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 Type designation is enclosed in the typical M-OV ellipse etched into the glass.
The top mica holds the electrodes in the shoulder of the envelope. The anode box is about 7 mm front to back. Note the small piece of paper to the left of the lettering. This has a gold border and a red interior.
Face view: the bright anode is ribbed for mechanical stability as the nickel is quite thin. The electrodes are placed high in the envelope and thus the lead inductance would limit the high frequency capability but as an audio valve the designers had no need of an RF response.
The top mica showing the secondary mica holding the four filament tension springs.
A closer view showing the filament loops in the hooks of the tension springs.
The pinch and lower mica. The remains of the Marconi brand label can be seen. There is a tiny piece of red paper left that allows the determination of Marconi rather than Osram branding for this M-OV valve.
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. See also1929 adverts.
