(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