The PX4 is one of the classic audio power triode valves and was first introduced in 1929. 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.
This balloon envelope example is very substantially made with the supporting frame secured to the pinch stem with a bolted collar in the same way as the PX25. This is an expensive form of construction.
The anode box is 36 mm wide 41 mm long and 8 mm thick, and the whole construction is like the larger powered PX25. It is possible that this is one of the upgraded PX4's that were made from 1931.
The glass rod electrode support originated with the Azide process and was superseded by the use of mica supports when the messy Azide process was phased out. Early PX4's were made by the Azide process, later ones used the oxide coating employed for the PX25.
The Osram or Marconi branding was achieved with a paper sticker but at the M-OV production line they all were identified by the etching on the dome. The filaments are hairpin shaped and held by the tension springs. The one at the top that is more spread out is where the filament has been destroyed.
The working faces of the anode are 5.4 mm apart. The filament springs are held in the glass rod and the grid cab be seen to bow out at the centre.
The pinch, lower filament supports and grid. Note that the filament wire is white indicating an oxide coating.
A closer look at the stem and anode support collar. It looks as if the metal and glass are separated by two layers of mica. The hole in the stem below the pinch is where the air was evacuated.
A closer look at the dome showing the good and damaged filament strands.
The balloon envelope is 60 mm in diameter, and excluding the B4 base pins is 124 mm tall.
References: Datasheet, 1005, 1040 & 1043. Type PX4 was first introduced in 1929.