Marshall Valvestate 8020 - Hums and Crackles

[nullsys]

Edit: Valvestate s80 8240 E Model

Hi all,
This should be fairly simple but has me stumped.

The marshall has only one pre-amp tube and one output transformer.

Now, at low master volume there is a hum no matter what you do on the preamp controls or plug in a guitar. If you turn the master up, it goes away.

Also, sometimes there is a crackle and a pop and the clean channel even overdrives slightly from time to time.

I believe it may be a pre-amp tube problem but the low hum makes me believe its a power transformer problem.

Could anyone give me any pointers?
Any help would be greatly appreciated, thank you!
Carl

[nullsys]

Update:
I tried tapping the tube, this made no change.

I removed the tube and the clean channel works normally, still with the problems tho...so this obviously means it isnt a pre-amp problem. The gain channel of course cut out entirely.

Still doesnt help much but maybe someone can make sense of it?

[Enzo]

Yes, I would say your wild guesses are not what is going on in there. I doubt it is the tube, since turning all the controls down would turn down anything the tube was doing.

That transformer is the power transformer, not an output transformer. Solid state amps don't have output transformers except in special cases - CV systems for example.

I doubt the power transformer is at fault, all it does is provide AC to the system, if the system doesn't filter the rectified AC very well, that is not the transformer's fault.

Does the hum come out both speakers? They each have their own amplifier circuit.

I'd be looking for potentially a funny MV control, or more likely a problem with the ground to it.

Make sure both the low and high voltage rails are good and clean. By high voltage I mean the ones for the power amps, not the tube one. But make sure the tube supply is clean also.

I would check IC11 output pins - 1 and 7 - for any DC offset beyond a few millivolts.

I suspect what is happening here is there is some system hum always there, and some hum in the signal path that happens to be out of phase with the system hum. As you turn up the MV, the signal path hum is turned up and the more it increases, the more it cancels the system hum, until you reach the point where the two out of phase hums exactly cancel. Does that fit the clues?

[nullsys]

I suspect what is happening here is there is some system hum always there, and some hum in the signal path that happens to be out of phase with the system hum. As you turn up the MV, the signal path hum is turned up and the more it increases, the more it cancels the system hum, until you reach the point where the two out of phase hums exactly cancel. Does that fit the clues?

Ah, sounds familiar.
I get a certain hum at low volume...as I increase MV the hum BLENDS in with a high end hum associated with high volumes. I think thats what you meant?

Ive spent a few hours spraying and cleaning pots and it did help as it was VERY scratchy at first.

Yes, as far as I can tell its coming out of both speakers.

so, its sounding like a dodgy pot, input or MV control?

[Enzo]

There COULD be a pot issue, but as I surmised, I think you have two sources of hum that meet at the MV stage to cancel due to phase differences and level differences. SO I'd look to make sure the power supply is clean first, then look to whatever is injecting hum into the signal path.