An electron gun assembly includes a cathode 1, control grid 5 located in front of it and a focus electrode 6. The control electrode 5 is integral with a cylindrical support 11 and formed of pyrolytic graphite. Similarly, the focus electrode 6 is integral with a cylindrical support 16 and is formed of molybdenum. The control grid 5 and focus electrode 6 are mounted on a common mount 15 located behind the front face of the cathode 1, giving a simple structure having low thermally capacity and being mechanically robust.