6SN7

The 6SN7 is a dual triode vacuum tube developed by RCA in the 1940s. It contains two independent triode sections sharing a common cathode and is designed for voltage amplification in audio gear. It uses an eight-pin octal base and is commonly found in glass-envelope variants designated 6SN7GT; metal versions were also produced by some manufacturers.

In audio applications, the 6SN7 is valued for its relatively low noise, high input impedance, and linear response, making it a popular choice for preamplifier stages, phono stages, and early hi-fi line stages. It has also seen use in guitar amplifiers for clean or driver stages. Typical operation involves a 6.3 V heater and plate voltages in the hundreds of volts, with each triode having its own grid, plate, and cathode connections.

Characteristics vary by manufacturer, but the tube is a medium- to high-gain dual triode with high plate resistance and a mellow, well-controlled sonic character in many audio circuits. It is not a direct drop-in replacement for 9-pin miniature dual triodes such as the 12AX7 or 12AU7 due to differences in base, pinout, and heater requirements.

Variants and legacy: 6SN7GT refers to the glass-envelope version; other envelope types exist from several producers. Although production declined with the rise of miniature tubes, the 6SN7 remains in production by specialty manufacturers and is widely available on the vintage market, retained for audiophile restoration and guitar-amp repair.