Marshall JVM 205H squealing

[Danglin' Fury]

Greetings fellow electron manipulators! So I have this Marshall JVM 205H on the bench. Amp sounds delicious... For anywhere between 30 seconds and a minute forty five. Then the signal starts to crackle and then all you hear is a very high pitched squeal. It happens on both channels, on all modes and with either of the master volumes engaged. Also happens with the effects loops on or off. I tested the power tubes and the input tube and the PI. They all tested good. (Buuuuut I have seen preamp tubes that test "good" and sound like garbage, so there's that) So I cracked her open. The solder on the tube sockets was atrocious. On the phase inverter, 2 pins didn't even look soldered. I sucked all of.it out on all the pins then resoldered it all, to no avail... Have any of you fine techs seen anything like this in a similar amp? Like I said, she sounds phenomenal til she heats up. I can turn the standby switch to "not playing" and let it cool down for a minute or so, flip the switch and she will sound very good for another minute or so and then crackles and squeals. Thanks for looking.

[Jon Snell]

Try tapping each valve gently and listen for the ringing sound, if it rings, that valve is noisy, probably causing your issue.