Marshall Amp Noise??

[Unclemeet]

How much noise is acceptable at idle? For example, I have a late 70's JMP50 MKII MV. When I crank the Master and use the Preamp to adjust overall volume, there is a considerable amount of hum, but if I have the volumes adjusted in the opposite manner, there is very little hum. I have been gigging this amp for the last two years with minimal maintenance.
What could be causing this hum? Caps? Biasing off??? Time for a tube change?

My preferred amp guy is a little backed up and I have gigs lined up through mid Oct. so a trip to the shop may have to wait.

Any info would be appreciated

Tim

[Birdy]

My $.02...power tube maybe.
I get a bit of hum from my JCM800 in the same manner.
Gonna follow this thread and see what the experts suggest.

[Unclemeet]

I was thinking power tubes or possible caps. The amp is close to 30 years old probably had little work done to it.

Let's see what some other folks think.

[Swarty]

Hum or hiss?

[roodog]

http://www.geofex.com/ampdbug/ampdebug.htm

Good luck

[Unclemeet]

Swarty..it is more of a hum. With the master above 5 there is a low hmmmmmmmmmmmmm up to 10 and it's pretty noticable. Keep in mind this is noticed at home. At the gig it's only mildly annoying.

The power tubes appear to fairly recent groove tubes and I baised them several months ago.

Any more thoughts?

[jezzzz2003]

I think this just may be normal.
Most amps will have some noise when the master is flat out and some gain is applied to the signal.
Naturally the louder you go the more this is amplified.
My reissue 50w has the same problem but has a seperate
bias pot for noise. {dont know the technical name for it}
In old Marshalls there are some wires coming from the
pre tubes that can be moved away from the chassis to reduce
this hum but not completely elliminate it.
The procedure is explained inside Aspen Pittmans
The Tube Amp Book.
Hope this helps..

[John Phillips]

If the noise is worse than normal it's either the main filter caps, or bad wiring layout - they do sometimes suffer from this as jez said. It isn't the power tubes or anything else to do with the power section (eg bias filter caps) because the noise goes away when you turn down the MV.

If the amp has its original caps they probably need replacing as a matter of course given their age - do the bias filters as well, even if they're not part of the problem. If it still hums after that you may need to experiment with moving the wiring (especially the preamp tube filament wiring) around a bit.