Grateful Dead Music Forum

I just started to have some issues with my MC50… I can best describe it as "farting-out". From a cold start, it operates fine… then after a short period of time under load (increasingly short) it starts to sound very restricted… like a thick blanket has been thrown over the whole system. If I turn if off and let it rest for a short time… it goes back to operating fine for a while.
Anyone know what the cause of this problem might be?

I'm sure wiser minds will weigh in with the correct answer, but when I experience something like this it is always the battery in the guitar failing.

Does it seem to affect all frequencies equally, or is it focused more towards the bass/high notes? Is it only when you play loud, or is it at any volume?


Thinking of things that it's not likely to be, we can pretty much rule out the trannys (they did a "how it's made" segment a while back... they use asphalt to hold the core in place...), and it doesn't sound mechanical (otherwise it would sound like crackling or cutting in and out).

Since these do not have a power switch, turning it off may be draining the filtering caps and turning it back on may charge them up briefly, only to drain again. If I had to venture a guess, you are probably using the original caps, and their time may have come. Fortunately, they are easy to test with a voltmeter.

The other thing I've heard happening with the old Macs is that the transistors may eventually fail, resulting in "ringing". In the 250, this typically manifests itself as one channel sounding bad, but you've got yourself a mono amp, so the whole thing would sound off. Unfortunately, this is real hard to diagnose without an O-scope.

Yeah, this is with a passive guitar setup - no battery in that loop.
It's definitely the highs that are falling off… results in a very muffled sound. I would say it pretty much any volume, I never play it whisper quiet… and when it starts to happen, I always turn up the volume with no improvement.
I'm wondering now if it might be a preamp tube issue?? I've using a very new FYD preamp, but I put some NOS tubes in it...

Definitely try putting the original or different tubes in the FYD just to eliminate that possibility.

Also, do you have another power amp you can try? That would be my next move. Then you would know for sure which one it is.

That's a good idea.. I do have another power amp I can plug in. I was originally convinced it was the Mac… not sure now, I'll try that tomorrow morning!
Thanks.