The Mullard 0/20 transmitting triode dates to 1921. It is constructed as a more substantial version of a receiving valve of the time, with a vertical cylinder as the anode, a single thread filament and a helical grid.
This is a good specimen a scarce valve. The anode is rather thinner than usual for a valve of this rating, with the result that it is also rather long in order to increase the external radiating area for adequate heat dissipation. This is probably why it had to be mounted vertically rather than horizontally as in 'R' types.
A feature of this type of vertical arrangement is difficulty in maintaining anode voltage insulation at the top of the pinch when metal (blackening) is evaporated and sputtered onto the pinch during operation. Mullard have obviously had some difficulties here and have gone to the trouble of making a craftsman-worked three-prong crown to the pinch. Nevertheless, the filament and grid supports look a bit less than rigid.
The standard horizontal arrangement devised by the French and used by M-OV and Ediswan in almost all early single-ended transmitting valves was, on the whole, much more practical and robust.
The filament is rated at 6 V at 1.7 Amps. The following picture gives the operating curves for this valve.
The balloon envelope is 70 mm in diameter and, excluding the B4 base pins, is 117 mm tall.
Updated December03, 2012.