It’s a basic principle of the superheterodyne receiver circuit that the incoming radio signal, coming from the antenna or radio-frequency amplifier, must be mixed with a local oscillator signal to produce a third signal of fixed value (called the intermediate frequency). This single intermediate frequency, which is easier for an amplifier to process than a range of frequencies, can then be demodulated to recover the original transmitted audio.
When the superheterodyne circuit moved from the laboratory to commercial receiver production in the late 1920s, there were various schemes in use to accomplish this frequency conversion. The first receivers to market used triode vacuum tubes as both the mixer and the oscillator, with varying degrees of success. The advent of tetrode- and pentode-type tubes made it possible to eliminate the separate oscillator tube and thus decrease the cost of the receiver. However, such circuits were difficult to align and often tricky to operate. They were capable of good performance, but only if careful attention was paid to circuit design, and this wasn’t always the case with cheap receivers.
The pentagrid converter tube solved the problem of inexpensive, stable frequency converter circuit design. As its name implies, this tube contains five grids, in addition to a plate, cathode, and heater (filament). It successfully combined the functions of the mixer and the oscillator tube, simplified circuit design, and was more stable than any other circuit configuration.
In operation, two of the tube’s grids form the oscillator section. As the electron stream passes through these grids, the oscillator action thus derived mixes with the incoming signal on the tube’s control grid. The intermediate frequency signal is then available, as noted, for amplification and demodulation. The tube can be controlled by automatic volume control (AVC) voltage, adding to the circuit’s stability.
The first such tubes released were the types 2A7 and 6A7, produced by RCA in 1933. These tubes were designed for use in radios operated by alternating current and were an immediate success. Types 1A6 and 1C6, for use in battery-operated receivers, followed soon thereafter. With these tubes, radio engineers could design receivers whose performance surpassed all previous models. A five-tube radio of average design could easily outperform an older style tuned-radio-frequency receiver employing eight or more tubes.
Design improvements continued on the pentagrid converter type, right up to the ubiquitous miniature types 6BE6 and 1R5 of the 1950s. The basic circuit used with the tubes, however, remained much the same as it was at its introduction. Some circuits kept the pentagrid principle, but used the 6L7 pentagrid mixer tube that required a separate oscillator tube, but this was normally found in more elaborate receivers where designers wanted maximum stability and sensitivity.
The pentagrid converter was not without its problems – poor circuit design could lead to "pulling" (where the oscillator frequency varies with changes in AVC). Also, some tubes were notoriously inefficient in providing overall gain, especially in shortwave receivers. The advantages outweighed the disadvantages, though, and the pentagrid converter tube remained in use until the end of the vacuum-tube era in the late 1970s. It was one of the many innovations that made "radio for the masses" possible.
SOURCES
Stokes, John W., 70 Years of Radio Tubes and Valves. Vestal, New York: The Vestal Press, 1992.
Langford-Smith, F. ed., Radiotron Designer's Handbook, fourth edition. Sydney, Australia: The Wireless Press, 1953.
Radio Corporation of America, RCA Receiving Tube Manual, various editions. Harrison, New Jersey: Tube Division, Radio Corporation of America.