I have a problem with a Peavey 6505+ Head. It has some sort of direct guitar signal leeking (bleeding?) through all the channels, the channel knobs doesn't affect it in the rythm channel, and when i turn up the post (volume), the sound I should get is here but adds on top of the direct signal, unchanged. (It is pretty loud). On the Lead channel, the "loudness" of the direct signal increase really fast with the PRE gain knob (impossible to get it past 3 - too loud), and sort of "changes phase" when I turn the Post knob instead of gaining in volume...
I already checked all the tubes, which seems to be fine (not 100% sure about preamp tubes), I'm pretty sure no mods have been done to this amp (I'm not first owner), and I have tested it before and it played great... The "direct signal" is there even with ALL the knobs at 0 and still same volume... Please help... I though at first it is a problem with the effect loop, but I'm not sure, nothing is plugged in the rear panel besides the output speaker cable of course. How can I find the problem? thank you, Patryk.
Look on the back of the amp, is it a USA made amp, or one of the newer one made in China?
diabolical I tried swapping the tubes with new one and nothing changed... I put a little jack cable used for pedals from the FX send to the return and i had a pop in the sound but the sound of the guitar never stopped :/ maybe it's a transistor failing?
Do you have some kind of distortion pedal? Try bypassing the preamp sectiin and going guitar to distortion pedal o fx return and report back.
Trying to isolate whether pre or power amp issue.
Also might try power amp tube swapping one by one.

Did this amp have 12at7 tube for fx loop?
Yep already tried, power section works fine, i get signal from the fx return, the most weird thing is that the lead channel controls don't work as they should, PRE add volume and POST does a weird filter thing (phase?) idk...
Personally, this is where I would take it to one of my local mom & pop Peavey dealers. Although all 3 within an hour of me have been around since before I was born (35 y/o last week), and I trust there knowledge about the products.