Triotron describe the WD4 as an RCC triode. It was this designed for use in circuits that had resistance/capacity coupling rather than transformer coupling.
The box anode is mounted at 45 °
The grid and filament enter the anode from the pinch end.
The gettering makes examining the internal structure very difficult.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 88 mm tall.
References: 1040, Data & 1043. Type WD4 was first introduced in 1927. See also1927 adverts.
