Editing 12AX7 (section)

== Similar twin-triode designs ==
[[File:Vintage Telefunken 12AX7 (ECC83) Vacuum Tubes, Made In Western Germany (15822838905).jpg|thumb|Telefunken ECC83 Tubes, Made in West Germany]]
The 12AX7 was 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). Most used heaters which could be optionally wired in series (12.6V) or parallel (6.3V), with respective current requirements of 150 mA or 300 mA.  Other tubes, which in some cases could be interchangeable, include the [[12AT7]], [[12AU7]], [[12AV7]], and the low-voltage [[12U7]], plus many 4-digit EIA series dual triodes.  They span a wide range of voltage gain, ruggedness, and transconductance.  

Those other designs offered lower gain (traded off for higher plate current) than the 12AX7 (which had a voltage gain or <math>A_v</math> of 100), for high-frequency applications. Others offered improved resistance to [[microphonics]] or were modified for specialized applications, such as [[digital computers]].

Some American designs similar to the 12AX7:

* 12AD7 (October 10, 1955 - 225mA heater - low hum)
* [[12AT7]] (May 20, 1947, dual 6AB4, <math>A_v</math> = 60)
* [[12AU7]] (October 18, 1946 dual 6C4, <math>A_v</math> = 17-20)
* 12AV7 (February 14, 1950 - dual 6BC4, <math>A_v</math> = 37-41)
* '''12AX7''' (September 15, 1947 - like miniature 6SL7, <math>A_v</math> = 100)
* [[12AY7]] (December 7, 1948 - <math>A_v</math> = 44, for audio preamp use)
* 12AZ7 (March 2, 1951 - 225mA heater, <math>A_v</math> = 60)
* 12DF7 (<math>A_v</math> = 100, low microphonics)
* 12DT7 (<math>A_v</math> = 100)
* 12U7  (<math>A_v</math> = 20, for use in automotive radios on 12-volt plate supply)

Although commonly known in Europe by its [[Mullard-Philips tube designation]] of ''ECC83'', other European variations also existed including the low-[[signal noise|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.