Heyyyy guys, so I'm new to this forum and just got back from lugging my heavy assed 5150 (on the bus) back from a local repair guy, who couldn't even fix my problem. Great use of $50...

Anyway the problem breaks down like this:

The master volume on both the clean and gain channels will not drop below 5. If I turn all the post volumes to 0, there is still a loud muddy guitar signal audible (not just noise, but the actual signal still goes through). This problem exists even when using the preamp out only (I've been testing it to see how close I can narrow the problem). If I use the effects I still get a loud muddy guitar signal coming out of the "send" jack (same signal that is present when the POST volumes are all the way down). So there must be some kind of signal crossover.

This leads me to believe (and the repair guy agreed, though I'm not sure how accurate his claims are) that the problem exists somewhere in the preamp stage. I've tried swapping the preamp tubes around but to no avail.

I was wondering if anyone has had similar problems with a 5150 or any tube amp really...any information would be greatly appreciated.
thanks guys
-Geoff

Did the repair guy check the caps from the preamp to the poweramp? There could also be a cold solder joint somewhere where the 2 meet..

Do you get alot popping/ crackel noises?

There are some crackling/popping noises on startup sometimes, but after that, nothing persistant. As far as I know he checked everything, but seeing as how he could not fix it, I sort of question his methods. He did suggest though, that it might be the preamp PCB that is having problems. I have no idea how to troubleshoot a printed circuit board though.

It could be alot of things but being that's it a tube amp it's easier to fix then a solid state. Find a good tech is a good start.

if you know about electronics get your meter and check the caps in the buffering sections of the amp and see if one has gone bad?

You will need a schematic for this amp