12AX7

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File:12AX7.jpg
RCA 12AX7A gray plate tube on right

The 12AX7 is a miniature dual triode vacuum tube with high voltage gain. It is believed to have been originally developed in 1946 by RCA engineers in Harrison, New Jersey under developmental number A-4522. It was released for public sale under the 12AX7 identifier on September 15, 1947. The 12AX7 was originally intended as a miniature form-factor follow-on to the 6SL7 family of dual-triode amplifier tubes for audio applications. Its ongoing 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 initial "12" in the designator implies a 12-volt heater requirement; however, the tube has a center tapped filament so it can be used in either 6.3V or 12.6V heater circuits.

History[edit]

The 12AX7 is basically two 6AV6 triodes in one package. The 6AV6 was a repackaging of the triode from the octal 6SQ7, which was very similar to the older type 75 triode-diode, dating from 1930.

Currently, the 12AX7 is made in various versions by two factories in Russia (Winged C, formerly Svetlana, and New Sensor, which makes current production tubes under the Sovtek, Electro-Harmonix, Svetlana, Tung-Sol, and other brands for which the firm has acquired trademark rights), one in China (Shuguang), one in Slovakia (JJ), and one in Serbia (EI Niš), 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.

Application[edit]

The 12AX7 is a high-gain (typical gain factor 100), low plate current triode. It is therefore best suited for low-level audio amplification. In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers. With its high Miller capacitance, it is not suitable for radio-frequency use.

Typically a 12AX7 triode is configured with a high-value plate resistor, 100k ohms in most guitar amps and 220k ohms or more in high-fidelity equipment. Grid bias is most often provided by a cathode resistor. If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12AX7 provides a typical voltage gain of about 60. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain.

Similar twin-triode designs[edit]

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-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.

See also[edit]

References[edit]

  • 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.

External links[edit]