The U5 has the distinction of being the first full wave rectifier. The U5 was introduced by the Marconi-Osram Valve company in 1926.
From the picture it can be seen that each filament is an inverted V. It looks as if the filaments are connected in series but the two outer connections are joined at the base pin. Each anode cavity is a flattened cylinder without any coating.
The etched lettering of Marconi Valve.
Here the thoriated tungsten filament wires can be seen entering the anodes.
On this side is the glass bead that is fixed to the anodes and holds the filament tensioning springs.
The U5 Type designation is etched with a circle that at the bottom carries the fact that the filament is 5.0 Volts.
The balloon envelope is 54 mm in diameter and, excluding the B4 base pins, is 105 mm tall.
References: 1005 & 1043. Type U5 was first introduced in 1926.