The CY2 is a rectifier designed for series heater chains. The two sections can be used together with a 110 V input to produce a 250 Volt HT rail with a voltage doubler circuit. Alternatively the pair of rectifiers can be run in parallel for greater current output.
In 1939 the CY2 replaced the UR3. The CY32 was the CY2 fitted to an IO base. Also the CY1 (Ct8) and CY31 (IO) replaced the UR1, also in 1939.
The two rectifier sections. The cathode tubes are closest to the centre and the rest of the anode forms a heat dissipating wing.
The insulated heater can be seen to be a tightly coiled wire. With a 30 Volt rating the heater needs to have a long length to consume only 200 mA.
The tungsten heater wire is clamped to make a connection and then the clamp is spot welded to the support. Tungsten having too high a melting point to weld effectively.
The classic envelope is 40 mm in diameter and, excluding the Ct8 base pins, is 100 mm tall.
References: Datasheet. Type CY2 was first introduced in 1939.