The UY85 is a half wave rectifier and would be used on small universal (AC/DC) mains appliances. The heater chain would draw 100 mA and this was to enable the small number of valves used in a broadcast radio receiver to be run without a large dropper (ballast) resistor as each valve required a significant voltage to heat it at this reduced current. The 100 mA heaters had to be wound from very fine Tungsten wire and wer consequently more prone to failure than heaters wound from a thicker gauge wire.
The UY85 can work into a maximum reservoir capacitance of 100 µF, as protection a series resistance of 100 Ohms minimum is required.
The image shows the anode pressed to form a small vertical cylindrical space for the cathode and wide flanges to dissipate heat. The outside of the Nickel sheet anode is carbonised but the inside is left bright and this is demonstrated in this valve by the end wings on the right of the image.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 60 mm tall.
References: Datasheet & 1040. Type UY85 was first introduced in 1954.