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
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
Re: Twin Reverb hums - hum zero trim pot
Roger's "opinions" here are generally worth more than you'd pay said "competent tech."
Re: Twin Reverb hums - hum zero trim pot
Quote:
Originally Posted by
phantomman
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!
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.