iv got another possible solution - ,
nothing guitar related turns out,
"(Note... the high pitched ringing sound your amp usually makes is not parasitic oscillation. It is a microphonic preamp tube, a problem that is far more common. Here's how to tell the difference: pull the guitar cord out of the input jack. Turn it up really loud and smack it. Does it make the amp ring more or stop rInging? Yes? Then reach around back and tap on each preamp tube individually. The one that makes the sound needs to be replaced.) "