The 6SF5 is a high-mu single triode vacuum tube designed for high-gain audio voltage amplifier stages and detector circuits in radio receivers. Units are available in both new old stock (NOS) and tested used conditions. Manufactured by companies including RCA, Sylvania, GE, and Raytheon, these tubes feature a 6.3V filament rating and an octal base design in a metal envelope without a top grid cap. Used for servicing vintage radio chassis, early phono preamplifiers, and audio equipment requiring high voltage amplification with grid-to-ground construction.