12AU7 is a miniature nine-pin (B9A base) medium-gain dual triode vacuum tube. It belongs to a large family of dual triode vacuum tubes which share the same pinout (EIA 9A). 12AU7 is also known in Europe under its Mullard-Philips tube designation ECC82. There are many equivalent tubes with different names, some identical, some designed for ruggedness, long life, or other characteristics; examples are the US military 5814A and the European special-quality E82CC and E182CC.
The tube is popular in hi-fi vacuum tube audio as a low-noise line amplifier, driver and phase-inverter in vacuum tube push-pull amplifier circuits. There are opinions which indicate that this popularity is caused by audiophile industry/media polarization, not by its performance.  It was widely used, in special-quality versions such as E82CC and 5814A, in pre-semiconductor digital computer circuitry. Use of special-quality versions outside of the purpose they were designed for may not be optimal; for example, a version for digital computers may be designed for long life without cathode poisoning when mostly switched to low-current mode in switching applications, but with little attention to parameters of interest only for linear applications such as linearity of transfer characteristic, matching between the two sections, microphony, etc.
This tube is essentially two 6C4s or two EC90s in the same envelope.
Double triodes of the 12AU7 family have a center-tapped filament for use in either 6.3V 300mA or 12.6V 150mA heater circuits.
- Lynn Olson. Illusion Engines (2005)