Park P50 Classic (1966 Black Flag Plexi)

The P50 Classic is the early version of Jim Marshall's 50 watt plexi circuit, the one known as the Black Flag. EL34 output valves give it colour and a hard focus in the mid frequencies, which is most of what people mean when they say British tone, and a valve rectifier behind them adds the squish and compression that arrives when the amp is pushed. Against the P45 it grew out of, expect warmer saturation and tighter filtering, with the bite and aggression that Angus Young and players like him built careers on.

Two differently voiced channels, one brighter than the other. Because this is the early circuit, the bright channel still has real bottom end, so you are not obliged to jump the channels with a patch lead just to get a full sound. Mitch Colby fits two push-pull pots: an upper mid boost that works across both channels, and one that applies independent bright cap values to both at once. PPIMV (Post Phase Inverter Master Volume) is now standard, so the amp can be overdriven at a volume that will not clear the room, and cranking the master fully takes it back out of the circuit.