Check the listing for details. Dr. Z PHD 10-watt Amplifier Head - Black. Condition: New. Listed at 1599.00 USD. Dr. Z's Ultimate Class A, Single-ended Powerhouse Inspired by the raw, raucous essence of history’s fiercest Class A combos, the Dr. Z PHD head takes the low-wattage, single-ended formula into the 21st century with expanded headroom and dynamic range. Tiny tube amps have reigned as studio secret weapons since the dawn of rock music for their ability to deliver raw, dynamic overdrive at reasonably low volumes. That said, for all their pint-sized power, most classic low-watters feature precious little headroom and a rather narrow bandwidth at high volumes. To rectify (quite literally) these historic challenges, the PHD replaces the standard 3-tube tube complement of Fullerton’s 5-watt champions with an octal-based 6SL7 preamp tube, a KT 66 power tube, and a 5U4 rectifier tube.