Twin Reverb build cuts out when Volume all the way up

[nicksorenson]

Hi there,
I just built a twin reverb out of an old Fender PA 100. All works well but the amp makes a flub flub sound and cuts out completely when the volume is turned all the way up on the normal channel. It works fine up to about 7 or 8 and then 8-10 it stops working. Any ideas?
Thanks,
Nick

[Satamax]

Check HF oscilation! :D As well as caps!

http://www.geofex.com/ampdbug/ampdebug.htm

[Mars Amp Repair]

Also check to see it the power supplies are remaining stable at that point. Might be dropping drastically at higher current demands. g

[nicksorenson]

How do I diagnose high freq oscillation?

Also, the PS caps are 34 years old. Could need a replace. I didn't read a Short accross any of them but they are old. Any way to tell if they're bad?

[stokes]

If the flub-flub you are describing is a "motorboating"type sound it is more likely a low frequency oscillation.This could be caused by the filter caps.At 34 years old they should be replaced,and are very likely the cause of your problem.

[ValveBytes]

sounds like filter caps to me also, I just built a much smaller amp and I accidentally put in 2x 22uF filter caps thought they were 32uF, anyway when volume was past a certain point it would sound almost like a tremolo with the sound coming and going as the caps were getting too much demand for the size.

[Andrei_T]

Hi, it sounds like oscillation to me.

Does it do this with no signal on the input or playing at normal room sound levels? If so it's not the filter caps.