Marshall JCM 2000 DSL 100 - tubes red-plating

[goldtop5]

I've got a Marshall JCM 2000 DSL100 that has a problem with the bias voltage drifting upward. It will keep going up until the tubes, v5 and v6 red-plate. I've replaced all of the bias components, new tubes, and went as far as replacing the 8-pin tube sockets, but the problem still persisted.

I've heard that there is a design flaw with this amp.
(Circuit Board # JCM2-60-00).

Does anyone know of any hints or mods that can resolve this? Thanks!

[Leo_Gnardo]

The "design flaw" is the use of a circuit board material that becomes conductive with heat and time. Really the only part of the board that's worrisome is right above the output tube sockets, where the board's conduction between pins 3 & 4 becomes an issue, pulling up the bias voltage via unintended resistance path to the screen grid pin. Lucky for you we're having a discussion about this on another current thread. To save me a lot of typing, have a lookee over here:

http://music-electronics-forum.com/t39832/

[Mick Bailey]

Did you check the trim pots on the bias board? The solder joints on these always need checking. Which revision board do you have?

[The Dude]

^^^^^^ What Mick said. That bias board is notorious for solder/connection problems.

Here are a couple more links that dig into the problem in more detail.

The Marshall TSL122 TSL100 thermal bias drift repair page
The Marshall TSL122 JCM2000 Repair/Mods Page