is it a tube problem?

EyesLikeGuitar · Posted: Tue Feb 06, 2007 11:17 pm Post subject: is it a tube problem?

I just recently traded in my marshall jcm2000 100 watt DSL head for a used jcm 900 50 watter and $75 cash out of my pocket, best deal I ever made. Problem is, one of the channels is acting up on me. Slight adjustments of the gain knob can make the preamp drop out completely, replacing beautiful meaty crunch with a very weak and undesirable sound with absolutely no depth. Is this something as simple as a blown tube, or is it something much more serious?

herb · Posted: Wed Feb 07, 2007 2:58 pm Post subject:

Could be a tube, could also be a dirty spot on the volume pot.

EyesLikeGuitar · Posted: Wed Feb 07, 2007 10:01 pm Post subject:

well, if i do full turns of the knob, it'll make a sort of crackling sound, and the parts that crackle are the only parts where it sounds like it's supposed to, and if you bump it even a tiny bit off those spots it drops out. does that help at all?

i wish i could play around with it, but it's at home and i'm at school

herb · Posted: Thu Feb 08, 2007 3:32 pm Post subject:

Sounds like a bad preamp tube.

SAguitar · Posted: Thu Feb 08, 2007 3:45 pm Post subject:

Yeah, that could be a bad preamp tube, but from what I'm hearin', it could still be a bad potentiometer in there. It's possible that a good cleaning of the pot might clear it up.

Regardless, you should have all your tubes tested, and the amp checked out and rebiased by a qualified tech, just so you know where you're at right now. There could be a good reason why the seller was willing to let it go so easy. Cool

herb · Posted: Thu Feb 08, 2007 6:43 pm Post subject:

The clearing of the crackling when the amp is thumped led me to the preamp tube. I never had a dirty or bad pot that would clean up when I thumped the amp. But it is still a possibility..... Wink

SAguitar · Posted: Fri Feb 09, 2007 6:35 pm Post subject:

Oh yeah! I forgot about that detail! Rolling Eyes

SunDog · Tadpole Joined: 01 Jul 2008 Posts: 20 Location: Arizona

Radio Shack sells some good contact cleaner called DeOxIt. A friend of mine swears by it. I've used it to clean up pots, switches, etc and it seems to work well.

SunDog

SAguitar · Posted: Thu Jul 17, 2008 11:33 am Post subject:

Yup, I'll second that. DeOxit is good stuff. Cool