12AX7 (also known as ECC83[1]) is a miniature dual-triode vacuum tube with high voltage gain.

Developed around 1946 by RCA engineers[2] in Camden, New Jersey, under developmental number A-4522, it was released for public sale under the 12AX7 identifier on September 15, 1947.

The 12AX7 was originally intended as replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications.

The 6AV6 is a miniature repackaging (with just a single cathode) of the triode and twin diodes from the octal 6SQ7 (a double-diode triode used in AM radios), which itself is very similar to the older type 75 triode-diode dating from 1930.

In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers.

It has relatively high Miller capacitance, making it unsuitable for radio-frequency use.

Typically a 12AX7 triode is configured with a high-value plate resistor, 100 kohms in most guitar amps and 220 kΩ or more in high-fidelity equipment.

If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12AX7 provides a typical voltage gain of about 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched.

The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times with a 100k plate load, and a center biased and bypassed cathode, and higher with a larger plate load.

The 12AX7 is the most common member of what eventually became a large family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout (EIA 9A).

Other tubes, which in some cases can be used interchangeably in an emergency or for different performance characteristics, include the 12AT7, 12AU7, 12AV7, 12AY7, and the low-voltage 12U7, plus many four-digit EIA series dual triodes.

They span a wide range of voltage gain and transconductance.

Different versions of each were designed for enhanced ruggedness, low microphonics, stability, lifespan, etc.