Princeton Reverb Hum

[RightLurker]

I'm working on a '67 Princeton Reverb that has a bad hum in the reverb circuit. I serviced the amp some time ago and all was well for about 6 months, then the hum started. With nothing plugged into the amp and the volume at zero and the reverb at zero, the amp is very quiet. Turning up the reverb knob introduces a bad hum - the higher the reverb setting, the louder the hum. Again, the hum occurs with the volume turned all the way down. The reverb itself works fine with the amp turned up and a guitar plugged in, but the reverb is unusable because of the hum.

I've replaced the reverb driver (12AT7) and recovery (12AX7) tubes with an assortment of new and known good tubes. I cleaned and re-tensioned the sockets. I've replaced the reverb transformer with a new part, and replaced the reverb pan with a new Accutronics pan, and used an assortment of known good cables. I replaced the RCA send and return jacks on the back panel of the chassis. I've tried moving the reverb tank position relative to the amp chassis. I've checked all of the voltages in the reverb circuit, and they are all right where they should be. I've checked all the grounds, and they are all fine. I put a new filter cap can in when I serviced the amp when I originally serviced it. Nothing has made a bit of a change. The only thing that stops the hum is to pull the send and/or return reverb cables, and then the hum stops completely, which makes sense because it removes the reverb circuit from the amp.

Obviously I am missing something. From what I've read on the internet, this seems to be a common problem with Princeton Reverbs, yet no one seems to know what or where the problem is. I'm thinking it must be a grounding problem (a loop?) somewhere in the circuit, but I'm not seeing it.

If anyone has any suggestions I'd sure appreciate it, and I think there are a lot of people out there with the same problem who'd appreciate it, too.

[martin manning]

Where is the reverb pedal in this scenario? Have the pedal RCA cable and jack been replaced?

[RightLurker]

The reverb/tremolo pedal and cables are long, long gone. One of the things I did while fooling with the amp was plug an RCA cable into the reverb pedal jack, and then shorted the other end of the cable - that is, I put a wire from the pin to the sleeve. The hum immediately stopped, but so did the reverb. I'm getting the sinking feeling I don't understand something rather basic and obvious. Which, in this case, may not be an all bad thing.

[martin manning]

Well you know that the hum appears before that point. What about the 220k to ground there? Is the ground connection good and does the resistor measure 220k? Which reverb pan? Is the ground connection continuous from the input to the output?

[RightLurker]

I checked the 220K resistor. The ground was good and it measured 230K. I replaced it with a new resistor anyway, rechecked the ground, and all seems to be in order. I checked for a continuous ground by touching one meter probe to the reverb output cable plug ground and the other to the input cable plug ground and there was strong continuity there, so I assume that's okay. The reverb pedal jack is properly grounded. I've interchanged reverb pans (the original Gibbs unit and a new Accutronics unit) and connecting cables, and it makes no difference at all. One thing I didn't mention is that when I worked on the amp originally, I installed a bias adjustment pot. I don't see how that could be causing the problem, and the amp worked fine for about six months before the reverb hum started. The amp has a poltergeist, I swear. Could it be a bad filter cap section?

[Stevem]

On the AT7 check how much AC ripple voltage ( pin 1 or 6 ) you have at that power supply node?

[RightLurker]

AC ripple voltage? Never heard the term. Learn something new every day. I set the meter to AC voltage and put it between pin 1 (cathode) and chassis ground, right? What should I look for on the meter reading? Thanks, Stevem.

[martin manning]

Steve is looking for how much sawtooth AC from the power supply is riding on top of the DC voltage. It's fed by the screen node so there should be little AC there, maybe a volt or two? Google it.

Try pulling one of the reverb cables at the amp and then connect just the tip with a clip lead. I'm wondering if the loop of shielding going around to the pan and back is picking up the hum.

[Stevem]

These amps are very light in term of the main filtering of the first power suppy node, In terms of that 20 uf main first cap I would up it to at least 40 if not 60 uf and also step it up to a 500 volt cap.
Also in the PI section ( for sound, not hum issue) I would check the 56 k resistor on the cathode and plate by lifting one end and measuring them, they should be within 1k of each other and in terms of getting better balance out of that PI section add in a series 1k 1/2 watt before that 56k plate resistor , or if one resistor checks in higher in resistance swap them!

Also if you play the amp above 5 or so the low end can fart out on you and if that takes place than those big ass .1 caps feeding pin 5 of each output should get changed out to .47 or maybe even .22 if at low volumes you still have enough bottom end for your taste!

Not making the output stage pump out a level of Bass that you do not need will give your sound more definition and help it to sing better , not to mention seem a bit louder!