6R7

Duodiode-Medium-Mu Triode Multi-Unit Tube, integrating two independent signal diodes sharing a common cathode with a medium-gain triode voltage amplifier. Operating from a 6.3V heater, it serves as the complete AM detector, automatic volume control (AVC) loop, and primary audio pre-amplifier in early console radios, housed in a standard large 8-pin Octal-style base glass or metal envelope.