The VT25 directly heated transmitting triode has a large four pin base and was made for the RAF. The original valve was from M-OV and a development of the DET1.
Originally the VT25 had a carburised thoriated tungsten filament that proved fragile in airborne applications. The filament was replaced by an M shaped oxide coated filament on a nickel ribbon as here. This exhibit has a single inverted V filament and is thus from the earlier design. Also unlike our other VT25's this example from Ediswan has a graphite anode.
The ceramic insulators and special base suggest a high operating voltage. Operating frequency would be in the HF range and the output power was of the order of 36 Watts.
The Maker and Type designation enhanced together with the old stores code of 10A/7312. The serial number is given as 500 4.
The graphite block anode supported by the side rods. The control grid is shaped to pass flat across the filament. At the top is s ceramic block that holds the helical filament tension spring.
The control grid and tension spring.
The reverse carries the Air Ministry crown and the letters AM.
The hook at the end of the tension spring with the thoriated tungsten wire loop present. The filament is good and at the rated voltage and current is indeed seen to be the bright orange of thoriated tungsten.
The grid and anode leads pass into glass tubes before reaching the pinch.
Here the filament and control grid are clearly seen together with the geometry of the graphite anode.
The wide glass tube envelope is 58 mm in diameter and, excluding the L4 base pins, is 154 mm tall.
