The 405/BU (405 B.U. Thrower) is a full wave rectifier from Cossor. It is unusual in that it is a full wave rectifier for EHT voltages, albeit 1,500 Volts maximum.
The design dates to 1937 and Cossor were well known for their oscilloscopes and this rectifier would be suitable to supply the high voltages used in the deflection amplifiers.
The construction is as a conventional FW rectifier. SeeSU2150A for the Cossor design of an EHT rectifier for use up to 5 kV.
The B4 base pins.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter and, excluding the B4 base pins, is mm tall.
References: Data & 1043. Type 405/BU was first introduced in 1937. See also1937 adverts.
