Ampeg B15 oscillations at high volume

[imo]

Enzo,
Makes sense. The amp is a PCB, part of the Magnavox 70's line. I am able to move the plate output wire as it goes to the OT.
I asked this earlier in this thread, but is there any possibility that my high voltage, particularly in the preamp, could be causing troubles? I am over 100v high in the preamp, close to to 235V, when it calls for around 135V.
Testing with my DMM, The HV and Bias appear to be pretty clean in terms of AC ripple.
I went through checking all coupling caps, especially from the PI down. All resistors and caps seem to check out. I have reflowed most of the solder joints.

New rectifier tube, new power tubes, new PI tube. New power section caps/

Any idea where to go from here?
Best
Ian

[Mars Amp Repair]

This might relate,
I had an old Silvertone 50Watt that squealed like crazy. The power tubes were new & all filters were fine. If I put in some used 70' Fender tubes, it worked just fine.

I found that only moving the screen wires around made any difference to the issue (I know your's is motorboating..this just an example of the sometimes inexplicable weirdness you can run into in an old amp).

This particular amp had no individual screen resistors, so on a whim & frankly a loss of anything else to do! I added them.

Voila, problem fixed. I think there must have been some weird difference in the orginal tubes & the newer 'style'. Don't know.

Most of us would agree that sometimes over time, enough components shift their values & cause the amp to behave in a manner in which it was never inteneded to, especially with parasitic oscillations.

So sometimes you just have to get creative to solve these wierd issues, like the ground loop one I mentioned in my previous post above. glen

[imo]

after the classic chasing the rabbit down the hole, i finally figured out what was going on! Thanks again for all of the help, it really helped me hone in on what was going on.
It was simply a ground loop that was made by the new cap can being grounded to the chassis. I pulled it off and am ordering an insulated washer for it.
Problem solved.
Sounds great!
Thanks guys
Ian

[Mars Amp Repair]

Well,
that's nearly the exact issue I mentioned in my prior thread (mine just created a major hum) but since I didn't know how the grounds were supposed to go in the first place, I just split the ground path at the higher current drawing devices & voila, issue resolved.
At lease now I know how one of those is supposed to go.

I believe if you trace the gound path you'll find that it terminates at the input jack. I just wonder how having all those higher current sources finding ground through the higher gain circuitry doe not somehow create a hum or feedback path seeking ground in near the higher gain preamp stages.
Anyway, glad you resolved your issue. glen