6AU6
The 6AU6 (European equivalent EF94) is an RF pentode for up to ~50 MHz with a miniature B7G 7-pin base, introduced in 1946.
The 12AU6 is the equivalent version with a 12 V heater.
The 6AU6 is used in the 513, 581, 585 and others as an error amplifier, in the Type 53/54C plugin, and others.