The VT-46A is the US Government procurement code for the 866A high voltage half wave mercury vapour rectifier. This valve is marked as 866 and 866A so able to match the high specification and be plug compatible with the older design.
A pair in a full wave configuration feeding a choke input filter can produce 500 mA of DC at 3,000 Volts. Serious power and the rectifiers will produce damaging UV radiation and must be screened.
The rectifier also carries the code 110E/45 which is an old stores code used by UK Government departments.
The filament leads pass into a screening cylinder.
The black anode is connected only to the top cap.
The classic envelope is 61 mm in diameter and, excluding the UX4 base pins, is 148 mm tall.