The 225DU is classed as a full wave rectifier but would be better described as two separate half wave rectifiers in a single envelope. The use could be as a full wave rectifier but the voltage and current ratings are then strange. However, Cossor made very good oscilloscopes and in the late 1930s and during the 1940s no post deflection acceleration existed so a bright trace required a high final anode voltage of up to 3,000 Volts. As a consequence the deflection sensitivity of the tube was low and high voltage deflection amplifiers were required. The 225DU is quoted as having a voltage on the anode of 750 Volts, this is given as the PIV and the RMS value. It is most probable that the RMS value is 525 Volts, a common voltage for triode amplifiers of the time. A deflection amplifier HT of around 1 KV seems correct so the 225DU for the HT and perhaps an SU2150A for the tube EHT supply.
The Type designation repeated on the reverse.
Not easy to see but each section is a bright flattened oval anode with a coated single strand filament within.
The balloon envelope is 49 mm in diameter, and excluding the B7 base pins is 115 mm tall.
Reference: 1043. Type 225DU was first introduced in 1937. See also1937 adverts.