Ok I'm going to post some things to try to describe my problem. This is a peavey 6505+ head that was new 2 years ago and I purchased used 1 year ago.

My main issue is this - When I put a noise suppressor into the fx loop it introduces a very noticeable hum and the guitar signal is never completely muted. I can still hear the guitar coming through even with my boss ns-2 on the mute setting or with my decimator threshold turned all the way up. It sounds just like when you play with the post volume all the way down from 0 to about 0.5 when the guitar is really muffled and dark sounding, except it should be dead silent instead. Like, I could have the post volume at 5 which would melt someones face off and theres this quiet signal thats at a constant volume which isn't affected by the post volume level that still makes its way through somehow. Weird.

This amp has not always been like this. I used to be able to have complete silence when a suppressor was used in the loop, until just a couple weeks ago. The hum that is introduced is actually louder than the static coming through the preamp. I have rotated preamp tubes around with no change so I don't think its a bad tube. I have tried hooking up a suppressor all by itself with no other effects with different spare 1 foot patch cables straight from the head to the pedal to the head again and the hum was still there, so I figured that would eliminate the possibility of having a bad patch cable in my pedalboard. I have tried shutting everything else off in the room to eliminate any possible interference from other electrical devices. And still no solutions.

So I'm not an amp tech but I figure it must be something inside my amp that has gone south. I have bias modded this amp and am comfortable with taking the board out and soldering in a new part if someone has experienced this problem before or knows what the deal is. I have at one time had a power tube fail causing a fuse to blow inside the amp that I replaced. It was a brand new tube that I played on for about 5 minutes and it shorted right away. If there is some test you want me to run on this amp to help troubleshoot anything just let me know as I have tried everything I can think of and am out of ideas. I dont have a good tech around me so I would like to repair this myself if possible.

Thanks everyone at UG for reading this and helping me out.