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. It is used in the 513, 581, 585 and others as an error amplifier.
Links
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6AU6A and 12AU6