The 6F5 and 6F5GT are high-mu triode vacuum tubes introduced in the late 1930s, utilized primarily as high-gain voltage amplifiers in audio preamplifier circuits and radio receivers. The metal 6F5 features an integrated metallic envelope with an external top control grid cap for shielding, while the 6F5GT uses a tubular glass envelope maintaining the same internal structure and top-cap configuration. Both operate on a 6.3V heater and fit standard 8-pin octal sockets. Available NOS and tested used.