This rectifier from Mullard is directly heated. The maximum reservoir capacitor is 4 µF and the minimum series resistance has to be 150 Ohms.

The classic envelope is 46 mm in diameter and, excluding the B4 base pins, is 122 mm tall.

References: Datasheet, ³⁰⁰² & ¹⁰⁴³. Type FW4-800 was first introduced^¶ in 1948.

 
Pin Connections

B4	1	2	3	4
	a(1)	a(2)	f	f

 
Absolute Maximum Operating Conditions^¶

Rectifier	Vh	Ah	Va	mAa
	4.0	3.0	850	125

Thanks to Frank Philipse for supplying the above PDF datasheet