The 12BY7A is a 1950s video amplifier output valve designed for boosting wideband TV video signals to a sufficiently high level to be transmitted over a considerable length of cable without loss of quality.
The heater is rated for 12.6 volt use and is centre tapped. Makers describe the 12BY7A as a pentode but the construction is as a beam tetrode.
Henning Hertz comments that the 12BY7A was widely ised by radio amateurs in the 1960's and 1970's as a driver stage for a pair of 6146B tetrodes in the PA (final stage) of HF transmitters up to 30 MHz.
The anode is in two active regions that are joined by two straps. The bright central box is the beam forming plate and the screen grid can easily be seen within. The control grid also passes flat across the cathode and at a very close distance to it as is required for a high gm.
The anode face view with the joining straps to the right.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 60 mm tall.
References: Datasheet, 1040 & Henning Hertz. Type 12BY7A was first introduced in 1955.