The 12AU7 is a standard low impedance double triode with separate cathodes. An amplification factor of 17 is quoted. The 6C4 is half of the 12AU7.
The American designation of 12AU7 is identical to the European ECC82 specification and here the valve carries both names.
The two small triodes with central screen.
Each triode is rated at 2.5 W anode dissipation and additional heat dissipation is provided by the side flanges of the anode. The control grid is wound on copper supports for better heat transportation.
The thin glass tube envelope is 21 mm in diameter and the length excluding the B9A base pins, is 57 mm.
References: Datasheet & 1040. Type 12AU7 was first introduced in 1951.