What made this AB763 Twin burn up two screen resistors on the same side?

[Randall]

TRRI comes in for "dreadful screeching noise". It makes no such noise in my shop. Guy tells me he changed all the tubes, and it still did it, so he brought it to me. I did find a white capped new reverb driver tube, but still no screechng. Then I noticed the two 1 watt 470R screen resistors on the right side were toast. I replaced them with 2 watts, and can find nothing else wrong.

So what puzzles me is, why two screen resistors? If one power tube went bad and took out it's screen resistor, why did they both go?

I figure one 6L6 was failing and roasting it's screen resistor, so he changed the tubes, and maybe the roasted resistor had it's last death throes while the new tubes were just installed, and had finally given up fully by the time it got to me. Becayse he will want to know why the noise happened before and after he changed to new tubes.

I checked the bias voltages to all pin 5's whilst wiggling and prodding wires and the two 220K resistors looking for a possible lost connection, but it all seems real solid.

https://www.ampwares.com/schematics/...verb_ab763.pdf

[Helmholtz]

If one power tube went bad and took out it's screen resistor, why did they both go?

Maybe because the guy swapped a bad tube between different sockets?

[Leo_Gnardo]

Parallel tubes - what happens to one, happens to the other.

[Helmholtz]

Parallel tubes - what happens to one, happens to the other.

As there are individual screen resistors and screen currents, a bad tube doesn't necessarily take out the other's screen resistor.

But a common reason could be loss of bias or a shorting tube that causes a positive grid voltage on both tubes.

[Chuck H]

Or coincidence? I know we look for reasons based on symptoms but then sometimes things just happen.

It could just be that one tube failed and left the other hanging on for dear life with wonky currents and a low load until THAT tube failed. We can't know the owner wouldn't/didn't just keep cranking the amp after a tube failure. But then two tubes on the same side are out and finally "Hey... I think somethin' ain't right.?.

[pdf64]

If one 6L6 became gassy or something and passed a lot of grid current, it could act to reduce the magnitude of the bias on that side, so both 6L6 would run hot, redplate, and pop their screen grid resistors.
Unless there’s a suitable HT fuse, uprating screen grid resistors can act to increase the stress on the HT supply etc when an output valve develops problems such as this.
Hence it may end up being a retrograde move; I suggest to stick to 1W MO with a flame retardant coating.

[drewl]

Also bad coupling cap from phase inverter, killing bias like Helmholtz said.

[Leo_Gnardo]

If one 6L6 became gassy or something and passed a lot of grid current, it could act to reduce the magnitude of the bias on that side, so both 6L6 would run hot, redplate, and pop their screen grid resistors.

I'm liking this scenario best. I find it's much more common that a parallel pair have both self-destructed, than to have one survive with no sign of distress.