The FW5 is a full wave rectifier that uses an oxide coated ribbon filament of the type pioneered by the 80.
The filament is rated at 4.0 Volts and the maximum anode voltage is 500 Volts. These details are on the envelope.
The dome is etched with identification data. Also on the left the top of the filament can be seen through the top mica.
One of the anodes. On the right the ribbon filament tension spring can be seen.
The coated filament ribbon can be seen in the left anode. The side flange on the anodes aids radiation as does the blackened surface. The filaments are run in series, the joining bar can be seen clearly.
The classic envelope is 45 mm in diameter, and excluding the B4 base pins is 103 mm tall.