This PX4 has blackened fins spot-welded to a shiny anode. This shows that it is of post-1938 manufacture and has been uprated from 250 V and 12 Watt dissipation to 300 V and 15 Watt dissipation. From around 1950 the fins were omitted but the outer surface of the anode was given a grey coating instead.
See also Ferranti LP4 also M-OV made and with with cooling fins.
The original anode design with added cooling fins. The ridges on the anode mate with the groove in the fins.
The side view showing the small anode front to back thickness and the large additional heat radiators.
The pinch and oxide coated filament.
The second of the pair in side view.
The top mica with tension springs leading to hooks holding the filament. The grid wire and spacing can also be seen.
The mica support has metal tape fitted that makes firm contact with the shoulder of the envelope.
The tension springs and on the filament can be seen the oxide coating. In fact some of the coating is breaking off near the top of the wires.
The lower mica and filament connections. The filament has the outer ends connected together with a strap. The connections are made such that the eight active lengths of wire running through the grid are in fact two pairs of four wired in parallel.
A close-up of the lower end of the anode. The flat strap passes behind the three inner filament connections. The middle support has no electrical connection.
This pair of PX4's were used in an amplifier made in the 1930s and salvaged when the amplifier was scrapped.
The classic envelope is 60 mm in diameter and, excluding the B4 base pins, is 122 mm tall.
References: Data-sheet, 1005, 1040 & 1043. Type PX4 was first introduced in 1929.