 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.The Mark IIB's front control panel is identical to that of the IIA.