RCA 2D21 Electron Tube

The RCA 2D21 is a miniature glass thyratron (gas‑filled tetrode) vacuum tube designed as a high‑speed switch for relay and grid‑controlled rectifier service. It uses an indirectly heated cathode and a miniature 7‑pin base, packing four electrodes (anode, cathode, control grid, and starter/trigger element) in a compact envelope. Typical operating characteristics cited in historical catalogs include 100 mA average current, 500 mA maximum peak current, and a 10 A surge rating, enabling it to switch load currents in timing and sequencing circuits. The device found use in jukebox systems and early computer equipment, and RCA marketed it alongside other manufacturers’ equivalents under the same 2D21 designation.