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10 turn pots for bias adjustment

Discussion in 'Glowing Bottle Tube Amp Forum' started by kbold, May 23, 2020.

  1. kbold

    kbold Tele-Afflicted Silver Supporter

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    In a recent thread I recommended using 10 turn pots for bias adjustment.
    This allows fine adjustment of bias.

    BUT .....

    A month or so back one of the 10 turn bias pots failed >>> redplating! A nice raspberry red.
    Replaced with a spare I had.
    Today I hear crackling from the speaker after about 5 minutes warm-up.
    That's odd I thought. Looked to see what may be causing this >>>redplating! A nice cherry red.
    Another 10 turn pot failure.

    So ..... from this experience, I recommend sticking with single turn pots (and the associated coarse bias adjustment). The added mechanics within a 10 turn pot seems to increase risk of failure.
    (I could have tried some Clarostat multi-turn pots, but $$$ ...... and I'm now shy on the idea.)

    Ordered some simple pots today - lucky I have a s/s amp to twiddle with.
     
  2. tubegeek

    tubegeek Friend of Leo's

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    Pots have a wattage rating. I think you're exceeding it.
     
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  3. kbold

    kbold Tele-Afflicted Silver Supporter

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    Hmm ... so a 2 Watt wire wound pot is insufficient? (I think it exceeded the original pot rating.)

    What Watts do you recommend?
     
  4. Paul G.

    Paul G. Friend of Leo's

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    One of the first things I learned in high school science was the concept of "precision".

    Now you're adjusting the bias voltage to vacuum tubes. Tiny bits in a glass tube, operating at different wall voltages, different temperatures, different loads, vibrating with the speakers, that change characteristics over time. If there's more than one tube, well...

    Tubes operate well over a fairly wide range of bias settings. Now, how precise do you think you need to be? .1V? .01? .001?

    After I set my bias, I usually readjust up or down a bit until I like the sound. How precise are my ears?

    Now, the short answer is you've exceeded the watt rating of your vernier pots. Shorter answer is "why", as in why do this other than some esoteric concept of "precision".
     
  5. tubegeek

    tubegeek Friend of Leo's

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    I meant the 10 turn pot - that was a wire wound? I was thinking those little wimpy cermet ones in the blue rectangular body.

    Don't know what you are working with but if you know the ohms and the bias voltage, Watts = V squared / R

    I'd pick a pot rated for at least 1.5 times your result just for reliability's sake.
     
  6. Asmith

    Asmith Friend of Leo's

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    What value pot are you using and what resistance do they end up set too? Also more importantly what bias are you setting?
     
  7. kbold

    kbold Tele-Afflicted Silver Supporter

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    The pot is wire wound (a 25K / 2Watt wire wound 10 turn pot).

    I can't recall the actual voltage, but lets say for example 50 Volts across the pot >>> 50 x 50 / 25K = 0.1 watt
    2W /0.1W = 20 So pot is 20 times the requirement. (And yes .... I do know ohms law)

    This leads me recursively to my original suspicion - that the mechanics of the 10 turn pot is causing these fails.
     
  8. tubegeek

    tubegeek Friend of Leo's

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    Huh. My guess about the rating is completely off base then. Weird that you'd have that result. Head scratcher.
     
  9. kbold

    kbold Tele-Afflicted Silver Supporter

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    No, I have not exceeded the watt rating of the pot. But yes I agree a multi-turn pot may be viewed as excessive precision.
    However, I did find adjustment within the recommended window of adjustment (45 to 55 mA) a bit too coarse to dial in easily.
     
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  10. kbold

    kbold Tele-Afflicted Silver Supporter

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    I scratched my head for about 10 seconds before deciding to order single turn pots.
     
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  11. tubegeek

    tubegeek Friend of Leo's

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    And, to be fair, it's not like you have a choice of a 2 turn or 5 turn pot for a "just a little more precision" option.
     
  12. tubegeek

    tubegeek Friend of Leo's

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    Yeah, I get that, but it'd also be nice to know wtf is going on. Otherwise you'll forever be plagued with 10 turn pot trust issues. Did you ever previously get those same tubes to bias up roughly using fixed R's? I'm wondering about a bad batch of tubes with internal shorts.
     
  13. kbold

    kbold Tele-Afflicted Silver Supporter

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    Maybe I'll add a suitable resistor above and below to create a "virtual" 2 turn pot.:)
     
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  14. kbold

    kbold Tele-Afflicted Silver Supporter

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    It's not the tubes (IMO). The first redplate occurred on a NOS British Mullard (survived). The second on a Russian Mullard. Both had "done time" in the amp previously.
    I stay with my original analysis/opinion, in the absence of a better explanation.

    I think I'll forever be plagued with 10 turn pot trust issues - when it comes to bias pots. (Tubes are so much more expensive than pots).
     
  15. Asmith

    Asmith Friend of Leo's

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    I doubt your using nearly any of that 25k track maybe closer to 1k which would make the disappation more like a 0.25W or more and on a very small length of track.

    What is the total current through the pot? What is the voltage across the pot?
     
  16. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Part of the problem is that the wattage rating is for the whole of the pot, so the full ten turns being able to dissipate the heat. But if you have the pot only set to one turn, only a tenth of the available area is having to deal with the full current going through the pot, so it is always a good idea to use as small a rating pot as possible. If you think you will need something in the 15K range, then use a 10K fixed resistor and a 5K pot.......Or better still, use a 33K fixed resistor with a parallel 22K resistor in series with a 10K pot - to give a 13.2K to 15K range, that way if the pot goes, or the wiper looses contact, the 22K resistor will still be there in the circuit to limit the danger.
     
    Last edited: May 23, 2020
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  17. kbold

    kbold Tele-Afflicted Silver Supporter

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    Both pots were adjusted to somewhere near mid travel for correct bias.
    I checked bias in fault condition: over 100mA for almost all travel, which dropped to zero near end of travel (pot failure).
    On the first redplate I swapped tubes: redplating stayed on channel, which led me obviously to the bias pot.

    Have not exposed the innards to check further: I'll probably wait for replacement pots.
     
  18. FenderLover

    FenderLover Poster Extraordinaire

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    I'm using a Bourns 10-turn pot for bias on one of my builds. No trouble, going on 7 years. If you take the bias voltage off the wiper and wire the pot with the end terminals from source going to ground (thru a suitable series resistor) the pot sees the same (low) series current regardless of bias adjustment. You don't need a lot of current; the bias circuit is for bias voltage, not bias current. If wattage is a problem, you did it wrong.
     
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  19. kbold

    kbold Tele-Afflicted Silver Supporter

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    I've heard of paralleling to limit current, but have not applied this idea. I may do that when I replace pots.

    Your recommendation of a combined series/parallel bias circuit seems the best idea.
     
  20. Asmith

    Asmith Friend of Leo's

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    Not really useful for diagnosing your issue but I would still bet the pot got burnt out with too much power. We also don't know what kind of bias configuration is going on. Is this a cathode bias amp and the pot is the cathode resistor or is this a fixed bias and the pot is dialing in the negative grid voltage?
     
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