12AX7 is a miniature dual triode vacuum tube with high voltage gain. It is believed to have been originally developed in 1946 by RCA engineers in Harrison, 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 a miniature form-factor follow-on to the 6SL7 family of dual-triode amplifier tubes for audio applications. Its ongoing wide use in guitar amplifiers (see valve sound) has caused it to be one of the very few small-signal vacuum tubes to continue in production since it was introduced.
The initial "12" in the designator implies a 12-volt heater requirement; however, the tube has a center tapped filament so it can be used in either 6.3V@300mA or 12.6V@150mA heater circuits.
Currently, the 12AX7 is made in various versions by two factories in Russia (Winged C, formerly Svetlana, and New Sensor, which makes current production tubes under the Sovtek, Electro-Harmonix, Svetlana, Tung-Sol, and other brands for which the firm has acquired trademark rights), one in China (Shanguang), one in Slovakia (JJ), and one in Serbia (EI Niš), for a total annual production figure of 2 million units (estimated). The vast majority are used in new-production guitar amplifiers or for replacement purposes in guitar or audio equipment.
Typically a 12AX7 triode is configured with a high-value plate resistor, 100k ohms in most guitar amps and 220k ohms or more in high-fidelity equipment. Grid bias and some negative feedback are often provided by a cathode resistor. In this mode each half of a 12AX7 can provide a voltage gain of about 60. The cathode resistor can be bypassed to reduce AC negative feedback and thereby increase gain.
Some variants offered lower gain (traded off for higher plate current) than the 12AX7 (which had a voltage gain or of 100), for high-frequency applications. Others offered improved resistance to microphonics or were modified for specialized applications, such as digital computers.
Some American variants of the 12AX7:
Although commonly known in Europe by its Mullard-Philips tube designation of ECC83, other European variations also existed including the low-noise versions 12AX7A, 12AD7, 6681, 7025, and 7729; European versions B339, B759, CV492, CV4004, CV8156, CV8222, ECC803, ECC803S, E2164, and M8137; and the lower-gain low-noise versions 5751 and 6851, intended for avionics equipment.
The 'E' in the European designation explicitly classified this as having a 6.3 volt heater, whereas the American designation of 12AX7 explicitly classified it as having a 12.6 volt heater. It can, of course, be wired for either operation.
The 12AX7's popularity has made a complete cataloging of all manufactured variations impossible. In past decades, versions were known to be made in the USA, Canada, virtually every European country, Australia, Japan, India, Argentina, Chile, Brazil, China, USSR, Yugoslavia and possibly in other places. 12AX7s are still being made in Russia, China, Slovakia and Serbia.