The 25RE is a pair of HT rectifiers with a 300 mA heater for series connection. The rectifier sections can be used in parallel for increased current, used as a full wave rectifier (unlikely in a set with no mains transformer, or as a voltage doubler to give 250 Volts from USA mains.
The identification contained within a circle on the envelope.
The UX6 base pins.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter and, excluding the UX6 base pins, is mm tall.
References: Data & Data. Type 25RE was first introduced in 1940. See also1940 adverts.
