The UD4 is an audio output triode. Triotron describe it as a Super Power type at a time when 200 mW could seem loud.
Initially most manufacturers made valves for two, four and six Volt LT supplies. The four Volt range was the first to be dropped. The UD2 still being advertised in the early 1930 whereas the UD4 was not.
The electrode assembly is mounted at 45° within the upper part of the balloon envelope.
The simple split-pin base pins showing the screw retaining collars.
The filament is a simple single strand in a V form. The tension springs can be seen in front of the anode.
The filament is held in a fold in the support and not welded - this suggests the base wire is tungsten. The grid wires can also be seen.
A better view of the tensioning springs.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 98 mm tall.
Reference: 1043. Type UD4 was first introduced in 1927.