jcm900 50w low volume break up

[psycho_a_gogo]

I'm new to tube amps. My first is a Marshall 4100 JCM 900 the 50 watt version. It sounds great when it's turned all the way up, like marshalls do, but it's not as loud as i would expect and at lower volumes it breaks up to the point that it sounds too terrible to use.

Any advice as to what the cuase of this could be?

Smart ass marshall jokes are also welcome.

Thanks!

[Amp Kat]

Could be several things but try patching the send to the return with a guitar cable. All of the grounds from the pots on this amp aren't tied together if I remember seeing a mod for this on the last one. Do all of the channels work correctly ? Footswitch jacks on these cause problems. The rectifier from the power tube heaters to the preamp heaters cause problems,gets hot and needs replacing with a larger rectifier. Bad tubes in either the preamp or power section do it also. Other than that you'd do better sending it to a tech who can go over it.

[Mars Amp Repair]

if it breaks up at low volume, it could be a deficiency in the dc supply voltage to one of the stages....could also be biased wayyyy to cold & actually distorting at lower levels in the output stage (cutting off the tubes).

As AmpKat mentioned, the bridge that supplies DC for the preamp tube filiaments can bad, but it ususally manifests itself as the sound intermittently fading out as the filiaments dim. I suppose a situation could come up where the bridge was partially bad were supplying less than the nominal 6.3v to the tubes. I've not run into that yet, tho.
I agree with Amp Kat, it's probably an issue for a tech. glen