The U52 is directly heated full wave rectifier with a rating of 500 Volts on the anodes. The U52 has a maximum reservoir capacitor rating of 16 µF and a minimum series resistance of 180 Ohms, capable of delivering 100 Watts of DC power. It would be used for all types of medium power equipment.
The 5U4G has ribbon filaments coated with oxide with the top of the filament extends beyond the top mica, their directly heated design and generous power rating means sag free HT+ is available instantaneously and with bags of reserve any application - even audio - could desire..
Their appearance is typified by the fat balluster shaped envelope and shaped micanol base ala KT66.
The classic envelope is 48 mm in diameter, and excluding the IO base pins, is 113 mm tall. Type U52 was first introduced in 1938.
As you can see, there are many variants of the U52 type but the GEC-Osram product is the most desired and well regarded both from an aesthetic and performance perspective.