From the Mesa 2:90 Power Amp manual:
Much of the 2:90's huge sound and spongy dynamic feel comes from its patented Simul-
Class operation. For those just now discovering the magic of Simul - a brief explanation is in
order. In late 1982 an experiment in tone shaping within the power section, using esoteric
power transformers and unique power tube wiring configurations, gave birth to the first tube
power amp that incorporated two types of tube power operating simultaneously. These two
types or "Classes" of operation were Class A Triode and Class AB Pentode. The older Class
A style of wiring was popular in the early tweed amps of the 50's and produced a sweet
bubbling and bouncy tone - though not much power. As electric music became more popular,
louder and more powerful amps were needed. Class A wiring was discarded in favor of the
more efficient, cooler running Class AB Pentode wiring scheme. So ended an era of sweet
tweed amps. As we all know, those gems of yesteryear are much sought after today. Well,
both styles of wiring have their advantages; Class A Triode for sweet tone and a spongy easy
to play feel, Class AB Pentode for power, punch and cool reliability. To us, it made perfect
sense to marry all these attributes and combine two simultaneous classes of operation into
one power section. Thus the name Simul-Class . A decade later Simul-Class has powered
three generations of well loved Boogie combos and two generations of tour proven power
amps. The Simul 2:90 Stereo carries on this tradition and improves upon its predecessors.
By utilizing an even better, more "head-like" driver stage, this Simul delivers all the urgency
and soul often missing in some so called "guitar" power amps.