1981 JCM 800 4210 Combo losing volume

sabre

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Hello, good people,

I have a unmolested 1981 JCM800 4210 50 watt combo that intermittently loses volume/power. I bought it used in '83. I can't say I've ever been thrilled with the stock sound of this thing, and mostly run a multi-effects through the clean channel. I don't gig, just a woodshed guy here. Anyhow, lately, I have had a hankering to see if I could go old school and look for some good tone. The amp goes from sounding just okay to mud. Sometimes if I let it sit on standby, it will come back. This amp has the infamous channel bleed issue, too, btw.

What I've attempted (I'm not an amp tech):
  • Tried two new old stock Groove Tube EL34s - matched pair
  • Tested all Mesa preamp tubes (two were crackling with a chopstick test and were replaced with new old stock RCA 12AX7s (tested 90% good)
  • Cleaned pots w/ deoxit
  • Cleaned all jacks and tube sockets
  • Had a buddy check the large original Daly caps
Also, I haven't used the dirty channel in 30 years. I forget, should the gain pot control the volume from 0 - 10? Because, that's what it does currently.

Thanks in advance for any insight!
 

Matthews Guitars

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Rebiasing new power tubes is not optional. It's mandatory. Get it done.

A volume fluctuation can be caused by ANY preamp tube going weak or intermittent.

It can also be a bad coupling capacitor.
 

sabre

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Rebiasing new power tubes is not optional. It's mandatory. Get it done.

I'm going to have to run it up to an amp tech.

A volume fluctuation can be caused by ANY preamp tube going weak or intermittent.

It can also be a bad coupling capacitor.

I think I can dig up a few more AX7s, test them, and give them a try.

Thanks for the tips!
 

tallcoolone

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No, I didn't. It's a bit out of my comfort zone.
I get it, me as well, unless there are external bias points. It’s a $60 job from my guy. Prob wouldn’t hurt to drop a few bucks into the old girl to tune her up sounds like anyway?
 

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