The gun comprises a modular beam-forming region (BFR) assembly (12) and a main focusing lens (22) affixed to two insulative support rods or beads (14). Ihe BFR assembly includes a plurality of cathode assemblies (16), and control (18) and screen (20) grid electrodes having aligned apertures (60, 62) therethrough for passage of a plurality of electron beams from the cathode assemblies. The cathode assemblies and the electrodes are individually held in position from a common ceramic member (50) having a first major surface (52) and an oppositely disposed second major surface (54), with a metallized pattern (56a,56b,56c) formed on at least a portion of each major surface. The control and screen grid electrodes are attached to the first major surface, and the cathode assemblies are attached to the second major surface. A transition member (66), disposed between the metallized pattern (56a,56b) on the first major surface and the screen grid electrode, includes a flat portion (74) attached to the metallized pattern (56b) and two upright portions (76) perpendicular to the flat portion and parallel to each other. The screen grid electrode comprises at least one plate-like member (20a,20b,20c) disposed between the upright portions and connected thereto by a plurality of step-like support members (82) each including a screen grid electrode contact portion (84), a bead support contact portion (86) and a central riser portion (88) extending between the contact portions. The screen grid electrode is longitudinally spaced from the control grid electrode, and the central riser portion is laterally positioned and attached to the upright portions of the transition member. The bead support contact portions are attached to respective bead support members (15) affixed to the insulative support rods, whereby the screen grid electrode is longitudinally spaced from the main focusing lens.