The U3 is a double ended half wave EHT rectifier with a rectified current of 27 mA max. The anode voltage is given as 1,500 Volts but also a maximum of 3,000 Volts. Thrower gives the ratings for the M-OV version as the U3 was made by both Cossor and M-OV.
The filament has detached and was seen within the envelope. It is a helix of, probably, tungsten wire.
The anode connects to the bayonet end and has a wire attached to the centre contact. Within the envelope the support from the pinch connects only to one side of the anode cylinder. The ES27 cap end connects to the filament. One support passes through the anode to the top of the cylinder and the other is positioned below the anode so that the filament passes through the centre of the anode.
The anode end. Note the side tube used to evacuate the envelope. The valve is not gettered.
The lettering etched into the glass is very difficult to see. This was the best image from a whole series of attempts.
Initially overlooked, with the right light the Cossor name shows clearly.
Thanks to Fin Stewart for help with this exhibit.
The balloon envelope is 55 mm in diameter, and including the base caps is 118 mm tall.
Reference: 1003. Type U3 was first introduced in 1925.