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That the 'Simul Class' eats tubes is very much related to much too low protection resistors for EL34s, take a look at the mkiii, there this resistor is about 6 times as high. This is for some reason 😉

Afaik class A only means that a tube conducts 100% of its cycle no matter if self biased or fixed bias. Class A can be triode or pentode Mode

If I'm right, the 60W class a/b pentode is available anyway and much cheaper than simul class.

Possible? - yes Worth the effort? - no, afaik

Of course there are different was to handle this: 1) put 1,5k/5W or higher resistors in the class A Socket 2) leave them as they are and take the risk ( which might be Boogie but not Gibson) 3) Leave them and no warranty if el34

I strongly believe that a typical el34 (be it NOS or not) ist abused because of much too small grid resistors. There is a reason why mk3 grad resistors are 6 times as high. These are current limiting resistors to protect the tubes.

Especially the Mk2c circuit is not built to run el34s safely. the screen grid resistors are much too low to keep the el34 in a safe state. In later mk3s they were like 2k7 which is roughly 6 times as high. So from a reliability PoV it is mandatory to NOT warrant any el34 usage.

I suspekt Mesa to be rekuctant wrt warranty as modern el34s are so many different flavors that its impossible to warrant every Kind of el34 especially when driven hard to their limit. Its not a quality issue but rather the fact that not every el34 although being in Spec can stand the circuit...

Most likely one ore more of the hv- diodes is shorted

Hi Team, does anybody eventually have a mk2c schematic without any mod or +? Same goes for a mkv 35 preamp for repair purposes? Thanks a lot guys!

I'm in for 1-2