Mark V went out at a gig, preamp problem

[ugly_guitar_guy]

Hey guys, I'm trying to diagnose a problem here. I was playing a gig last week and at the end of the second to last song my Mark V went completely quiet. I fired it up the next morning when I was at home and it's not dead, but REALLY quiet. To rule out the power amp section I plugged into the effects return and it was at full volume, so the issue is obviously in the preamp section somewhere. If the rectifier tube was bad I would assume that I would be blowing fuses, and the amp is running and powered just fine.

I swapped all the preamp tubes, one at a time to see if one of them was bad, and there was no change. Then I swapped them all and it's still doing the same thing. I can hear all the channels and gain stages switching just like they're supposed to, but there's literally like 5% volume when it's cranked all the way up.

Any thoughts on what could still be the culprit? Thanks!

 

[kingb]

The fact that you have switching channels AND full volume when running through FX return - sort of covers the pre and power sections (sort of, of course). So it sounds like something in the middle. Maybe the reverb. Try switching it in/out and check that the cables to and from are seated properly. What about FX loop, if you have something in the loop, take it out. Also try the bypass switch on the back (bypass the loop and solo volumes) - I believe it's right under the FX volume.

 

[jb's 52]

I did something like that before. I can't remember the exact circumstances or issue though, but I remember it happening when I was plugging pedals in/out of the amp and experimenting if one or more should be in front or in the loop. Sorry, I'm not much help...but I wanna say it was a non-true bypass pedal w/o power...or some pedal placement that didn't jive with the amp and/or loop.