The 12AU7 is a double triode and this exhibit the 12AU7A is a special quality version of that design specification. Probably a rugged version for operation in a vibrating environment.
An amplification factor of 17 is quoted, the same as the 12AU7.
The working face of the anode is in the centre of the image. The end of one grid support can be seen. The outer portions of the anode plates, outside the stitches, are present to dissipate the 2.75 Watts of heat that the valve is rated for.
Here the plane of the grids is left to right.
The thin glass tube envelope is 21 mm in diameter and the length excluding the B9A base pins, is 47 mm.
Reference: Data-sheet. Type 12AU7A was first introduced in 1956. See also1956 adverts.
