An electron collector assembly for an electron beam tube comprises an electron collector (2), which is at least partially surrounded by dielectric material, such as a dielectric fluid (5). A heat pipe (6), in communication with the dielectric fluid, is provided. Heat energy from the collector is transferred to the dielectric fluid, and hence to the heat pipe. By pumping coolant through the heat pipe, heat energy is removed from the vicinity of the collector. Previous proposals for cooling electron collectors necessarily involved non-conducting fluids as coolants. The provision of a dielectric fluid as an intermediary between the collector and the heat pipe allows greater freedom in the choice of coolant.