Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MacGwyn on May 05, 2015, 03:10:09 pm
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Completed a Brownface Deluxe kit 6G3 and it sounds great. Only problem is a slight hum is present even with both volumes turned all the way down. It sounds like 120 hz to me. You can pull the first two preamp tubes and the hum is just the same... pull the PI tube and it goes quiet. The hum is not tremendous... just more than my other amps at "idle". Any suggestions to kill the hum varmint?
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Start with a forum search on "hum", or PI hum. probably one of the most covered subjects here. The next biggest, grounding. Guessing when you pull the PI the hum quits? Do all the easy stuff, 6vdc battery for your filaments, change power sources, like your buddies home, moving tubes around, re-inspect the grounds, measure ohms from main chassis ground to all other grounds, looking for higher than normal.
All but 2 of my builds have low level hum, my criteria, if I can't hear it with the guitar vol on 1, or the cats are sound asleep next to the idling amp, good enough
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Thanks Shooter - weird thing is the hum actually decreases a bit when you turn the volume UP to 3-4 or so. It's loudest when the pots are at zero. Definitely going to look at the grounds though. I tried subbing a fender choke for the big resistor between the first two filter caps (as in the blackface deluxe) but that didn't help the hum at all.
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UPDATE: pretty happy with things now... biggest help in hum reduction was changing rectifier tubes. Put an old Ruby 5AR4 in place of the TAD GZ34 and it cleaned-up substantially. Had experimented with differences in grounding points... remelting solder joints... but the rectifier tube swap made the biggest difference in hum reduction. Never suspected that a rectifier could make that much difference in sound quality.
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"Never suspected that a rectifier could make that much difference in sound quality."
Nor I, but it wouldn't be the first time on this forum where someone fixed their hum by swapping GZ34's. Hazards of an indirectly heated cathode tube, I guess.
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Still playing with the Brown Deluxe and had another breakthrough on hum reduction:
After doing some research online, I tried grounding the early filter caps at the power tranny. Originally, all four were grounded at the "audio" star ground by a single wire, since that ground point was close to the filter cap 'doghouse'. Now the preamp filter cap is grounded there, but the others are grounded at the power supply end, and the amp is now dead quiet with respect to hum. :icon_biggrin:
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That's how it's supposed to be done! You've learned the lesson, which is great.
" Originally, all four were grounded at the "audio" star ground by a single wire, since that ground point was close to the filter cap 'doghouse'."
The [area of the] doghouse is EXACTLY the kind of "Gee this seems like ground" thing/area/place that you DON'T want to be grounding the filter caps to. The power tranny is "the original and still champion". We want to avoid having the heavy currents of the first several filter caps "running around" or "running across" the chassis with indeterminate paths to the best ground they can find. We don't want them to "find" (meaning; "search for") anything. That's the crux of the issue.
Well done!