Abstract: A magnetron comprising a large vane-type, double-strapped anode structure with the vanes being cooled by liquid flowing in a channel immediately behind the surface bombarded by electrodes and the two pairs of straps passing through openings in the vanes, a liquid cooled secondary-emission cathode positioned centrally of the anode, said cathode incorporating a starting filament connected to a heating current power source, means for engendering an axial magnetic field in the region between anode and cathode, and power output means connected to at least one slot between vanes, said means including a coupling slot, a ridge waveguide transition connected to said slot and a window between the transition and an output waveguide.