Origins
The first devices designed to change frequency in the manner described above seem to have been developed by the French who simply put two grids into what would otherwise have been an ordinary triode valve (the bi-grille). Although technically a four component electrode device, neither the term tetrode nor the tetrode valve as we know it today had yet appeared. Each grid was able to accept one of the incoming signals and the non-linearity of the device produced the sum and difference frequencies. The valve would have been very inefficient but, most importantly, the capacitive coupling between the two grids would have been very large. It would therefore have been quite impossible to prevent the signal from one grid coupling out of the other. At least one reference claims that the bi-grille was self oscillating, but this has not been confirmed.
When Edwin Armstrong invented the superheterodyne receiver in 1918, although the tetrode had been invented a couple of years earlier, he nevertheless employed only triodes in his design. Armstrong employed a pair of triodes as his mixer stage. One operated in a conventional oscillator circuit, but he was able to employ the other as a mixer by coupling the oscillator signal into the mixer's cathode, and the received signal to the grid. The sum and difference frequencies were then available in the mixer's anode (or plate) circuit. Once again, the problem of coupling between the circuits would be ever present.
Shortly after Armstrong invented the superhet, a triode mixer stage design was developed that not only mixed the incoming signal with the local oscillator, but the same valve doubled as the oscillator. This was known as the autodyne mixer. Early examples had difficulty oscillating across the frequency range because the oscillator feedback was via the first intermediate frequency transformer primary tuning capacitor which was too small to give good feedback. Also keeping the oscillator signal out of the antenna circuit was difficult.
The invention of the tetrode demonstrated the idea of screening electrodes from each other by using additional earthed (grounded) grids (at least as far as the signal was concerned). In 1926, Philips invented a technique of adding yet another grid to combat the secondary emission that the tetrode suffered from. All the ingredients for the pentagrid were now in place.
Read more about this topic: Pentagrid Converter
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