The 25X4G is a half wave rectifier. The 300 mA heater rating indicates use as part of a series heater chain and confirms that this rectifier was aimed at sets without a mains transformer and so could be used on both AC and DC mains. The reservoir capacitor has a maximum value of 35 μF and a series resistance is required to limit the peak cathode current.
This view shows that there are two rectifier sections within the envelope - connected in parallel.
Thanks to Mark from Vintage-parts for sending these images.
The thin glass tube envelope is mm in diameter, and excluding the IO base pins is mm tall.
Absolute Maximum Operating Conditions¶
Updated March 20, 2017.