Marshall JCM 900 4100 Fading

[cawoodm]

Our 4100 started fading while we were playing the other night - the volume would drop unexpectedly to a fraction of normal and it wouldn't recover. We'd switch the standby off and wait half a minute and then it would be OK for a few minutes and then fade again and be very soft.

Is this a common problem with tube amps? Should we just replace all tubes or do we need a proper service. We have technical expertise in electronics but it's hard to fix a problem which is sporadic and you can't reproduce it in the lab...

[mac1amps]

Since you describe the sound as "fading", I would suspect a faulty heater connection somewhere. Is this "common"? Well, with respect to connections being faulty in general, my pro-opinion would be yes (especially with respect to P.C. board connections). With respect to "heater" connections specifically (which, BTW are not P.C. board connections in this case), I would say not as much.......It's very typical to want to replace tubes first off, but I'd say it's more likely that re soldering will be needed somewhere.

[JerryP]

Dity effects loop jacks would be my first guess. Power tubes would be my second guess.
Jerry

[cawoodm]

Originally Posted by JerryP

Dity effects loop jacks would be my first guess. Power tubes would be my second guess.
Jerry

Do you mean the Send/Receive out/in jacks?

[JerryP]

Originally Posted by cawoodm

Do you mean the Send/Receive out/in jacks?

Yes, they can cause a break in the signal path if they are dirty. You can plug a jumper cable in the loop to rule out the jacks. A quick shot of contact cleaner and plugging a cable in and out a few times usually cleans them up if they are the problem.
Jerry