Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on April 23, 2024, 09:38:58 am
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Hi all, could use some help. I just finished a Princeton Reverb circuit (trem deleted) that performs flawlessly apart from a distracting and not so nice oscillation sound that occurs when I turn the volume from about noon to 3 or 4 p.m. Other than that, the amp works well and all voltages are to spec. No other issues I can find. Mods on this amp: Solid state rectifier, thermistor to slow down surge, larger transformers, mid pot and passive effects loop.
I have tried rerouting wires, swapping tubes, swapped out the three tone stack caps one at a time to see if one was leaking DC, etc etc etc but I haven't found anything yet that makes any difference. I do not currently have a scope unfortunately. Can anyone offer advice on how to find and address this issue?
Pix below.
Thanks!
Schematic here (accurate except for changes noted above)
https://www.schematicheaven.net/fenderamps/princeton_rev_gz34_aa1164.pdf
Short video clip of the issue:
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I don't hear an oscillation. All I hear is a noisy pot, maybe dc on the pot? Measure dc volts on V1 pin 7 and note if there is a small dc voltage when the pot is rotated. Also connect a short instrument cable between your fx jacks. Any joy?
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Thanks Sluckey.
The fx loop doesn't have any impact.
At 0 volume there's 0 mv on V1 grid 2. When the noise starts it jumps up from about 4mv and continues to about 24mv, before it backs off and quiets down and ends up at about 6 mv fully cranked.
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Pull V1. Do you still have that small dc voltage on the socket pin 7? If so, put a 1M resistor directly between pin 7 and chassis ground. If the dcv goes away try different tubes in the V1 socket.
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Thanks Sluckey. Just tried eight or nine 12AX7s, and was not able to find one that didn't cause the noise. With the tube pulled I was still getting very slight voltage, a few mv that rose with the volume pot turned clockwise. So I put in a 1M at your suggestion and tried again. I still got the noise on all but two of the tubes. But those two were pretty quiet. Thank you!
So what is going on here? Do all the tubes w/noise have DC leaking internally from plate to grid?
The 1m seems like a really handy trick to know about, thank you. Safe to use as a preventative measure even if I managed to find a nice quiet tube? Thanks again.