The UY41 is a half wave rectifier designed for universal domestic equipment and replaced the UY21. By universal we mean AC or DC mains input and therefore no mains transformer.
All valve rectifiers have a maximum current rating when charging a reservoir capacitor, this will be several times the average current. To limit the current to a safe value for the UY41 the series resistance needs to be at least 210 Ohms. The maximum reservoir capacitor must be limited to 50 μF
The thin glass tube envelope is 20 mm in diameter and, excluding the B8A base pins, is 59 mm tall.
References: Data-sheet, 3002 & 1040. Type UY41 was first introduced in 1948.