#1
Hi

I've got a Marshall Valvestate 8080 (80 watts) amp. When I bought it (second hand) it worked perfect. Then I brought it to a gig and the distortion channel has never worked properly since.

Well, the distortion channel does actually work, but the volume is very low. If you cranked all the knobs, it'd have about the same volume output as if you turned the gain knob on the clean channel almost all the way down. I have checked it for loose connections and stuff. I think it might be a transistor or IC (maybe even the tube) that's faulty, but I'm not sure. Could anyone tell me what to do?

-Schematics for the preamp (think this is the faulty one)
-Schematics for the power amp

Thanks
#2
A lot of times its a bad pot. Try rocking the gain and volume pots around a little, or possibly the contour pot... the contour pot on my 8040 is pretty rough, and it can kill all sound if I let it.

To clean up old pots, the easiest quick fix is to just turn the thing off, and spin the pots from zero to ten and back over and over several times. This will clean some corrosion up and get it working a little better.

If that doesn't work, then I try rocking the knob against its spinning axis, seeing if leaning it one way improves sound or cuts it off completely. If this is happening, then replacing the pot is the next step.

On the Valvestates, a lot of times the jacks will get pretty corroded, so cleaning them can help. If its only the boost channel doing it, though... then I'm worried it could be more like what you're saying... a tube possibly?

Try opening it up and see if the tube is even lighting up. Mine gets smoking hot... but you can't really tell unless you expose it.

if the tube is bad, you will have the problem you're having.

12AX7 tubes don't cost so much, and replacing it can't hurt... unless you buy a crappy one. It would be cheaper for you to put in a new 12AX7 and give it a go than to take it to a repair shop.

If you get pissed and decide to sell this thing, please contact me as I will take it off your hands.
All my photobucket pics are dead so no links to my guitar build threads.
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#3
Thanks for the response

I've checked the tube. When the amp is turned on, nothing happens to it. No light, heat or whatever might happen when a tube is being warmed up. But I noticed another wierd thing: when I turn off the amp while it's set to the dist channel and I've got a chord ringing, the volume slowly fades out and when it's completely faded out, it gives a little, short boost with the right volume and is then completely turned out.

Should I get a new tube?
#4
You could just gently pull it in and out a couple of times to see if it is just bad connection from oxidation.
This is what you'ld do with IC's to if they are socket mounted.
#5
i had that exact same problem with my marshall, and what i honestly did, was kick it, n it worked perfectly, it had wound me up that much to make me kick it, but it did the trick, so that may be your problem. give it a good boot-but at your own risk, if u screw it even more, none of it is my fault
#8
Thanks for the help, everyone. I've tried kicking and pulling it in and out, which didn't work, but I'm pretty sure it's dead so I'm buying a new one - sorry, no amp for you this time, zeroyon
#9
Haha... I knew that. I was just throwing that out there...
All my photobucket pics are dead so no links to my guitar build threads.
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