Microwave amplifier having cross-polarized cavities

A multi-stage microwave gyrotwystron amplifier is disclosed having two or more stages comprising rectangular cavities to facilitate tuning. The first two stages comprise rectangular tuneable cavities that are oriented orthogonal to each other which produces a homogeneously modulated electron beam that is directed to a cylindrical output cavity which increases its power output as compared to prior art devices havig an output cylindrical cavity excited with a non-homogeneously modulated electron beam.