The 25Z5 is an American TV rectifier. Within the envelope, sharing a common heater, are two independent rectifiers. The heater current of 0.3 Amps is one of the standard heater chain currents.
The independent rectifiers make three configurations possible: the pair as a full wave rectifier, the pair to produce 250 V from 117 V mains in voltage doubler configuration, and lastly in parallel as a half wave rectifier.
The maximum reservoir capacitor is quoted at 16 µF.
The twin rectifiers. The folded anode plates are stitched together.
An edge on view of the top of one anode. Note how the plates grip the support rod at the stitches and not at the top. The mica is bent as the assembly is pushed into the shoulder of the envelope and the grip prevents internal vibration.
The classic envelope is 38 mm in diameter and, excluding the UX6 base pins, is 87 mm tall.
References: Datasheet & 1040. Type 25Z5 was first introduced in 1936.