The 5Z3 is a heavy-duty, directly heated full-wave vacuum power rectifier designed for high-current power supply service in early radio chassis, public address amplifiers, and industrial electronic equipment. Featuring a classic shoulder-type (ST-16) glass envelope and a 4-pin UX base, it operates on a 5.0V/3.0A filament. It is electrically equivalent to the later octal-based 5U4G and 5X4G, and was widely specified in 1930s and 1940s high-power radio receivers manufactured by RCA Victor, Zenith, Philco, and Crosley. Available in NOS and tested used conditions.