The 12AX7 is a twin triode basically composed of two of the triodes from a 6AV6, a double diode triode. The 6AV6 is a miniature repackaging of the triode and twin diodes from the octal 6SQ7, which itself is very similar to the older type 75 triode-diode dating from 1930. the 12AX7 was made in various versions by one factory in Russia, at least two in China, and one in Slovakia, for a total annual production estimated at two million units. The vast majority are used in new-production guitar amplifiers or for replacements in guitar and audio equipment.
Application
The 12AX7 is a high-gain, low-plate-current triode best suited for low-level audio voltage amplification. In this role it is widely used for the preamplifier stages of audio amplifiers. It has relatively high Miller capacitance, making it unsuitable 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 the plate impedance must be matched. Thus half the voltage is across the tube at rest, half across the load resistor. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times with a 100k plate load, and a center biased and bypassed cathode, and higher with a larger plate load. = μ * Rtot / Where = voltage gain, μ is the amplification factor of the valve, rP is the internal plate resistance, RK is the cathode resistor and Rtot is the parallel combination of RP and Rload. If the cathode resistor is bypassed, use RK = 0. The initial "12" in the designator implies a 12-volt heater requirement; however, the tube has a center-tapped heater so it can be used in either 6.3V or 12.6V heater circuits, and in most current applications is used in 6.3V circuits.
Characteristics
Similar twin-triode designs
The 12AX7 is 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. Most use heaters which can be optionally wired in series or parallel. Other tubes, which in some cases can be used interchangeably in an emergency, include the 12AT7, 12AU7, 12AV7, 12AY7, and the low-voltage 12U7, plus many four-digit EIA series dual triodes. They span a wide range of voltage gain and transconductance. Different versions of each were designed for enhanced ruggedness, low microphonics, stability, lifespan, etc. Those other designs offer lower voltage gain than the 12AX7, and are more suitable for high-frequency applications. Some American designs similar to the 12AX7:
12AD7
12AT7
12AU7
12AV7
12AX7 dual 12AV6
12AY7
12AZ7
12DF7
12DT7
12DW7
12U7
Although commonly known in Europe by its Mullard–Philips tube designation of ECC83, other European variations also exist 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. In European usage special-quality valves of some sort were often indicated by exchanging letters and digits in the name: the E83CC was a special-quality ECC83. In the US a "W" in the designation, as in 12AX7WA, designates the tube as complying with military grade, higher reliability specifications. The 'E' in the European designation classifies this as having a 6.3 volt heater, whereas the American designation of 12AX7 classifies it as having a 12.6 volt heater. It can, of course, be wired for operation off either voltage.
