The U30 is a full wave rectifier from the 1930s. It is specified for a 250-0-250 input and can deliver up to 120 mA. It is interesting that the valve has separate cathodes brought to base pins. Technically this gives flexibility in that full wave, parallel half wave and voltage doubler circuits are all possible. No value is given for the heater to cathode voltage and so it may not be assumed that voltage doubling was possible at the full 250 volt input. With the 300 mA heater requirement and the 120 mA output it is possible that this valve would have found use in the Television receivers of the late 1930s.
The mesh anode can be seen to have two additional support rods pass through it on either side of the central cathode.
This clearly shows a dead valve. The cathode connections terminate short of the cathodes in a sphere of metal. Quite some overheating to melt the nickel tape.
The support rods to the side of the cathode are connected to the anode. The interesting thing about these rods is that they support grid wires. It appears that a pair of triode sections were the basis for this rectifier.
The classic envelope is 50 mm in diameter and, excluding the B7 base pins, is 104 mm tall.
References: Datasheet & 1043. Type U30 was first introduced in 1934.