The 6C4 is a 7-pin miniature medium-mu single triode designed for high-frequency oscillator, phase inverter, and RF amplifier applications in radio receivers, transmitters, and audio equipment. Introduced in the 1940s, it functions electrically as half of a 12AU7 twin triode within a compact 7-pin envelope. Its low interelectrode capacitance and dependable high-frequency performance make it a common choice for local oscillator stages in VHF equipment, as well as driver and phase-splitter stages in vintage audio power amplifiers. Operates on a 6.3V heater and fits standard 7-pin miniature sockets. European equivalent is EC90. Available NOS and tested used.