Editing 12AX7 (section)

==Similar twin-triode designs==
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). Most use heaters which can be optionally wired in series (12.6V, 150&nbsp;mA) or parallel (6.3V, 300&nbsp;mA).  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.

Those other designs offer lower voltage gain (traded off for higher plate current) than the 12AX7 (which has a voltage gain or <math>A_v</math> of 100), and are more suitable for high-frequency applications.

Some American designs similar to the 12AX7:

* 12AD7 (October 10, 1955 - 225mA heater - low hum)
* [[12AT7]]<ref>killerrig.com,{{cite web |url=https://killerrig.com/12ax7-vs-12at7/ |title = 12AX7 vs 12AT7 Tube Types| date=13 April 2022 }}</ref> (May 20, 1947, dual 6AB4, <math>A_v</math> = 60)
* [[12AU7]] (October 18, 1946, dual 6C4,<ref>{{Cite web|url=https://www.thetubestore.com/preamp-tubes/6c4-m8080-tube-types|title=www.thetubestore.com - Tubes with part numbers beginning with 6C4 - M8080 Tube|website=www.thetubestore.com}}</ref> <math>A_v</math> = 17-20)
* 12AV7 (February 14, 1950 - dual 6BC4, <math>A_v</math> = 37-41)
* '''12AX7''' (September 15, 1947 - dual 6DR4, also like octal 6SL7, <math>A_v</math> = 100) dual 12AV6 (6AV6)
* 12AY7 (December 7, 1948 - <math>A_v</math> = 44, for audio preamp use)<ref>{{Cite web|url=https://www.thetubestore.com/preamp-tubes/12ay7-6072-tube-types|title=www.thetubestore.com - Tubes with part numbers beginning with 12AY7 - 6072 Tube|website=www.thetubestore.com}}</ref>
* 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)
* 12DW7 <ref>{{Cite web|url=https://www.thetubestore.com/preamp-tubes/12dw7-7247-tube-types|title=www.thetubestore.com - Tubes with part numbers beginning with 12DW7 - 7247 Tube|website=www.thetubestore.com}}</ref>(First triode: <math>A_v</math> = 100, Second triode: <math>A_v</math> = 17)
* 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 exist 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.

In European usage special-quality valves of some sort were often indicated by exchanging letters and digits in the name: the E83CC was a special-quality ECC83.
In the US a "W" in the designation, as in 12AX7WA, designates the tube as complying with military grade, higher reliability specifications.

The 'E' in the European designation classifies this as having a 6.3 volt heater, whereas the American designation of 12AX7 classifies it as having a 12.6 volt heater.  It can, of course, be wired for operation off either voltage.