Hi all, I need a little help diagnosing a problem with my vintage PR. It is a '68 that has been recapped (by me) and adjustable bias added (edit: about 5 years ago-just saying it has been maintained). While I was playing, it started making a ring-modulator type sound (I'm assuming some sort of oscillation?), and there is a loud hum now. I changed tubes--no difference. I left the rectifier and power tubes in and powered in up and it is still humming loudly. Pulled the chassis and nothing looked wrong visually to me. Where do I need to start checking first? Thanks!