Is it really that important to debate the method by which Mesa biased the Mark V? The only time the Mark V operates in pure class A mode is in the 10W power setting, amp is reconfigured to operate at a cathode bias and uses one inside tube and the outside tube next to it. In 45W mode, the amp is operating in class A/B (push pull) but the operating point of each side is biased closer to class A than it should be for class A/B. 90W mode just pulls in the outer pair also in Class A/B. Quite similar to the Mark IV but does not have a 10 W mode. If I recall, the differences were in the plate voltages, screen resistors, and the voltage divider network on the pair of tubes. Too bad I deleted the schematics for the Mark IV after I sold the amp (you can find that one doing a search). I still have the schematics for the Mark V (not complete as it is missing the relay control circuits and logic for the footswitch, but is good enough for simple repair if needed). In essence, the Mark V in 45W and 90W is operating Class A/B. If you consider each portion of the Class A/B amplifier, each is composed of a push pull format, extended class A refers to the operating point or load line characteristic of the inner pair of tubes that is closer to that of a Class A but is not a true class A amplifier, it is a push pull or A/B. How this differs from a traditional class A/B amp is that the inner pair of tubes have a lower bias voltage than the outer pair. Inner pair of tubes have a -43.6V on the grid, outer pair have -46.6v on the grid. If you feel the Mark V is operating in Class A in the 45W mode, you are mistaken.
If you want it in writing by the manufacturer, look at bottom of page 3 of the Mark V manual.
copied from the manual:
45 WATTS turns off the outer pair of 6L6s so only the middle two are running. These are the ones with the lowered bias so, while they are still Class AB, their Class A region is extended. In Channel 3, they can be switched to run in Triode configuration which cuts their clean headroom roughly in half.
Combining these two opposite styles of wiring in one amplifier gives you the best of vintage and modern amplifier styles. Headroom and power are there when you need it… but there is always a naturally pleasing and musically curvaceous quality to the sound that
Overview: front panel (Continued)
PAGE 3
is magic to your ears and to your hands. Simul feels great and is inspiring to play!
10 WATTS reconfigures the whole set of 6L6s so that the two nearest the 5U4 run pure Class A, single-ended—no longer push-pull. This is the ultimate low wattage output circuit that duplicates the essence of the best really old vintage circuits. Here, the second harmonic (an octave above the note played) is NOT cancelled out (as it is in push-pull circuits) and provides a magical halo surrounding the notes. Onset of clip is so gradual that it’s hard to pin down the transition from clean to overdriven.
I was under the impression that the 45W mode was class A as well. It is not.