12AX7
Encyclopedia
12AX7 is a miniature dual triode
vacuum tube
with high voltage gain
. It was developed around 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 replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications. It is popular with tube amplifier enthusiasts, and its ongoing use in such equipment makes it one of the few small-signal vacuum tubes in continuous production since it was introduced.
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), for a total annual production figure of 2 million units (estimated). The vast majority are used in new-production guitar amplifier
s or for replacement purposes in guitar
or audio equipment.
, and 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 30; the amplification factor is basically twice the maximum stage gain, as you have to impedance match the plate. Thus half the voltage is across the tube at rest, half in the load resistor. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain. Maximum gain approaches 60 in these tubes.
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.
, 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
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:
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.
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
with high voltage gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
. It was developed around 1946 by RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
engineers in Harrison, New Jersey
Harrison, New Jersey
Harrison is a town in Hudson County, New Jersey, United States. As of the 2010 United States Census, the town's population was 13,620. The town is a suburb of the nearby city of Newark, New Jersey.-Geography:Harrison is located at ....
, 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 replacement for the 6SL7 family of dual-triode amplifier tubes for audio applications. It is popular with tube amplifier enthusiasts, and its ongoing use in such equipment makes it one of the few small-signal vacuum tubes in continuous production since it was introduced.
History
The 12AX7 is basically two 6AV6 triodes in one package. The 6AV6 was a miniature repackaging (with just a single cathode) of the triode and twin diodes from the octal 6SQ7 (a double-diode triode used in AM radios), which itself 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
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
(Winged C, formerly Svetlana, and New Sensor
Sovtek
Sovtek is a brand of vacuum tube owned by Mike Matthews's New Sensor Corporation and manufactured in Saratov, Russia. They are often used in guitar amplification and include the popular variants of the 12AX7, EL84, EL34, and 6L6 models...
, which makes current production tubes under the Sovtek, Electro-Harmonix, Svetlana, Tung-Sol
Tung-Sol
Tung-Sol was a manufacturer of lamps and vacuum tubes in Newark, New Jersey.Tung-Sol developed the first successful car headlight in 1907 and the first two filament high and low beam headlight in a single bulb in 1913. Tung-Sol was also responsible for the first flashing turn signal.In the 1920s...
, and other brands for which the firm has acquired trademark rights), one in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
(Shuguang), one in Slovakia
Slovakia
The Slovak Republic is a landlocked state in Central Europe. It has a population of over five million and an area of about . Slovakia is bordered by the Czech Republic and Austria to the west, Poland to the north, Ukraine to the east and Hungary to the south...
(JJ), for a total annual production figure of 2 million units (estimated). The vast majority are used in new-production guitar amplifier
Guitar amplifier
A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...
s or for replacement purposes in guitar
Guitar
The guitar is a plucked string instrument, usually played with fingers or a pick. The guitar consists of a body with a rigid neck to which the strings, generally six in number, are attached. Guitars are traditionally constructed of various woods and strung with animal gut or, more recently, with...
or audio equipment.
Application
The 12AX7 is a high-gain (typical amplification factor 100), low plate current triodeTriode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
, and 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
Miller effect
In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals...
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 30; the amplification factor is basically twice the maximum stage gain, as you have to impedance match the plate. Thus half the voltage is across the tube at rest, half in the load resistor. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain. Maximum gain approaches 60 in these tubes.
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.
Similar twin-triode designs
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 12AT712AT7
12AT7 is a miniature 9-pin medium-gain dual triode vacuum tube popular in guitar amplifiers. It belongs to a large family of dual triode vacuum tubes which share the same pinout and of which 12AX7 is the most prolific type....
, 12AU7
12AU7
12AU7 is a miniature nine-pin medium-gain dual triode vacuum tube. It belongs to a large family of dual triode vacuum tubes which share the same pinout . 12AU7 is also known in Europe under its Mullard-Philips tube designation ECC82...
, 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
of 100), for high-frequency applications. Others offered improved resistance to microphonicsMicrophonics
Microphonics describes the phenomenon where certain components in electronic devices transform mechanical vibrations into an undesired electrical signal...
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)
- 12AT712AT712AT7 is a miniature 9-pin medium-gain dual triode vacuum tube popular in guitar amplifiers. It belongs to a large family of dual triode vacuum tubes which share the same pinout and of which 12AX7 is the most prolific type....
(May 20, 1947, dual 6AB4,
= 60) - 12AU712AU712AU7 is a miniature nine-pin medium-gain dual triode vacuum tube. It belongs to a large family of dual triode vacuum tubes which share the same pinout . 12AU7 is also known in Europe under its Mullard-Philips tube designation ECC82...
(October 18, 1946 dual 6C4,
= 17-20) - 12AV7 (February 14, 1950 - dual 6BC4,
= 37-41) - 12AX7 (September 15, 1947 - like miniature 6SL7,
= 100) - 12AY7 (December 7, 1948 -
= 44, for audio preamp use) - 12AZ7 (March 2, 1951 - 225mA heater,
= 60) - 12DF7 (
= 100, low microphonics) - 12DT7 (
= 100) - 12U7 (
= 20, for use in automotive radios on 12-volt plate supply)
Although commonly known in Europe by its Mullard-Philips tube designation
Mullard-Philips tube designation
In Europe, the principal method of numbering vacuum tubes was the nomenclature developed and used by Mullard in the UK and applied Europe-wide thanks to their parentage by Philips who had subsidiaries in Germany and France , plus the support of Telefunken, from 1934...
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.