12AX7

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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 6SL7 family of dual-triode low-cost amplifier tubes for audio 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.

12AX7 (1947)
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12AX7 (1947)

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

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[edit] History

The 12AX7 is basically two 6AV6 triodes in one glass package. The 6AV6 was a repackaging of the triode from the octal 6SQ7, which was very similar to the older type 75 triode, which goes back to the mid 1930's.

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.

[edit] Application

The 12AX7 is a high-gain, low plate current triode. As such it is best suited for low-level audio amplification. In this role it was and is widely used for the input and mid-level stages of audio amplifiers. With its low plate current, it is not suitable for anything much above audio frequencies.

But as a high-gain audio amplifier, it is very good. Typically it is configured with a high-value plate resistor, in the range of 100,000 to 1,000,000 ohms. 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 50 to 80.

[edit] 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. The variations include the 12AT7, 12AU7, 12AV7, and the low-voltage 12U7, plus many 5XXX series dual triodes too numerous to mention. The variations span a wide range of voltage gain, ruggedness, and transconductance.

Image:EIA-9A.png


Some variants offered lower gain (traded off for higher plate current) 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 - miniature 6SL7 ɥ100 gain of 100)
  • 12AY7 (December 7 1948 - ɥ48 gain of 45 for audio preamp 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. They are still being made in Russia, China and Yugoslavia.

[edit] References

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