6922 / 6DJ8 / ECC88 / E88CC / 6N23P / 6N11 A miniature nine-pin medium-gain dual triode with very high transconductance. First developed by Amperex / Philips for use as an amplifier in TV tuners, and later used in high-quality oscilloscopes.

The 12AX7 is typically used as a class-A amplifier in the hi-fi, professional line and microphone preamps and is usually, always found in the preamp sections of tube guitar amps including Fender, Marshall, Boogie and Soldano.

Lee De Forest invented the audion, a vacuum tube device that could take a weak electrical signal and amplify it into a larger one. The audion helped AT&T set up coast-to-coast phone service, and it was also used in everything from radios to televisions to the first computers.

The 6J6 is a miniature medium-mu twin triode designed for use as a radio- frequency amplifier, oscillator, or mixer. With the grids connected in push-pull and the plates connected in parallel, each type may be used as a mixer at frequencies as high as 600 megacycles.