An electron gun for use in electron irradiation equipment. The gun comprises a cartridge including a Wehnelt electrode and a filament with two plug electrodes mounted on an insulating plate. The filament is mounted between the Wehnelt electrode and a conductive plate by two fasteners and can be adjustably aligned with respect to the Wehnelt electrode by moving the electrode with respect to the filament before the mounting is made fast. The electrical potential supply terminal has two jacks which receive the plug electrodes thereby mechanically and electrically coupling the cartridge to the potential supply and the Wehnelt electrode receives its potential through a resilient contact on the potential supply terminal.