Pentagrid converter

The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.

The pentagrid was part of a line of development of valves that were able to take an incoming RF signal and change its frequency to a fixed intermediate frequency, which was then amplified and detected in the remainder of the receiver circuitry. The device was generically referred to as a frequency changer or just mixer.

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 it is known 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.

In 1918, Edwin Armstrong used only triodes when he invented the superheterodyne receiver. One triode operated in a conventional oscillator circuit. Another triode acted 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 circuit. Once again, the problem of coupling between the circuits would be ever present.

Shortly after Armstrong invented the superheterodyne, 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.

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