Scott Johnson identified this valve that we previously could only see as a 3B2. He writes: it is a common HT rectifier used in Radar sets in the 1940s and 1950s. We also have the variant 3B24W and 3B24WA.
The cylindrical anode from the side.
The filament connects from the base to the top of the central support. The support is the centre tap and the filament is a double helix.
The wire is uncoated indicating thoriated tungsten wire. On test the filament emits a very bright orange light.
Using the microscope camera to show the double helix.
The wide glass tube envelope is 38 mm in diameter and, excluding the base pins, is 103 mm tall.