The PX4 is one of the classic audio power triode valves and this one is branded Marconi. The design belongs to M-OV and so Marconi and Osram are used to brand the valves.
This exhibit lacks the treated anode of later versions and features a bright nickel anode.
The filament is a single wire formed vertically within the anode cavity into four inverted V's. The grid is a single wire formed between two vertical notched rods 1.5 mm in diameter. The grid zig-zags around the filament.
The electrode system is only about 5 mm from front to back, but is 35 mm wide (including support rods) and 45 mm high.
The PX4 is directly heated and the coated filament wires can be seen below the bottom mica.
A closer look at the lower mica and above it can be seen the filament strands and one length of grid wire.
The top has three mica supports. The first is an interference fit to the shoulder of the glass envelope and holds the electrodes in place and thus minimises the microphonic effects of vibration. The second adds strength and the top sheet holds the coiled filament tension springs.
The classic envelope is 60 mm in diameter, and excluding the B4 base pins is 122 mm tall.
References: Datasheet, 1005, 1040 & 1043. Type PX4 was first introduced in 1929.