The DFA9 is the last in the Dull Filament Amplifying range from Mullard. The filament was thoriated tungsten and we have verified this on test. The two halves were connected in parallel and in use the filaments emit a very bright orange light.
Mullard describe the DFA9 as 'For use as power amplifier for operation of the largest speakers, or as a low power transmitting valve', The output power was 840 mW.
Our example is missing the composition B4 base.
The dome carries the identification in clear etched form.
A face on view of the lettering. The shadowing is caused by reflection from the silvered inner side of the glass.
The filament is a pair of inverted V's. The centre connection as well as the two outer connections are brought out through the pinch, the anode is ribbed for mechanical strength and the grid wires can be seen in the image.
The balloon envelope is 52 mm in diameter, and excluding the missing B4 base pins is 94 mm tall.
References: 1043 Type DFA9 was first introduced in 1928.