This is a mystery that has been sat in a drawer in my desk for some years. It has no markings at all but an interesting construction. It probably dates to the mid 1920s. The filament is intact as a single vertical wire. These is no oxide coating and so it is either thoriated tungsten (dull emitter) or pure tungsten (bright emitter). On test with 1.4 Volts on the filament the current was 0.2 Amps. The DER has a thoriated tungsten filament and runs at 1.8 Volts and 0.35 Amps. THe best guess, therefore, is that is a thoriated tungsten filament.
The base cap is pitted and carries no markings. The pins are of simple split pin construction. The pins are threaded and secured by a slotted nut.
The inside of the glass is silvered from the action of firing the getter and only at this angle can the construction be seen. The main support above the pinch is the glass bead that the top filament support passes through.
The lower filament is held in a fold of the support. The grid is the interesting part, the ring at the base has strips running vertically. There is no wire grid.
The ring is formed from a flat sheet. The vertical strips seem to number five or six.
In different light is seems clear that the grid has been stamped from a thin sheet of metal.
Getting serious with the photography now. A dark room with a single light source and the macro lens fitted to the Nikon Df. The grid is four vertical strips between two rings pressed from a single sheet and folded.
Another view of the glass bead.
The only light is from the filament. Over 30 seconds exposure.
What a delight!
The balloon envelope is 44 mm in diameter, and excluding the B4 base pins is 89 mm tall.
