Abstract: In a spin-tuned magnetron a plurality of anode vanes disposed in an annulus form resonant cavities and a spinner, carrying a plurality of apertures rotates coaxially with the annulus to pass through the cavities and vary the resonant frequencies. This expedient gives a frequency variation which is predictable. This invention provides a less predictable frequency variation by modifying the shape of at least one of the vanes and at least one of the apertures. The aperture is in one example modified by cutting a slot between one aperture and the edge of the spinner.