18Watt.com

I've owned a late 70s Princeton Reverb for about a year and since has been my main amp. I gigged it last night and ran it a bit hot, and today find that nearly anything played on the bass strings in a lower register produces a very pronounced sawlike fuzz that sustains as long as I keep the note fretted. Other than that the middle-upper registers seem true. Any clues on where to start diagnosing this one? Thanks in advance

My first guess is your filter caps are old. Electrolytics typically have a 2000 hour lifespan (for the better ones), and average play over a decade can easily pass that. Your amp being as old as it is, I wonder if the electrolytics have been changed at all.

That being said, I'd swap the tubes out first to rule them out.

Thanks! I'm a plumber and have limited capacity for capacitors so thanks for giving a starting point, hopefully it's as simple as a tube swap

I agree that the Electrolytic filter caps should absolutely be replaced if they’re the original ones.. This should be done regardless, whether or not it’s the cause of the issue. Especially if you want a reliable gigging amp you can count on!
Second, check out the preamp tubes (Carefully! With the amp on/warmed up) take a wooden chopstick lightly tap each tube starting with V1, if you can hear noise or get feedback on any you know that’s the culprit.
Next I would check out the output tubes and verify the bias voltage is within spec and that the 6v6’s are balanced correctly..
If all that checks out fine, you’re going to need to get into a more specific and difficult series of tests.. but a couple other simple things you can do is just visually look over the inside of the chassis.. often times with these older amps you can see burned, charred, or melted components (old CC resistors in particular) Also remember to take a look at the speaker and make sure that you haven’t blown it!