amp sounds like a ring modulator? why?

[yunger]

well, it's a pro jr and used to sound great but now it is starting to sound like a ring modulator when cranked. maybe i just didn't notice it before but i don't think so. i don't remember it having that "digital" or octave-up sound before. at least i don't have to break out the green ringer anymore.
also, as my stock speaker finally gave out, i noticed that when i hit the low e string, the note would grow/fade in pulses. this was easy to hear with a blown speaker. i wondered if this was some sort of phase cancellation.
any thoughts?

[pdf64]

Maybe the reservoir cap etc on the B+ is worn out. How old is the amp? However, the symptom with this is usually a sub (lower) note (out of tune), rather than a higher note. Try measuring the ac and dc on the B+ at no signal, and then when cranked. Carefull - Peter.

[Steve Conner]

Yes, worn out filter capacitors are the most likely culprits.

(A lack of filtering results in your guitar signal being amplitude-modulated with twice the AC line frequency, which amounts to much the same thing as a ring modulator.)

[Don Moose]

Man, if a Pro Jr is old enough for the filters to be worn out, they must have been some low-quality parts. Wouldn't a bad solder joint on the filter be a more likely cause? ( does the Pro Jr have the same bad low-voltage supply design as the Blues Jr and the HR series? )

Hope this helps!

[yunger]

the amp is a 94 model revision b. the problem seems to become evident at high volume. i guess the caps arent able to keep up. i was thinking of upping the filter caps from 22 to 33. any reason why i shouldn't?

[pdf64]

No reason not to go to 33 that I can think of, other than that you liked how it sounded when the 22s were working ok.
Don, I had a 70s Orange 100 watt amp in awhile ago, with dreadful 100Hz modulation. There was a dry joint on the connection to the reservoir caps, so the main B+ was totally unsmoothed. A quick cheap fix with a massive improvement. Peter.