There is always the first time for everything

[rzenc]

Howdy!!!!

A friend of mine bought a new Marshall JCM800/2203KK model. Very nice sounding amp for hardcore/metal.

He used it for around 200 hours and it quit working. We opened the amp to find blown H.T. fuses. Heaters were lighting up. Bias was ok. SS regulators are doing there job as well. Turns out onde of the KT88 had a puncture on the glass container. Changed it to 6550's and the amp came alive.
That's when I went to measure cathode current - according to schem, it has 1 ohm for 2 power tubes of the push/pull side and it was reading something like 10-15mA's @ 455V on plates and -54V on G1. I would never expect such low current reading on a quad monster 6550. So I decided to turn the heat up.
Turning the bias pot full gave me around 40mA's and about -44V on G1. No redplating. Amp sounded way up fuller and more defined. Put it back and called my friend.

While he was rehearsing the amp started to squeal when engaging the beast mode or whatever boost/OD/distortion pedal he kicks in. I was there and witnessed the whole thing.
So we opened the amp again and started troubleshooting. Changed power tubes - 3 different quads/brands - changed all preamp tubes and no joy.
I chopsticked the whole amp while he was playing and appart from 1 ceramic capacitor there was no noise at all when chopsticking components and tubes.
After a lot of poking I decided to return bias to original setting (-54V on G1 and low current reading on cathodes) just to eliminate it as a source of trouble.
Turns out it stopped squealing. Now we can crank the shit out ov it and no freaking noise anymore.
We changed tubes again, power and preamp tubes, and no noise. Chopsticked it again and no noise - except for the ceramic cap mentioned earlier. 
The amp only seems to be happy when cold biased, otherwise it starts to squeal - it's a high pitch squeal that comes on when beast mode or other gain structure is introduced into the signal flow.
It's the first time time I see something alike.

The One Million Dollar Questions: What could cause such a thing? Why it squeals when 'properly biased'? 

I compared the schem I found on the web to a standard 800 and there are a few differences but none that catches my eyes to such a weird behavior. I was not able to find a readable gate/eq. schematic but only the main amp schematic.

Have you ever seem it? All input is greatly appreciated!

Thanks in advance.

Best Regards,

R.