Metal Rectifiers

 Below you can see a miscellany of seven types of rectifier, half-wave, full wave and bridge.

In the centre are five RM4; below these are three RM3 and two RM1A half wave rectifiers.

Along the top is a small bridge rectifier, a 280/LU1093 or AP53170 together with a larger bridge, the "5307"

To the right is a full wave 12-2 and below it a full wave STC STK MG

The smaller, circular types could be placed on a threaded rod and built into full wave rectifiers.

 

Ratings for the above are as follows:

Brimar RM1A: 100mA @ 125VRMS

Brimar RM3: 120mA @ 125VRMS

Brimar RM4: 275mA @ 250VRMS

 

 The earliest mains radio sets used a valve rectifier. So did the earliest battery eliminators. Generally speaking, for an HT supply, one would find a double diode valve with a directly heated filament.

Valve rectifiers were used for both HT & LT in early battery eliminators, such as those produced by Philips, although because of difficulties in producing a clean hum-free low voltage supply, it was a rule that an accumulator was employed for the LT supply of radio sets.

The use of a valve for rectifying AC for HT was the more usual choice in the 1930s. One needed an extra transformer winding for the rectifier heater of course, but this needed only a handful of turns of copper wire. Efficiency was not wonderful, but anyway, not really of concern to the setmaker. The cost of providing a valve was not inconsiderable and involved a payment to the Marconi Company, who had come up with a wizard idea of extracting cash from radio listeners. Every valve in a set attracted a sort of tax to be paid by the manufacturer who passed on this to the purchaser.

A selenium rectifier was the "modern" alternative to the valve when it came to rectifying AC for battery chargers, mains eliminators and power supplies in general, but I should say, became the second choice by setmakers, probably due to considerations of the final selling price of the radio.

I suppose this type of rectifier was seen in equipment designed from the late 1920s until the 1960s, and provided an alternative manufacturing option for providing an HT supply compared to that of a valve. The cost of a selenium rectifier was more expensive than an equivalent valve but it did not require a heater supply or a valve socket. One major advantage over a valve however was reliability. Whereas a valve guarantee was often only 90 days, that of a selenium rectifier was a year. The life of a typical valve rectifier was around 1,000 hours, whilst that of a selenium rectifier, operated within its ratings, was rarely quoted but certainly a lot longer than a year, however a typical purchaser of a radio would be looking at its ticket price and its appearance rather than being concerned about its maintenance cost.

Later versions of the selenium rectifier dispensed with fins and were very thin in construction, often only around 5mm or less for 250V output. These were bolted to a metal chassis which served as the heatsink. Their price would have reflected the new design and, for a short time, this type of rectifier became competitive and was used in both radio and TV sets.

After the selenium rectifier came chunky germanium diodes followed shortly afterwards by physically smaller silicon diodes. The price of a silicon diode dropped to a few pence. Physical size and construction is dependent on their rating, but as the forward voltage drop across the diode is relatively small, so is the heat dissipation, and so in many cases the diode needs no heatsink.

Older germanium diodes needed to be treated differently to modern silicon diodes as the electrical characteristics of the construction material of the former changes quite dramatically as it gets hot. This may not seem particularly important, but as a designer who used germanium products I can tell you that this was a major aspect.

The design of a simple single transistor amplifier based on a germanium transistor was not easy. It involved calculations based on supply voltage variation, tolerance of resistors and the study of the transistor's characteristics to take account of gain at differing temperatures and particular frequencies. It may also involve calculations regarding the efficiency and performance of a heatsink including allowances for ambient temperatures. If the amplifier was for radio frequencies one also had to juggle with capacitances, feedback and the like to avoid instability. The advent of silicon transistors came as a huge relief to designers as one could virtually ignore many of the intricate calculations. The struggles to find components to fit the requirements of the germanium transistors became a thing of the past.

Back in the 1950s, Plessey designed huge computers using germanium transistors and diodes. If I define "huge", let me say you have to imagine a vast room containing 1000 seven foot high stove enamelled racks. These racks comprised around only 20 or so computers, each using countless germanium transistors and diodes. These were used to make simple bistable circuits or "toggles" with maybe half a dozen toggles mounted on a printed circuit board. There were around 16 circuit boards mounted in a frame and perhaps 7 frames in a rack. To make the designer's job marginally simpler transistors were selected by manufacturers to our specification and given Plessey codes. Thus, some of the variations were removed. There was a nice black-coloured TO5 transistor used in memory core stores made by STC that made an excellent top band transmitter power amplifier and some rather good gold coloured transistors that had an fT in the hundreds of megahertz. I'll have to dig around the junk box and see what I can find...

Why did we use germanium transistors and diodes even when silicon types were common? Would you believe that MoD didn't trust these "new fangled devices" and insisted we stuck to what they knew. It had been hard enough weaning them off ECC31 double triodes!

see some more metal rectifiers

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