Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: texwest on September 28, 2025, 10:24:05 am
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Hey guys My old 66 princeton reverb has hum on the reverb circuit. Hooked it up to a different tank with different cables. Changed v2 and v3. Still have the problem. Amp works fine with the reverb turned all the way down, but has increasing hum as you turn up the reverb pot.
I do hear the same pitch hum a tiny bit with the reverb turned all the way down. But the amp is totally useable as long as I do not turn up the reverb, even with the amp cranked up.
The hum gets out of control as you turn up the reverb pot. Seems like the reverb circuit is amplifying a hum, but the rest of the amp is not involved.
Ideas about what I should try? Thanks!
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That kind of hum can have several sources and it’s good you’ve tried all the obvious quick fixes.
So methinks it probably the old filter cap can and bias supply cap.
But it could also be some deterioration of connections (oxidation) between the brass grounding plate/pots/jacks and the chassis.
You could also try rolling some other preamp tubes in case some noise is coming from one of them (which the reverb is then picking up on)
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Thanks a lot. Yes I changed out the can filter cap to internally wired F&T around 10-15 years ago. I have already rolled V1 to a known good tube with no joy. I have not tried V4, but didn't think it would have much to do with the problem considering the symptoms.
I think my next step is to use alligator clips to clip in some caps over the filter caps and cathode caps and see if it makes a difference. Does that sound like a good idea?
How would I test the brass plate? Alligator clips again?
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I find the Princeton Reverb especially susceptible to hum. The only way I've found to drive it out is to move the reverb HT connection to the next node while reducing the first 18k dropping resistor to 12k.
If you sub the 2.2k cathode resistor on the 12AT7 with 1.5k you'll still have plenty of clean drive despite the lower plate voltage.
Granted, this is a significant modification, but it definitely kills the hum.
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... I have already rolled V1 to a known good tube with no joy. I have not tried V4, but didn't think it would have much to do with the problem considering the symptoms. ...
From the symptoms, I think the valve in V3 socket seems the most likely thermionic suspect.
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_aa1164_schem.pdf
Is the shield of the reverb pedal RCA well bonded to the chassis metal?
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Yes cap cans are inherently prone to grounding noise. One option for trying to get around this is separating the HT supply for the last two filter cap nodes off into two separate stand alone filter caps, and ensuring the parts of the circuit that are supplied from each of these is returned to the ground return lead of the respective filter cap - all returning at the input Jack ground (I.e., a split ground) but no guarantees as much depends on the quality of workmanship to pull this off.
Re cleaning behind the grounding plate - this requires stripping out all the parts going through the plate, removing the plate, and thoroughly cleaning all surfaces before putting Humpty Dumpty together again.
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Thank you! I'm not using a can cap any more for this amp. I have hard wired in separate F&T caps, but this was done at least 10 years ago. I already checked V3 tube but I will try this again.
I will clip in a few caps over the existing filter and cathode caps and see if this helps. I'll get back to you and let you know.