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Thread: Twin Reverb hums - hum zero trim pot

  1. #1
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    Twin Reverb hums - hum zero trim pot

    I'm having a slight but annoying problem with my silver face '84 (possibly) Twin Reverb.

    When I switch it on an tubes are warming up (with both switches, power and standby thrown into the up position), I'm hearing a hum that start a bit louder and then fades to become fainter, but it does not totally disappear.

    Only when I then use the zero out hum trim pot on the rear and turn t with a screw driver such that the hum totally disappears - and when I say "totally", I mean totally. I can zero out the hum so that the amp sounds like not being switched on at all.

    So far so good.

    But next time I switch on the amp the hum is there again and I have to find another point with the trim pot to zero out the hum.

    In the old forum when I posted the problem (under user ChristophK, which can be back 7 or 10 years ago), someone said, I should do a modification to the wiring of the pilot lamp (add some resistor(s) or so).

    Any clues?

    --
    Christoph

  2. #2
    Forum Member phantomman's Avatar
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    Re: Twin Reverb hums - hum zero trim pot

    Problems such as you describe are rarely rectified by a mod. Your issue should be diagnosed and isolated by competent troubleshooting and analysis. Were the amp mine I'd start looking at the power supply, specifically the HV filter array and/or the biasing circuit. It appears you have some AC ripple bleeding though during the start-up sequence due to one or more tired electrolytic caps.

    JMO
    "When injustice becomes law then rebellion becomes duty."

  3. #3
    Forum Member OldStrummer's Avatar
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    Re: Twin Reverb hums - hum zero trim pot

    Roger's "opinions" here are generally worth more than you'd pay said "competent tech."
    Striving to be ordinary

    Proud to be a TFF Dumbass!

  4. #4
    Forum Member Don's Avatar
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    Re: Twin Reverb hums - hum zero trim pot

    Quote Originally Posted by phantomman View Post
    Problems such as you describe are rarely rectified by a mod. Your issue should be diagnosed and isolated by competent troubleshooting and analysis. Were the amp mine I'd start looking at the power supply, specifically the HV filter array and/or the biasing circuit. It appears you have some AC ripple bleeding though during the start-up sequence due to one or more tired electrolytic caps.

    JMO
    My opinion as well!

  5. #5
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    Re: Twin Reverb hums - hum zero trim pot

    The "Hum Balance" is a pot that works like the 2 100 ohm resistors to ground used to provide an "artificial center tap" on the heater circuit. Earlier circuits had a center tap.
    I suspect that the pot needs cleaning/replacing.
    Last edited by Bill Moore; 01-13-2022 at 04:46 PM.

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