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12AX7 is a miniature dual triode vacuum tube of high gain. It was believed to be originally developed in 1946 by RCA engineers in Harrison, New Jersey under developmental number A-4522. Release date for public sale under the 12AX7 identifier was September 15, 1947. 12AX7 was originally intended as a low-cost amplifier for audio applications. Its wide use in guitar amplifiers has caused it to be one of the very few small-signal vacuum tubes to continue in production since it was introduced.
The tube has a center tapped filament so it can be used as in 6.3V 300ma, or 12.6V 150ma heater applications. Other tubes with the same base include the 12AU7, 12AT7, and the fairly unknown 12AZ7. Of these the 12AX7 has the highest gain which is typically 100.
Hundreds of different variations of the 12AX7 have been manufactured all over the world. Variations include the low-noise versions 12AX7A, 12AD7, 6681, 7025, and 7729; European versions B339, B759, CV492, CV4004, CV8156, CV8222, ECC83, ECC803, ECC803S, E2164, and M8137; and the lower-gain low-noise versions 5751 and 6851, intended for avionics equipment. Its 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, and possibly in other places.
Presently the 12AX7 is made in various versions by two factories in Russia, one in China, one in Slovakia, and one in Serbia, 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.
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