The HD22 is a battery double diode triode for receiver use. Marconi alone produced an HD21 that only differed from the HD22 in that it operated with a bias point of -1.5 Volts.
This exhibit has very feint lettering that outlines the M-OV ellipse but the centre is almost blank. A slight hint of an H is all I could find. Time was then spent looking at all likely valve images. Initially this looked like a double triode, however, the second box looked to only have rods and not a grid. A double diode triode! identification soon followed. The main image was shot looking directly at the ellipse.
The Osram Brand label.
The top mica showing the single inverted V oxide coated filaments. The HD22 has a filament rating of 0.2 A at 2 V whereas later versions HD23 and HD24 had lower current consumption. On test this valve ran at 0.18 Amp.
With the microscope the tension spring and filament are easy to see. The rods are the diode anodes. The copper lead from the top cap to the triode grid can be seen in the out of focus foreground.
The triode section. The anode box can be seen through the hole in the mica lower left. The anode support is just out of shot on the left.
The classic envelope is 45 mm in diameter, and excluding the B5 base pins is 101 mm tall.
References: Data-sheet, 1040 & 1043. Type HD22 was first introduced in 1934. See also1934 adverts.
