The 12BH7A is a modern variant on the original 1950s design of double triode. Originally the valve was designed for use in television vertical deflection stages as well as a Class A voltage amplifier.
The A variant of the 12BH7 has a defined warm-up time of 11 seconds.
The National Electronics lettering is supplementary to the original Type designation. The presence of the pattern of dots suggests that the manufacturer was General Electric.
The two long triodes with the grids visible through the holes in the anode sides.
The National Electronics logo.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 59 mm tall.
