A magnetron comprises an anode (2) having vaner (3a, 3b) which define a plurality of cavities. A dielectric resonator (7) is located such that it is in communication with at least one of the vanes (3a, 3b). In use, the dielectric resonator (7) at least partially absorbs spurious radiation generated in a predetermined mode of operation of the magnetron, such as the π - 1 mode. Power generated in the π -1 mode, if transmitted, may interfere with other electronic devices. The resonator (7) may be of ceramics material, such as alumina.