The article discusses the specifications and pinout of the electronic vacuum tube from the 12AX7 series. According to the description, it is a small-sized low-frequency double triode. Despite the rather high linear distortion and reduced parameters relative to its predecessor 6SL7, in the 50-60s of the last century it was widely used in a variety of analog Western audio equipment.
In European countries, along with its full counterpart ECC83, 12AX7 is very often found in audio equipment released in the United States. It was mainly used in electric guitar amplifiers, including high-end ones. Against the background of other radio tubes, it stood out for its rather low transconductance, increased voltage gain, warm and sonorous transparent sound.
12AX7 Pinout
The device is made in a vacuum glass 9-pin finger package, 12AX7 pinout and pin assignment are shown in the figure. For installation on the board, a B9A 9-pin tubular base is used. The operation of the radio tube is possible both in vertical and horizontal position.
12AX7 Pinout by pin numbers:
- Anode Triode Number 2;
- Grid Triode Number 2;
- Cathode Triode Number 2;
- Heater (Ttiode 2);
- Heater (Ttiode 1);
- Anode Triode Number 1;
- Grid Triode Number 1;
- Cathode Triode Number 1;
- Heater Center tap.
Specifications
The specifications of the 12AX7 tube, namely the very high low-frequency gain, allowed it to become one of the most popular and recognizable tubes used in analog equipment in the 50s and 60s. At that time, it was she who began to be widely used in inexpensive record players, guitar amplifiers. Since 1956, it has been de facto considered the industry standard in the production of audio equipment in the United States.
Specifications 12AX7:
- Filament voltage, V
- for parallel connection – min 6, max 6.6;
- Plate voltage, V – max 330;
- Cathode – heater voltage, V ± 200;
- Cathode current, mA max 9;
- Power dissipation at the plate of each triode, W max 1.2;
- Grid circuit resistance for each of the triodes, MΩ
- fixed bias – max 1,0;
- self – bias max 2.2.
Exceeding the values of the above parameters leads to the failure of the device.
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Electrical parameters 12AX7 |
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Parameters, conditions and units |
Nominal |
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min |
max |
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Heater current,mA |
350 175 |
390 195 |
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Grid reverse current, , ( at: filament voltage 6.3 V or 12.6 V, plate voltage 250 V, grid voltage minus 2.0 V, resistance in grid circuit 1.0 MΩ ) |
– |
0.2 |
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First and second triodes plate current difference, % ( at: filament voltage 6.3 V or 12.6 V, plate voltage 250 V, grid voltage minus 2.0 V) |
– |
± 18 |
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Plate current at the beginning of the characteristic µA ( at: filament voltage 6.3 V or 12.6 V, plate voltage 250 V, grid voltage minus 4.5 V) |
– |
30 |
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Slope of characteristic, mA/V ( at: filament voltage 6.3 V or 12.6 V, plate voltage 250 V, grid voltage minus 2.0 V) |
1.4 |
– |
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Amplification factor ( at: filament voltage 6.3 V or 12.6 V, plate voltage 250 V, grid voltage minus 2.0 V) |
83 |
– |
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Cathode – heater insulation resistance, MΩ ( at: filament voltage 6.3 V or 12.6 V, cathode -heater voltage ± 200 V) |
20 |
– |
12AX7 Equivalents
The efforts of many manufacturers to develop an acceptable equivalent for the 12AX7 tube have paid off. In 1952, Philips began production of the almost identical ECC83 series. In 1958, the engineers of the German “Telefunken” developed the ECC803S, in which the microphone effect was almost completely leveled.
It is known that in China in the 80s, full copies marked CV4004 were produced on the equipment of the American Mullard.
As functional equivalents, 12AT7 and 12AU7 radio tubes can be recommended, in which the gain is much lower – by 40% and 80%, respectively.
Manufacturers
The first versions of the 12AX7 vacuum tube appeared back in 1940, and were included in the technical specifications of the RCA developer company in 1948. Subsequently, production was established at the factories of the American company Sylvania Electric Products. The design of the device has improved over time. Until the end of 1970, the American companies Tung-Sol and CBS remained the main manufacturers. Along with the decline in demand for these vacuum tubes, these companies have gradually phased out production.
Until about 1990, mostly full clones of the Hungarian “Tungsram”, the Chinese “Shuguang” were distributed on the market, and old stocks continued to be sold. Also, outright fakes with fake labels “Mullard” and “Telefunken” were sold in large quantities, which have nothing to do with these brands. Since 1995, copies of similar parameters began to be made at Chinese (Sino), Serbian (Electronics Industry), Slovak (JJ Electronic), Czech (Tesla) enterprises. Currently, their release is almost stopped, so they are extremely rare.
In the European market, the tube in question is now known under a different designation – ECC803. These are products of the Slovak manufacturer of electronic comмponents “JJ Electronic”. On the body of the product there is a typical for the company marking “JJ”. It is believed that in terms of gain and frequency range it is very good, but in the dynamic range it is slightly inferior to analogues from other companies. The price is quite attractive and affordable, up to about 30$ for 1 piece.
In this article, we reviewed the specifications of the 12AX7 vacuum tube, its pinout. As well as the history of its appearance and manufacturers with their markings.
You can download Datasheet for 12AX7 in a special section by clicking on the name.
