The DET3 is a large baloon transmitting triode with a thoriated tungsten filament and operation to 3 MHz. It has an anode dissipation of 250 Watts and is a larger version of the DET2.
This particular valve came in a Marconi polythene bag with a paper label identifying it as having been used at the Brookmans Park regional transmitter.
The DET3 was designed for RF amplification and the sister valve DEM3 was designed for modulator service. The DET3 was widely used in the amplifier chains of Marconi transmitters including the short-wave SWB8, SWB10 and SWB11.
The anode is at the bottom of the main image. Note the loops at the end of the copper wires and the evacuation pip on the left side. The supports are clamped to the re-entrant tubes at either end.
The paper circle seems to suggest where in the transmitter this valve was sited.
The Osram Logo and the bright box anode. The end of the grid can also be seen.
The anode connection is on the right and the filament tension springs are insulated with ceramic collars. On the left are three glass tubes. The outer two connect to the ends of the filament and the centre one holds the bottom of the middle loop.
The grid winding and the top loop of the thoriated tungsten filament.
The lower filament loop can be seen. The filament passes as four strands within the grid. The box anode can be seen to be riveted together.
The paper label that accompanied this exhibit.
The balloon envelope is 162 mm in diameter, and excluding the leads is 430 mm tall.