The brainchild of Pete Millett designer of the Wheatfield Audio HA-1/HA-2 tube headphone amps, i first saw this low-voltage (24VDC) headphone amp in an AudioExpress article which included the schematic and detailed build instructions. Pete designed the amp to use valves that originated in 50s car radios, reasoning that these valves would still be available (through surplus stores , Ebay etc) as well as cheap and safe to use for the novice builder.