#1
Just a couple days ago, after flipping the standby to "on" for my Peavey 6505+ Head, it started emiting this horrid, screeching white noise. I noticed it died down as I turned down the "Presence" on both channels, but then noticed the volume was dramatically affected by whatever is causing this to happen. I tried replacing the cable which links my guitar cabinet and amp head, but the screeching persisted even with the cable unplugged.

Could anyone explain to me what may be wrong? As I said, when I turn the "presence" up to and past 7, it starts screeching. And My "Post" (Volume) is never set above 1-2. Otherwise my house would explode.
#2
Quote by OfForceAndChaos
I tried replacing the cable which links my guitar cabinet and amp head, but the screeching persisted even with the cable unplugged.


Very bad idea indeed. Don't do it again, you could damage the output transformer, which will turn your amp into a very large and expensive paperwight until you can afford the $100+ to have it replaced..

I'd have guessed you have a microphonic tube, but if you can hear noise coming from the amp even when it's not plugged into a speaker it could be something considerably more complex.
Take it to a tech.