Used in many guitar amplifiers around the world. Predominantly used as a phase inverter.
12AT7 (also known in Europe by the Mullard–Philips tube designation of ECC81) is a miniature 9-pin medium-gain (60) dual-triode vacuum tube popular in guitar amplifiers. It belongs to a large family of dual triode vacuum tubes which share the same pinout (EIA 9A), including in particular the very commonly used low-mu 12AU7 and high-mu 12AX7.
The 12AT7 has somewhat lower voltage gain than the 12AX7, but higher transconductance and plate current, which makes it suitable for high frequency applications.
Originally the tube was intended for operation in VHF circuits, such as TV sets and FM tuners, as an oscillator/frequency converter, but it also found wide use in audio as a driver and phase-inverter in vacuum tube push–pull amplifier circuits.
This tube is essentially two 6AB4s or two EC92s in a single envelope.
The tube has a center-tapped filament so it can be used in either 6.3V 300mA or 12.6V 150mA heater circuits." - Taken from Wikipedia