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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! |
