The Heartbreaker's four 6L6s are supplied high-voltage from a dynamic tube rectifier or can be switched to the silicon-diodes for added punch and headroom.  The on-board Bias Selector allows substitution of either EL34 or 6V6 output tubes for the shipped quartet of 6L6's capturing a more Euro vibe or, if you prefer, a true low power retro feel.  Each selection has its own distinctive feel and forte.