheap flimsy pot. The whole life of the amp depends on that pot.
    So I will usually get a precision mil grade pot which is expensive, and install it to replace the cheap ones.
     
    Last edited: Feb 9, 2019
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  5. Delete!!

    Delete!! Active Member

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    Thank you for the information, I may get it out again tomorrow and try that. I have to admit, the pot doesn’t exactly strike me as being robust or precise.
     
  6. MarshallDog

    MarshallDog Well-Known Member

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    Your wall voltage may be drifting. I see this happen when I bias my amps. The drift coencides withe a couple plate voltage chances. Just an idea.
     
  7. Delete!!

    Delete!! Active Member

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    That makes sense, I know some of the lights dim anytime the heater kicks on. I imagine it’s always changing somewhat. I took some of the numbers I remembered seeing on the meter, and when I calculate the actual difference in fluctuation it comes out to about 4.8% (up or down).
     
  8. Ralph

    Ralph New Member

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    I am very positive that is wall voltage drifting, check the wall voltage with the meter for an hour, an see the fluctuations.
     
  9. Delete!!

    Delete!! Active Member

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    I am going to do this when I get some spare time, been busy so haven't gotten back to the amp yet. Did you say your amp runs well (not overheating or melting down) at 600mv? How is the tone at this setting?
     
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    It is always a good idea to write down the wall voltage, plate voltage, negative grid voltage and idle current each time you check bias. Make a chart for comparisons and keep as a record for that amplifier.
     
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  11. ampmadscientist

    ampmadscientist Well-Known Member

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    The pot can definitely become oxidized which will cause a drifting voltage.
     
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  12. Delete!!

    Delete!! Active Member

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    The amp is at the tech now, will be interesting to see what he finds. I used the amp again before taking it over and it was working very well (and sounded great). Time will tell....
     

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