Maury, my "NOS" Mesa preamp tubes are just killer. In fact, my NOS SPax7a's can easily go head to head in performance and tone with the (new) Tung Sol 12ax7a's, which gives me a couple of benchmarks. The newer SPax7a's and 12ax7a's from Mesa have been kinda spotty...some killer, some not so. From what I've been told by the folks at Mesa, the NOS's are Chinese tubes that underwent extensive testing. Not sure why the newer ones aren't up to par..maybe a drop in quality control at Mesa?
From what I understand, Mesa is still using Chinese (Ruby) tubes. That might be changing in the very near future from what I understand.
Of course, everyone has to consider what shipping can do to a vacuum tube and all it's tiny components inside the bottle....it's any wonder ANY tube makes it safely from the manufacturer to the consumer without total failure
Going back to biasing the output tubes. You have to consider the fact that Mesa biases their amps "cold". This is to perserve tube life (i.e. using the same set for over 1 year under heavy use).
All tube amps can be rebiased.
Of course, if you're Mesa is still under warranty, the warranty won't be voided if you use Groove Tubes brand tubes with ratings between 4 and 7.Otherwise, per Mesa, you will void the warranty. This can be viewed several ways. Most of us see it as a way for Mesa to get you to buy
only Mesa's tubes. My clients with under warranty Mesa's have forgone the ploy and have their amps biased to their new tubes...but that's why they get the killer tone that they have.
For what it's worth, we are currently finishing up our own amplifier design and have decided to go with the Tung Sol 12ax7a's for our v1 and v2 positions, the TAD (Tube Amp Doctor) 12ax7a-c for the phase inverter tube, and are at a toss-up between the SED (Winged C) and TAD EL34 for output tubes. We are currently testing different types of GZ34's for the rectifier (the amp is switchable; either tube or solid state). Right now the Shunguang GZ34 is in there and kicking butt!
