by Norm Wilson N6JV – Visit the museum at N6JV.com
In their build up after WWI, the Royal Air Force developed a new triode transmitting tube that was capable of higher power than earlier types. The DET25 was a developmental tube similar to the “50-watters” being made in the US. In 1925, the RAF was still flying open cockpit biplanes so the DET25 had the L4 base, with its long voltage breakdown paths, for use in wet applications.
I have found two early RAF transmitters that used the DET25. The TR 4 is a single DET25 oscillator that has plug-ins to operate on the range 143 to 500 KHz or 3 to 15 MHz. It uses either CW or interrupted CW (ICW) where the keying lines are fed through a motor driven wheel like a spark transmitter. The TR 1091 was a MOPA design with the DET25 as the amplifier. It operated in 1.2 to 1.5 MHz and 2 to 3.4 MHz using plug-in capacitors and coils. It used the same CW/ICW keying system, but also had grid voice modulation. This was a larger unit and required a larger aircraft.
The DET25 has a published filament voltage of 7.5 volts but other sources give it as 6 or 8 volts. Filament current is somewhere in the 1-to-2-amp range. Plate voltage has a maximum of 1200 volts and a current given as 36 ma dissipation maximum. The user needed to know the average of the mode they were using. Fifteen WPM CW has an average of 45% depending on the fist. Hand written notes indicate that in practice about 80 ma was used for a 60-to-70-watt output.
The designation DET25 was changed to VT25 and then CV1025 (Common Valve) when the Inter-Service Technical Valve Committee in 1941 started their version of JAN numbers. The British Army also used the designation AT35 for the VT25.
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