Marshall 6100 noise

[Valvehead]

Ive got a Marshall 6100LM here , and its making a very faint low freq hum when the amp is in standby, and this is with no tubes in it!

Its the same if i put all the tubes in------and of course the noise gets a bit louder with the amp on/ running at idle.

There is no dcv at the speaker outs.

What could cause this ? The OT itself ?


http://www.drtube.com/schematics/mar...04%20iss10.pdf


http://www.drtube.com/schematics/mar...-04%20iss6.pdf

[drewl]

Probably physical hum from the power transformer.

I had a DSL100 reissue with that problem last week.

Remounting the trans and removing the top helped alot.

[Enzo]

Two thoughs:

One is that the issue is probably the OT core coupling magnetically with the field from the power transformer. Turning one of them 90 degrees might help, unless the current arrangement is already the quietest.

The other is that you describe the hum as very faint. So is this an actual problem we need to solve? or is it just something you wish wasn't there?

[Valvehead]

I think the amp should be dead quiet in standby--especially with no tubes.

But yes Id like to fix it because it gets worse when the amp is on

[Enzo]

The worse part when it is running is probably from other sources of hum. I suggest fix the hum when on noise and once that is squared away, determine if this faint hum is worth the effort.

[Valvehead]

The filter caps could probably be replaced , being the amp is from 93

[Valvehead]

ok, i replaced all the B+ filter caps , new power tubes , i found a burnt 10 ohm resistor in the power supply (goes to the heaters) , i pulled all the heater wires and twisted them up tight and isolated them ,

and found something else-- C401 , a 2200uf / 35v , in the power supply is getting very hot , and looks to be expanding on top, like its about to burst open. what might cause this?

and yes , i still have the noise

[pdf64]

For R401 the 10 ohm heater reference resistor to ground to blow, there was likely a power tube short, and without that reference, the heater line may have been pulled up to VB+, damaging the heater-cathode insulation of any / all tubes. Leading to the heater supply breaking through into signal circuits.
So suspect all tubes to be bad, try replacing all with known good ones.

And maybe from such an incident, that the high Vac ripple from the stressed B+, if shorted to the heaters, may have damaged the raw supply prior to the 5V regulator REG401, eg C401, C402, C403 and W005 (unless it's a very voltage type).