Univox Oscillation, bias runaway

[Gaz]

I had a short thread already on this amp. Univox U-1101

My friend handed it over to me initially and I found one of the "plate stoppers" was blown.

Well today he told me the volume had dropped while playing, and sure enough one of the 100R/3W plate stops was blown again. I noticed he had the bright volume all the way up, which means he was running this thing full blast. I figured maybe the 3W was not up to task...

I then checked the bias on that tube and it started to run away. I could hear an oscillation (through the OT/vibrations since the amp was into a dummy load). I quickly turned down the bright gain that was on "10", and the bias dropped and stabilized. I then turned the presence and treble all the way down and turned the gain back up... no oscillation. It appeared again as I turned up the treble.

So now I presently have a "chicken or the egg" type of thing where I don't know if the oscillation caused the 100R/3W plate stopper to blow, or if the 3W blew under normal cranked playing conditions and the amp only started oscillating when I jumped the blown resistor (meaning that the 100R was stopping the oscillations in the first place as was suggested in the last thread).

Any ideas where to take it from here?

Thanks!

[Gaz]

Turned out OT was bad, and the windings were either shorting or opening at high signal levels.

[Enzo]

Arcing, probably.

[Gaz]

Arcing, probably.

Yeah, it would look good on the scope then power would drop when the gain got to a certain level. After that, power would be super low no matter the gain setting. If I powered down, then back up, same thing. It sounded okay at low volumes too, but once it started presumably arcing low power and bad crackly sound.

I'm surprised the tubes survived all this, although the owner told me he shut it down right when the power went low. How do you think this happened in the first place? I was thinking simply the OT was cheap and with 700vdc through it for decades it was bound to die being pushed as hard as it was.

What might be a good replacement? It was doing about 120W clean with this one I had on hand for another project and tacked in. ClassicTone Project Style 100W Output Transformer, 2.2K to 4/8/16 Ohms

My concern is that it's a bit heavy for the thin steel chassis, which has already bent a tiny bit from the weight of the transformers.