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. It was also used in professional test equipment such as Tektronix oscilloscopes.
The heater is rated for 12.6 volt use and is centre tapped.
The Hallicrafters name and Made in USA. Hallicrafters main products were radio equipment.
The beam tetrode construction with the two inner grids visible.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 57 mm tall.
References: Data-sheet, 1040 & Henning Hertz. Type 12BY7A was first introduced in 1955. See also1955 adverts.