Abstract: An electron gun having a stacked construction is provided, comprising a cylindrical shell, a cathode concentrically disposed within the shell, and at least one spacing ring disposed within the shell in abutting engagement with the cathode. The cathode is electrically insulated from the shell by the spacing ring, which is comprised of an electrically insulative material. The cathode and spacing ring are held together within the shell by simple mechanical force, such that the cathode and spacing ring can be readily detached from the shell as desired. An additional electrode, such as an anode or grid, may also be concentrically disposed within the shell, with a second spacing ring disposed in abutting engagement with the cathode and the electrode. A third spacing ring may also be disposed between the electrode and a shoulder portion of the shell disposed at an end thereof.