Marshall 8100 head hums!

[wrvh]

Hi everybody, I've ben reading on this forum for a while and now that I have a problem with my amp I decided to finally register. Maybe you guys can help me out.

Anyway, here's my problem:

I play a (early 90's) Marshall valvestate 8100 head. It's a hybrid amp: 12AX7 preamp tube and a solid state poweramp. It's old and kind of cheap, but I like it a lot. But over the years some hum started to emerge. If possible, I would really like to get rid of this hum (without having to buy a new one). I will try to be as clear as possible.

- The hum is there when no guitar is plugged in. It's only barely noticable, but it's there.

- When I plug in a guitar, it's gets worse. Much worse. In the clean channel it's not that annoying, but when I switch to the lead it's pretty bad. So it's there on both channels.

- I've changed the tube, no effect. I've tried it on a different cab, not it. I also tried some different cables, just to be sure. No difference.

So what could it be? Old capacitors? Hum from the power section? Probably a lot of things, but I hope you can help. Thanks in advance!

[52 Bill]

Welcome to the board.

- The hum is there when no guitar is plugged in. It's only barely noticable, but it's there.

As you noted, it could be any number of things, input jack ground, bad pre-amp IC, failing filter cap.

Do any of the controls change the hum, volume, tone, etc.?

Find yourself a copy of the schematic and try to isolate which section the hum is coming from; before the volume control or after the reverb, etc.

[wrvh]

Sorry, I forget to mention some things.

- The hum is still there when I turn the reverb knob all the way down.

- Turning down any of the tone/volume knobs does not remove or change the humming sound.

Of course when I turn up the volume the hum gets worse, but it's definitely present when all the knobs are turned all the way down and no guitar is plugged in.

If I'm not mistaken, this should mean the problem is in the power-amp section (after the main volume pot). I have the schematic here: http://www.drtube.com/schematics/marshall/pc0689p.pdf but I'm not that good with schematics. Any help?

[52 Bill]

If I'm not mistaken, this should mean the problem is in the power-amp section (after the main volume pot).

No, if the hum increases when the volume is turned up, then the power amp is probably not the problem.

I would suggest having the filter caps in the power supply checked first.

[wrvh]

Thanks for the response. I will get the caps checked out. Hopefully it will work!