Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: retrorod on August 22, 2025, 02:58:32 pm
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Hello, I just did a full cap job on a working '69 Bassman. No circuit mods done other than adding a bias trimpot to the balance pot and an associated res change to help with the range. The amp is operating with expected voltages.
The issue is extreme microphonics associated with the pre-amp, especially V3. Chopstick taps cause loud pinging on the tube and chassis. This is occuring even with new Mullard replacements. In fact, the new tubes are even louder. I have replaced with an older 12ax7 that seemed to lessen the noise some, but it is still annoying.
Could anyone give me a clue to what to look for or test that may help with this?
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other than adding a bias trimpot to the balance pot and an associated res change to help with the range
what was the bias reading when you finished??
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Yes, It is sitting at -42.4
443 PV 37ma according to the biasrite.
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There's a lot happening at that grid: DC mixing, local negative feedback, decoupling, a relatively large grid leak, and no gridstopper.
I would try using a shielded cable for the run to that grid and add a 10-22k gridstopper right at the socket.
The real fix might be a bit more invasive. I'd probably want the two channels and local negative feedback decoupled before mixing. But try the shielded cable and small gridstopper first. That might get you where you want to go.
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Does the amp feed back or do the tubes ring when you are playing guitar through it, or are you just experiencing the pinging sound when you tap the tubes? When you tap the tubes, does the pinging create a feedback loop that keeps ringing through the speaker?
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BrianS, the amp doesn't ring or feedback when playing. The most notable thing is a pingy sensitivity on the chassis ie: when plugging into the amp and flipping the standby switch. I notice this with new Mullard 12ax7's.
It diminishes greatly with a new JJ tube. I did install 22k gridstopper res on pin 7 of V3 as suggested. I can't tell much change.
I can live with the function of the JJ but curious why the Mullards act that way.
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I did install 22k gridstopper res on pin 7 of V3 as suggested. I can't tell much change.
The shielded cable was the suggestion that I think would get you the most mileage. The gridstopper suggestion was because you were already in there, and it wouldn't harm anything--but might help.
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Could be microphonic tube, or something else like a loose input Jack tip switch that’s not properly connected when there’s no plug in. (This can happen if someone tried tightening the jack the wrong way).Try plugging a guitar cable into each input Jack to see if that stops it.
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Thx stratomaster! I should have explained that this Bassman already has a shielded wire from the vol pot to pin 7 on the normal channel V3. It has a regular wire to the grid on the bass channel however.
Using the JJ tube in V3 has cured the overly microphonic nature in the amp at this time.