The 3C4 (also designated as the 2G21) is a 7-pin miniature dual-shielded triode-heptode converter vacuum tube designed specifically for low-drain, battery-operated portable AM and shortwave radio receivers. Featuring a 2.8V filament rating with a center-tap option for 1.4V operation, it combines a local oscillator triode and a heptode mixer in a single envelope for efficient frequency conversion in compact radio designs. Available in new old stock (NOS) and tested used conditions.