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 tops of the electrodes are held in two glass rods.
The label suggests that this exhibit is only fit to be a rectifier. Needs testing!
The dome carries the identification and basic operating parameters.
The filament can be seen to be oxide coated and it passes four times through the body of the valve.
Here the hooks holding the filament loops can be seen together with the bare wire of the loop.
The balloon envelope is 67 mm in diameter, and excluding the B4 base pins is 143 mm tall.
Reference: Data-sheet. Type DO25 was first introduced in 1930. See also1930 adverts.
