The Nelson Multi is a bright emitter triode with three straight wire tungsten filaments and a switching arrangement on the base. The idea was to both increase the life of the valve and to provide a double functionality.
Thanks to Ross Beamer for sending these excellent images.
One filament could be selected and thus provide three times the normal life for a low power valve operating as a detector or AF amplifier. The top operating characteristics table then applies. Additionally a pair of filament could be switched into circuit at once. This provided extra emission and allowed the valve to operate as a power valve.
The instructions with the valve state:-
The valve contains three filaments which can be brought into use either one or two at a time, thereby lengthening the life of the valve and adapting it for power amplification.
The switch consists of two arms pivoted scissors-like on one of the cap legs. When it is desired to use one of the filaments the arms should be brought together with the tips in contact with one of the three studs in the valve base.
When two filaments are required the arms should be opened in opposite directions so that the tips are in contact with any two of the studs.
The trade mark stuck onto the envelope.
The type two BBC stamp dates the valve to between September 1924 and 1925 when Nelson stopped making valves. On the left can be seen the pair of switch arms connected to one of the filament pins. The pins themselves are simple split-pins. The inside shows the golden colour of phosphorus gettering.
A closer view of the BBC stamp.
The base with the levers to bring one or two filaments into circuit.
A close-up view of the two levers. The tracks in the base material show that these were moved more than once. The base pins are of simple split-pin construction with the connecting wires soldered to the tops of the pins.
This image shows the screw heads in the base that are the connections to the three filaments.
Another view of this innovative valve.
An almost face on view of the base. The copper of the pins can be seen on the anode pin.
The balloon envelope is mm in diameter, and excluding the B4 base pins is mm tall.
Reference: 1003. Type Multi was first introduced in 1924.