 More importantly, it marked the introduction of Mesa/Boogie's "Simul-Class" system, where two of the power tubes (always 6L6s) run in class AB pentode while the other two tubes (either 6L6s or EL34s) run in class A triode.In a simul-class amp, running all four tubes generates approximately 75 watts RMS of power; running only the class A tubes produces about 15 watts.Also available were non-simul-class Mark IIBs in both a 60 watt version and a 100 watt version that allowed shifting down to 60 watts by turning off a pair of power tubes.