The GZ37 is an indirectly heated full wave rectifier with a considerable rating. This exhibit has dual labeling as the Cossor type 53KU and as the GZ37 which is a Mullard type. The later GZ34 has a maximum current rating of 160 mA whereas this can rectify almost twice that.

The reservoir capacitor should not exceed 16μF and the valve needs a minimum effective series resistance of 75 Ohms to protect it from excessive peak currents.

The two sections both have a tube cathode with insulated heater wires within. The cathode is strapped to one side of the heater to avoid the need for the additional heater power that would have been needed to overcome the increased thermal resistance of the substantial insulation that would be required if the heater cathode insulation had to withstand 500 plus volts. The image has been enhanced to show the yellow lettering.

The classic envelope is 44 mm in diameter, and excluding the IO base pins is 126 mm tall.

References: Datasheet & ¹⁰⁴⁰ Type GZ37 was first introduced^¶ in 1958.

 
Pin Connections

IO	1	2	3	4	5	6	7	8
	-	h	-	a(2)	-	a(1)	-	h,k

 
Absolute Maximum Operating Conditions^¶

Rectifier	Vh	Ah	Va	mAa
	5.0	2.8	500	250

Thanks to Frank Philipse for supplying the above PDF datasheet