Hi, I have a quartet that I use in my YAH100, I use it for the first time with my band (so not at home at 0,000001 MV  ). I checked my bias before (60 and 60 each couple), after playing I find the bias of one couple at 90. I reset the dias to 60, but today I find it real low (something like 40), but after playing at high volume, after, I find it high.

I tryied inverting the tubes and the problem follows the problematic duet. I tryied with my old tubes (JJ) and no problem at all.

So I think could be one of the 2 tubes of the problematic duet, I can't hear sonic problem.

Any Idea?
Thank you

If possible, please find and post the schematic.

Yes, it seems one of the Mullards has a problem.

Yes, https://tube-town.net/cms/userfiles/downloads/yah100-schem.pdf

Ok, so the amp uses a 1R current sense resistor per side (i.e. for 2 parallel tubes).
If you insert only one EL34 per side (e.g. in V5 and V6 positions) you can monitor the idle current of each tube separately. Should give you 30mV to 35mV across the 1R resistors.
This way you should be able to identify the tube with instable idle current.

Thank you, I hope to find a matched mullard to save the quartet