This is the "Class A/Simul-Class" switch or the "60/100" watt switch, depending on amplifier type. In the Simul-Class amplifiers the
power (before clip) is much less in the Class A position - mere 15 watts. But the unique way the system is wired allows you to achieve a much greater degree of power tube distortion than with most other amplifiers ... and it is still very loud! In Class A, only the outside (outer left & outer right) pair of tubes - the EL-34's will be on. The inner pair - which are 6L6's - will also be turned on when the switch is in the Simul-Class position. In addition to this obvious difference, the biasing and control circuitry of the socket pairs are radically different ... and this is the heart of the now famous (and patented) Simul-Class circuitry. It is worth noting that almost all of the major players who have gone over to Boogie have chosen the Simul-Class power design!