The Type 45 is a directly heated audio output triode. This exhibit is from a late production run and is housed in a classic envelope. See also45 for the original balloon production.
The side view shows the box construction and as is typical with this type of valve with a wire filament the width is much greater than the depth. The rods at the right hand side are the supports for the filament tension spring.
The production of the 45 ended in around 1932 but has recently been put back in production as it has a well deserved reputation for good audio fidelity.
The top of the envelope. The straps visible at the top and bottom of the picture are the top mica fixings to the anode. The serrated edges of the mica cross are to enable an interference fit to the envelope. The hole in the mica reveals the filament and grid.
In close-up the filament can be seen as two inverted V's of oxide coated ribbon held by the tension springs.
The classic envelope is 45 mm in diameter, and excluding the UX4 base pins is 102 mm tall.
References: Data-sheet & 3002. Type 45 was first introduced in 1929.