12AX7 is a miniature dual triode vacuum tube of high gain. It is believed to have been 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 miniature form-factor follow-on to the 6SN7 family of dual-triode low-cost amplifier tubes for audio and computer applications. Its 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 12AX7 is the most common member of a huge family of miniature twin-triode vacuum tubes.

The tube has a center tapped filament so it can be used in either 6.3V 300ma or 12.6V 150ma heater circuits.

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.

Twin-triode variations

The 12AX7 was the most common member of what eventually became a huge family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout (EIA 9A) Most used a 150 Ma heater which could also be connected to run on 6.3V at 300 Ma

Some variants offered lower gain than the 12AX7 (which had a voltage gain of about 100); others offered improved resistance to microphonics.

Some American variants of the 12AX7:

• 12AD7 (october 10, 1955 - 225ma heater - low hum)
• 12AT7 (May 20, 1947, dual 6AB4 ɥ62 - gain of 60)
• 12AU7 (October 18, 1946 dual 6C4 ɥ17 gain of 19)
• 12AV7 (February 14, 1950 - dual 6BC4 ɥ48 gain of 41)
• 12AX7 (September 25, 1947 - minature 6SL7 ɥ100 gain of 100)
• 12AY7 (december 7 1948 - ɥ48 gain of 45 for audio peramp use)
• 12AZ7 (March 2, 1951 225ma heater, ɥ 60)
• 12DT7 (ɥ100)
• 12DF7 (ɥ100 low microphonics)

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 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, and possibly in other places.

References

• Information came from the RCA Archive in Gold Hill, OR, courtesy of Ludwell Sibley of the Tube Collector's Association, from the GE ESSENTIAL CHARACTERISTICS tube manual of 1973, and from personal files.
• http://www.tubecollectors.org Tube Collector's Association website
• http://www.mif.pg.gda.pl/homepages/frank/sheets/137/1/12AX7.pdf Original Sylvania data sheet for 12AX7.

Vacuum tubes | Guitar amplification tubes

ECC83 | ECC83