This development sample U10 is a directly heated full wave rectifier. U10 looks to be a later development with a coated cathode and heated strapped to the cathode. U10 is probably the form of the first U10 but the inner structure is not visible. The development must be the introduction of the coated ribbon filament.
The maximum reservoir capacitor is not quoted, but at 60 mA maximum output current the U10 was clearly designed for broadcast radio reception in 1930s all mains sets. The 4.0 Volt heater is characteristic of the 1930s British heater standard.
The anodes are carbonised for better heat dissipation and are folded and stitched together. the anode was spot welded to the support rods. The filament is an oxide coated ribbon.
The face view of one anode.
The envelope has the U10 etched into the glass. The ribbon filaments can be seen to be welded to the supports.
A closer view of the base of one rectifier section.
The ribbon filament is just bent over the top mica. Production version U10 have filament tension springs at the top of the filament.
The classic envelope is 45 mm in diameter and, excluding the B4 base pins, is 100 mm tall.
References: Data-sheet, 1040, 3002 & 1043. Type U10 was first introduced in 1930.