Wowee, where to start with this bad boy. Well, let’s take the baddest of the lot, the Cossor 53KU often labeled CV378. Introduced in 1948, this octal based indirectly heated full wave rectifier in which the reservoir capacitor should never exceed 16μF and to avoid peak current spikes and minimum load series resistance of at least 75 Ohms is recommended.
Let's start with the exquisite electrode structure which has a beautiful staggered parallelogram twin anode cage. Each anode within the cage is pressed to shape and stitched together enclosing the anode support rod. The top mica shows the oval cathodes and a sequence of insulated heater wires that pass the length of the cathode four times. To the rear of the staggered cages we have the elegant swan neck getter plate all contained within a gorgeous full baluster envelope – it looks like no other rectifier and so my ears sounds like no other too. The envelope rides on a beautiful curved micanol base ala KT66. The classic envelope is 56 mm in diameter, and excluding the IO base pins is 122 mm tall.
Cossor went on in 1958 – along with Mullard to manufacture the no less impressive performing but nowhere near as visually stunning tall form GZ37 also labeled CV378. In this format, each section posesses a tubular cathode which is strapped to one side of the heater to avoid the need for the additional heater power and consequent increased heat output which could be detrimental with such a narrow envelope. The tall form envelope is 42 mm in diameter, and excluding the IO base pins is 130 mm tall.
The GZ33 shares the same tall form envelope with the GZ37 (Mullard)/U54 (GEC) which in turn shares a family base connection sequence with the GZ32 and the GZ34. All of these devices are full wave rectifiers having a 500 Volt anode rating. As the numerical progression might suggest, the GZ32 is a 120mA rated device, the GZ34, a 160mA rated device and the GZ37 is a 250mA rated device. The GZ33, like it’s identical twin GZ37 is a 250mA rated device however, has a maximum reservoir capacitance of 120uF as compared to the GZ37’s maximum reservoir capacitance of 16uF.
Whichever brand or numbering is your preference, be assured that these are the daddies when it comes to octal rectifiers – simply the best - as you might expect as these are used in current high end amplifiers such as: -