The ZD2 is described by Triotron as: an output stage in two or three valve sets or first stage low frequency if followed by transformer coupled UD2. Triotron also describe the ZD2 in their 1931 booklet as a super power valve. A slightly improved version was issued in 1932.
The base has the reference number 3287 moulded into it.
The brass pins have a single split in the open end. The lead out wire is attached within the base. The anode is an open U shape with ridges pressed into the bright metal to give rigidity. The filament top supports are held in a glass bead, seen midway along the anode.
The envelope is mainly covered internally by the silvering from the gettering process but the internal structure can be seen by looking upwards from the base. The anode support is on the right and can be seen to be welded to the anode near the bottom of the U section.
The filament has two vertical strands in parallel that each pass through their own helical grid. Both grids are attached to a single vertical support. With the angle of the picture it is not clear that the grid support is separate from the lower support for the oxide covered filaments.
The balloon envelope is 44 mm in diameter, and excluding the B4 base pins is 91 mm tall.
References: Datasheet & 1043. Type ZD2 was first introduced in 1927.