Mesa Boogie DC3 red plating

[greenmeanh1]

Moving right along. The next amp im digging into is this DC3. I remember it being a fairly interesting amp tone wise. Its the only el84 powered tube amp i have had. It has a issue with two of the tubes red plating. I changed out to a new set as per the advice from Mesa and the same thing happens after about 3 minutes run time. I have already cleaned up everything like jacks and pots on the amp, just need to sort out the tube issue.
What causes the red plating and where is the best place to start looking for the problem.
Not sure if i have the schem for this or not. Have to look.

[Chuck H]

I couldn't find a good (legible) reference either. But I did find the illegible one below. It'll have to do.

I've seen red plating reported on these amps before.

Are the two tubes on one side only? One on each side?

Have you checked the actual bias current yet?

This amp uses Mesa's "dyna watt" thing on the power tubes grids. I don't like this circuit for a couple of reasons. Not the least of which is that it elevates the power tube grids out spec from the bias circuit. I'd surely undo that by lifting the 2.2M resistors, bypass the 220k series resistors to the grids, provide individual grid stop resistors to all the grids and change the screen grid resistors to 1k (470 is too small for modern el84's at high voltage).

That amp runs at or over 400V on the plates of el84's. Too high for only -12V bias. Unfortunately I couldn't find a schematic for the bias supply because it will need to be modified for more negative bias voltage. I don't know what your chops are when it comes to this stuff. These amps are a PITA to work on.

Contact Mesa for a schematic. They have been very accommodating for me in the past.

[g1]

Here's a more legible schematic

[Chuck H]

I think that schem must have been made when Mesa was still practicing collusion. I don't see how the D/E supply could function as it does the way it's shown. It shouldn't be too hard to modify the bias circuit in the actual amp though. Once the actual voltage divider is located.

[g1]

Did I say schematic? I meant to say 'rough guideline' (SOP for messa).