help with repair- Boogie Mark II

[Aaron Smith]

Hi everyone. I have a reasonable amount of experience with tube amps; I have build a single-ended amp from a kit, and I've done a refurb of a 60's Silvertone 1482.
My other amp, a Mesa Boogie Mark 2 (version A) from 1980, is a beauty but I'm having trouble with the overdrive. Whenever switched into lead mode, it hums like crazy- LOUDLY. It is fine in rhythm mode.
I'm going to start by swapping tubes to see where that gets me. The schematic on Schematics Heaven seems to represent my amp very accurately.

http://www.schematicheaven.com/boogieam ... e_mkii.pdf

I have already tried V1 with no success; looks like V5 is the phase inverter; V3b and both halves of V4 are for reverb; V2ab and V3b all appear to be gain stages. I must admit- the whole pre-amp escapes my understanding, with all the relays and EQ controls.
Are any particular gain stages more likely to be the culprit? There is very little room to work inside of this chassis and I'm thinking that this will be a difficult repair if I'm doing it blindly.
Thanks in advance...

[Structo]

The schematic is difficult to read

But to me it looks like V3A is the OD channel.

And V2 looks like another gain stage.

V3B and V4 is reverb and yes, V5 is the PI.

On any tube amp the first suspects should always be the tubes.
If it has been a while since this amp has be retubed, I suggest you get all new tubes or at least substitute a known good 12ax7 for the pretubes, one at a time to see if you can identify the bad tube.

Also, while you are inside, check for burnt resistors or anything else obviously wrong.

If it is not a tube causing the humming, then it may be something in the solid state wizardry in the amp.

[DonMoose]

Since the hum is specific to having the OD on, I would want a look at the plate supply for the OD section - and maybe the grounding in that section. On the Mk II C schematic I have, the OD plate supply is a node in the B+ string that only the OD uses. In the IIB schematic, it's shared with V1. In your II(a), it's not visible.

You may need to put a cap in series with your meter to read AC on high DC. This WILL charge the cap to a painful-to-touch voltage, so discharge it with a 220k resistor as soon as you're done taking your/each reading.

Hope this helps!

[Aaron Smith]

So it appears that V3a is indeed the overdrive gain stage. I replaced V3 and the hum went away. I had the amp serviced last summer, re-capped and re-tubed; too lazy to do it myself with such a crowded chassis. He used cheap Russian tubes in all the pre-amp spots; guess I should have known. Put an old Philips 5751 in V1, and an Ei ECC83 in V3; sounds like a new amp.
Incidentally the B+ for V3 is shared with V1. Thanks to you both for taking the time to help me out.