Strange JCM800 Problem ... Solved but ....

[kagliostro]

At DIYItalia a guy was asking how to fix a big squeal on his DIY JCM800

He told the amp was build 5 years ago and used without problems

3 weeks ago he powered the amp and had a big squeal as result

He tried various things and discovered that if the NFB was disconnected the squeal vanisched

I suggested to reverse the power tube plates wires and the problem was solved

But this seems odd to me, very odd ...

How is possible that an amp that was build years ago and used without problems one day show this kind of problem ?

The only thing I can think is that he did something to the amp then didn't used it for some time, so he lost the memory about what he did to start the problem

Did I deduced correctly or there are reasons that can start such a problem ?

Thanks

Franco

[pdf64]

A positive feedback loop doesn’t automatically result in oscillation.
An amp may be marginally stable with one cab or resistive load, yet oscillate with another.

[PRR]

Yeah, change a tube load, change the gain and thus the squeal.