A magnetron has an anode body (1) and including a ceramic sleeve (7). In higher power generators, stray radiation is emitted from this sleeve in addition to the main power launched from the antenna into the waveguide (2), and RF absorbing material is provided. Such absorbers, however, tend to be frequency-selective, and can overheat. According to the invention, a non-metallic jacket (13) containing a dielectric liquid such as water surrounds the sleeve. This provides absorption over a broad band of frequencies, and it is easy to make the jacket have a sufficiently high thermal capacity, for example, by arranging a flow of liquid through it.