A co-axial magnetron in which in order to provide attenuation of slot resonances the axial coupling slots in the anode member are opened out at either end into round apertures into which resistive attenuating material is introduced in the form of discs having thicknesses corresponding to the thickness of the wall of the anode member. The discs are held in place by thin copper cylinders inside and outside of the anode wall member and overlapping the attenuating discs on either side.