The U25 is a dedicated television EHT rectifier for picture tube supplies. The valve is rated at 19 kV peak inverse volts, and this suggests an output voltage suitable for a 12 inch black and white tube of say 16 kV.
The valve is also rated for sine wave use, and in this case an input voltage of 9,500 volts would be the maximum. The reservoir capacitor would need a value of 0.001 µF or less. In practice the capacitor would be formed between the internal and external graphite coatings on the picture tube itself.
The valve construction follows conventional practice in that the anode is bell shaped and there is a considerable separation between the cathode and the anode. The valve has flying leads so that it can be fitted directly across the line output transformer, where it would also derive its filament supply.
In the early 1950s Mazda had problems with flaking cathode oxide coatings and a high failure rate of its rectifiers in service.
The anode bell is secured to the top of the envelope and the heater & cathode to the pinch end.
The coated cathode tube should be central within the bell of the anode. This exhibit is new and came in the original box, so it is possible it was made this way.
At this angle the cathode and heater can be seen in their entirety.
An enlarged view.
The thin glass tube envelope is 18 mm in diameter and excluding the base wires is 55 mm tall.