Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: John_D on September 06, 2010, 06:40:35 pm
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I just finished building a "Super Reverb" design with Doug's AB763 board. The amp has sounded great since the first time it was fired up with one small annoying buzz as I turn up the reverb intensity. It's pretty quiet until I hit 5 on the dial then what sounds like a 120 cycle hum/buzz gradually intensifies as the dial moves from 6 to 10. By the way, the reverb works fine, it's just the buzz that mixes with it at the higher intensities that is the problem.
I have tried substituting the 12AT7 & the 12AX7 with a variety of known good tubes and the buzz persists. I have changed all the coupling caps in the reverb circuit, still buzzes. I have replaced every signal carrying wire in the reverb circuit with sheilded cable (yes it's grounded only at one end) still buzzes. At the suggestion of Gerald Weber I installed the brass strip between the pots and the chassis, eliminated the buss wire ground running to the pre-amp cathodes and ran individual wires soldered directly to the brass strip to all contacts where the buss wire was connected. Buzzzz!
It buzzes with the reverb pan disconnected or connected. I've tried 3 sets of reverb cables, no difference. I used Mercury Magnetics transformers & chokes. I've run out of ideas! Any suggestions?
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It buzzes with the reverb pan disconnected or connected.
Key data point. It's not the driver tube (12AT7), the reverb tank, the reverb transformer, or the reverb cables.
Where is the reverb pot grounded in relation to the ground of the filter cap supplying the reverb recovery stage? Try grounding that pot directly on the negative end of its respective filter cap.
Don't know if that's the answer, but it's worth a try (and based on some experience).
Chip
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Thanks for the input on moving the ground to the filter cap. I followed your suggestion but the noise prevailed.
Since then I tried an experiment, I plugged the output of the reverb tank into the input of my normal channel with no noise whatsoever.
So that verified the fact that it was the recovery stage that was creating the noise. I replaced all the components in the recovery stage and resoldered all the connections. I also thought that the 12AX7 sounded like it may have had a bit too much gain so I replaced it with a 12AT7 and all that seemed to do the trick, the noise has pretty much disappeared.
Thanks for your reply Chip!