12AX7
| 12AX7 | ||
|---|---|---|
RCA 12AX7 |
||
| Classification: | Dual Triode | |
| Service: | Class A amplifier | |
| Height: | 2.2 in (56 mm) | |
| Diameter: | 0.8 in (20 mm) | |
| Cathode | ||
| Cathode type: | Indirectly heated | |
| Heater voltage: | Series / Parallel 12.6 / 6.3 |
|
| Heater current: | Series / Parallel 150 / 300 mA |
|
| Anode | ||
| Max dissipation Watts: | 1 per section | |
| Max voltage: | 300 | |
| Socket connections | ||
| Typical class A amplifier operation | ||
| Amplification factor: | 100 | |
| Anode voltage: | 250 | |
| Anode current: | 1.2 mA | |
| Bias voltage: | -2 | |
| Anode resistance: | 62500 | |
| Reference | ||
| Sylvania 12AX7 datasheet | ||
12AX7 is a miniature dual triode vacuum tube with high voltage gain. It was developed around 1946 by RCA engineers[1] 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.
Contents |
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 (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 amplifiers or for replacement purposes in guitar or audio equipment.
Application
The 12AX7 is a high-gain (typical amplification factor 100), low plate current triode, 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.
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 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 
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, = 60)
- 12AU7 (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 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.
See also
References
- ^ Jung, David (September 2010). "Draw! A 12AX7 'Shooutout'". Vintage Guitar: 114.