Pentode

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The termionic valve formed by five electrodes is called pentode. Functionally very similar to the triode, it has three grids instead of just one. It was invented by the Dutchmen Gilles Holst and Bernardus Dominicus Hubertus Tellegen, of the Philips company in 1926.

Features

The reason for adding a third grid to the four-electrode valve or tetrode is that while the second grid increased amplification, there was a downside: it produced secondary emission on the plate. The electrons released in this secondary emission are captured by the screen (positive) grid, introducing a great distortion in the amplified signals.

That is why, to avoid this secondary emission, a new grid was added, called a suppressor that, properly polarized (more negative than the plate), eliminates this unwanted effect, repelling the secondary electrons back towards the anode. In many pentodes the suppressor grid is internally attached to the cathode.

The second grille (screen) makes it work better at higher frequencies and the third (suppressor) eliminates distortion, by secondary emission.

Abrupt cutoff pentodes

There are pentodes that seek great sensitivity to voltage variations in the grids, not just the control one. This allows the blanking grid to be used as a second control grid, for example in mixers. Type 5636, created by the Radio Corporation of America, is an abrupt cutoff pentode. They are also known as sharp cutoff pentodes.

Far-cutting pentodes

In radio receiving equipment, signals of very varied intensities are usually handled. In a fixed gain tube, within the normal limits of use, large signal variations will produce notable volume differences and even clipping or clipping problems. Volume differences were corrected with circuitry that varied the bias of the midrange tubes and even the high end. In the early 2000s it was known as "automatic gain control" (C.A.G) or "automatic volume control" (C.A.V.), but later the name "automatic sensitivity control" was adopted; (C.A.S.). However, a tube whose cutoff is well away from the 0 voltage of the control grid and which maintains a more or less uniform output with respect to very strong or weak signals is desirable. This is how the remote cutoff, highly variable or supercontrol pentode was born. These tubes for small-signal voltage amplification reach the cutoff with a very negative grid voltage, which would correspond to a strong radio signal. Before that limit, the special construction of the grid makes the gain for very negative grid values low compared to less negative values. Usually the winding of the grid is not uniform with respect to the spacing of turns. By having a higher amplification with less negative values of the grid and inversely for very negative values, it is possible for the output voltage to remain constant in the face of current variations. In English it is often called "remote cutoff". As an intermediate type there are also semi-remote cutoff valves.

Technological applications of pentodes

Pentodes are particularly appreciated in the power amplification stages of professional and amateur radio fixed transmitters, where they are used for several reasons:

They present a better linearity than transistors amplifiers
They are simpler construction
Unlike triodes, they have better linearity and it is not necessary to enter the call neutralization
They have an excellent power/cost ratio.