The DO25 from Mullard is a directly heated triode audio output valve with a 25 Watt anode dissipation.
Originally introduced in 1930 the filament consumption was 1.8 Amps, however, in 1931 an improved version was released that at 6.0 Volts required only 1.1 Amps of filament current. This exhibit is of the later type and has an oxide coated filament.
The lower part of the anode is supported on both sides
The filament volts are given as 6 and the max anode volts as 400. Construction follows the earlier style of having the electrodes supported away from the centre by glass rods. In the days of the Azide process the supports had to be kept clear so that they would not be sprayed with emissive material when the filament was heated to reduce the barium azide to the oxide.
The white oxide coating on the filament is clearly seen.
The lettering on the dome.
The balloon envelope is 67 mm in diameter, and excluding the B4 base pins is 145 mm tall.
Reference: Datasheet. Type DO25 was first introduced in 1930.