Components such as tuning capacitors, inductors (both fixed and variable), resistors and fixed capacitors were in common use during the Spark Era, at the start of last century. However, valves (when they finally made an appearance) were initially extremely expensive and fragile. Suffice to say, they didn’t have a long life.
Initially, obtaining a high vacuum inside a valve was quite difficult and most of the early valves were of the “soft” variety. This meant that they had a small amount of gas left inside, due to manufacturing limitations. As a
result, these valves were rather variable in their performance, even between supposedly identical types.
Fig.1: the IF (intermediate frequency) amplifier, detector and reflexed audio stage of the Kriesler 11-41 & 11-49 radio receivers. The detected audio signal at pins 7 & 8 of the 6AD8 are fed back to the grid via the primary of IFT2, a 47kΩ resistor, the volume control, a 4.7nF capacitor and the secondary of IFT1.
Another problem that had to be overcome was how to maintain a good vacuum. This could only be achieved if the glass and metal leads through the glass envelope had the same coefficient of expansion. If the coefficient was different, air would eventually leak into the valve and it would become gassy. Occasionally, even today, a valve with a purple glow inside it will be seen and this is often an indication that the glass to metal pin seal is not perfect and air has leaked into the valve.
Incandescent light globes were the first items to have metal pins or wires protruding through a glass envelope. However, this created no real problem, since the vacuum created was satisfactory for their operation and the glass-to-metal seals were not as critical. In some cases, the globe was filled with an inert gas such as nitrogen to prevent evaporation of the filament.
One problem with valves was that the metals used inside them (ie, for the elements and filaments) had to be carefully selected, otherwise they could emit gases when they became hot. These gases could then “poison” a valve and adversely affect its performance.
So early attempts at making valves into viable amplifying devices encountered many difficulties. However, their potential to revolutionise radio was obvious and so a great deal of effort was put into solving these problems.
It is for these and other reasons that valves were by far the most expensive and fragile components in early valve receivers. As a result, the engineers and experimenters of that era searched for ways in which each valve could be made to do more than one job, to minimise cost.
One of the first to achieve dual usage of valves was W. H. Priess, a US Navy worker who patented the principle of reflexing during WWI. This technique involves passing a signal through the same amplifier twice, at two significantly different frequencies – usually once at a radio or intermediate frequency (ie, RF or IF) and then at audio frequency (ie, after the modulated signal from the IF stage has been detected).
Initially, there was little interest in reflexing for a couple of reasons. First, it was no doubt kept a military secret during the war and so was not widely known during those years. Second, the principle, although sound, initially proved tricky to implement. However, valves were still very expensive in the 1920s and this led some manufacturers and enthusiastic experimenters to refine the technique and this eventually yielded good results
In theory, reflexing meant that one valve did the job of two. However, as there was always some compromise in the operating conditions of the valves for each different frequency, the actual improvement was always somewhat less that this.
Reflexing was used for only a relatively short time overseas but in Australia, it was still being used in some receivers as late as the early 1950s. It was initially used in high-end receivers in the early 1930s, then in receivers at the bottom end of the market to reduce valve count (and thus cost).