marshall VS265 - high pitched noised at high volumes

[littlered]

i have a marshall VS265, and when the overdrive is on and i turn it up past 7 or so, it squeels at me, it sounds like microphone feedback, any advice?

[cortezthekiller]

Try swapping the preamp tube, it may have become microphonic. I think it uses a single 12ax7, and they're easy to find.

-Dave