My Mark IIC+ is at the doctors...

[supramax]

... and the tech found a hum when using the 2pcs. 6L6 + 2pcs. EL34 - but not when using all 6L6.

What could be the problem? Is not the hybrid power section quite common i the Mark?

It is a C+ combo, all options minus reverb, with an export/international transformer.

Could anyone be at help?

 

[stokes]

I dont think the earlier Mesa's were intended to use the 6L6/EL34 outputs.Although it can work,the later simul-class amps have some mods in the bias supply that makes it work better.