Princeton Reverb Crackling/Zapping Sound

[Bob]

I got a 71 Princeton Reverb from a buddy not too long ago, knowing it needed some work. It sounded ok until you cranked it, and then you got a loud buzzing/crackling/zapping kind of sound. At first, I was sure it was mechanical, or even a bad speaker, but after trying a few speakers and through a separate cab, it's still there.

It came in with the bypass cap leads broken because the tag board had bowed pretty badly. I replaced the bypass caps, a couple cathode resistors, changed the filter cap can, replaced the bias cap, and installed a bias pot. I checked to make sure every component's lead wasn't broken, and touched up any solder joint that didn't look too good. I replaced all the tubes, and checked to make sure they weren't the problem by putting some other spares in.

I'm still getting that sound, and I'm getting frustrated. It happens when it's set to about 6 and above and you hit a bass note. It's the same crackling/buzzing type of sound.

Any thoughts on where to look next? I've put together amps before, but fixing old ones is new to me. I'm stumped.

Thanks for any help.

[UR12]

Sometimes if the amp has carbon comp resistors they start to go bad and make these kinds of sounds. These are the 100k plate resistors in the preamp section I'm talking about. Try replacing those and see if that helps.

[Bob]

Thanks for the reply.

Turns out that when I unplug the reverb cables, the problem goes away. Upon consulting some others, sounds like it might be the reverb drive transformer.

I'll try to switch that out and see if that fixes it.

[David Root]

For future insurance I would still check out those plate resistors as Dana suggests, and also the filter cap resistors, especially the first one. I had the 2W screen filter cap resistor burn out on my '62 Deluxe shortly after I bought it 3 years ago, same failure symptoms. Carbon comps are not exactly ideal filter cap resistors.

[Bob]

Thanks. I actually plan on making sure that I have all values of the resistors on hand to change those out as well if needed, paying close attention to those in particular.

I was close to just ordering a princeton reverb turret board and just replacing the whole circuit

Thanks again.

[bentonar18]

Bob, I had the same thing happen to me recently on a Princeton Reverb, but it was only when the reverb was on. I figured it was the reverb cap, ordered another one, and when it came in started to take out the chassis, then realized one reverb cable was longer than the other, which seemed to be stretched. I swapped them and the cracking/noises went away... who knew?