The 81 is a classic 4-pin UX-base high-voltage half-wave power rectifier vacuum tube introduced in the late 1920s. Engineered for heavy-duty power supplies in large vintage radio receivers, public address amplifiers, and industrial equipment, it converts high-voltage AC into DC power. Featuring a robust, directly heated filament, two tubes are commonly used in tandem for full-wave rectification circuits. It operates on a 7.5V filament supply and fits standard 4-pin sockets. Available NOS and tested used.