The design, manufacture and commissioning of an airborne radar transmitter amplifier is described. The microwave valve employed is a crossed-field amplifier (c.f.a.), cathode modulated by a hard-valve shunt modulator. The peak output power is 15 kW, and pulse lengths up to 5 μs may be used. The e.h.t. supply is derived from a high frequency transistor invertor stabilised by a feedback loop.