The DET1 designed by M-OV /HRC around 1924 was the first dull emitter, thoriated tungsten filament transmitting valve, certainly in the UK, possibly in the world. It was a conventional triode with flat nickel anode, ribbed for strength and capable of dissipating a power of 35 watts at anything up to 1,000 Volts. The cathode operated at 6 Volt, 1.9 Amp.
The valve had a pear-shaped bulb with seal-off pip on the top and sported the characteristic square (L4) base with 4-pins, one at each corner as required by the Royal Navy. Originally this base was handcrafted out of relatively thin fibreboard type material but later a more substantial moulded plastic base was used.
The valve was suitable for use in along or medium-wave self-oscillator circuit and also as a power amplifier or modulating valve. As a power valve it soon became a favourite with experimenters, replacing the smaller 10 Watt LS5. It was often used as a master oscillator to drive the larger valve in a MOPA (Master Oscillator-Power Amplifier) circuit arrangement. The relatively low filament consumption made it attractive for airborne equipment and it was adopted by the Air Ministry as type VT25, A few years later, a short-wave version (DET1SW) was produced.
Many years later when the VT25 was being used in larger quantities in rugged military service, the fragility of the carburised filament caused considerable losses and, as a result, a plug-in oxide-coated filament version was produced with a nickel ribbon filament of the same rating but about five times the length of the thoriated tungsten original. For this reason the original simple 'V' formation was replaced with a new filament mounted in an 'M' form which produced a much greater mutual conductance, so that the grid and anode had to be very much wider in cross section; the anode of carbonised nickel, in particular, was very large, only just small enough to be inserted through the neck of the bulb. This modified type was named the DET25 (CV1025).