Help--Volume Drop in Amp

[AimingHigh]

Hey guys,

Seeking advice from the experts as always. I have to admit I am a dummy when it comes to tube amps. I have three--an H&K Tube 50, a Sovtek MIG-60, and a Marshall JCM 800, 50 Watt. The Marshall is actually a 4210 combo converted to just the head.

I'm having problems with the Marshall--it plays great for about 15 minutes, and then the volume starts to gradually drop off. It doesn't go up and down, it just gradually drops off over time. I swear I've heard of a similar problem before. The issue is that I need to replace tubes, right? (specifically, the preamp tubes) Can this be symptom of something else?

Thanks guys!
George
P.S. here she is: (I know I have to replace some knobs )

[CharvelRocker]

I'm no expert, but it sounds like your tubes are going out. Do they keep glowing when the volume drops?

[AimingHigh]

I can check...good pointer, I'll need to see if the glow is fainter.

George

[JACKSONFREAK]

Sounds like the output tubes are dying. Just get a new pair and have it biased. Unless you already have a "matched pair of tubes in there. Say like Boogie tubes or Groove tubes. Then they should just pop in with a problem.
Otherwise get it checked out by a certified tech and get it biased.
Or it could be the output tranny. depending on how long it's been running with bad tunes in it.
Gil

[AimingHigh]

Thanks, Gil.

George