12AX7

12AX7 is a miniature dual triode vacuum tube with high voltage gain. It is believed to have been originally developed in 1946 by RCA engineers 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 a miniature form-factor follow-on to the 6SL7 family of dual-triode amplifier tubes for audio applications. Its ongoing wide use in guitar amplifiers (see valve sound) has caused it to be one of the very few small-signal vacuum tubes to continue in production since it was introduced.

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.