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The Tetrode

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The tetrode was developed in response to the need for valves that could operate at higher frequencies than the early triodes. In the US Bureau of standards in 1919 a mathematical analysis of the triode revealed the detailed nature of the inter-electrode capacitance problem that caused a falloff in performance as frequency of operation was raised.

Armed with this knowledge various solutions were tried. Wide separation of the connecting leads gave rise to the Marconi-Osram valves QV24 that were effective at up to 2 MHz.

Some bi-grid vales had been produced. Either a space charge grid of close wound fine wire was placed between the filament and the main grid to reduce the space charge and enable lower anode voltages to be used or two open concentric open helix grids were used one to amplify RF and the other to amplify AF so trying to make best use of the single electrode stream. See Early Bi-Grid Valves K4 and Q

The eventual solution was a development of the work carried out in Germany in 1919 by Walter Schottky working for the Siemens and Halske Company and trying to develop a higher gain type of valve. His solution was to put a protective net between the grid and the anode and to bias it with a positive voltage less than the anode voltage. His valve did not itself solve the problem as the net was not a complete screen and the valve did not improve high frequency performance.

In 1926 in a research lab in the General Electric Company in the USA produced two types of experimental valves that tried to screen the anode and control grids and achieved a very low inter-electrode capacitance. The first successful screened grid valve produced in the United Kingdom was the Marconi-Osram S625 in 1927. The design, by Round, of the S625 was radically different to the German work. A metal cylinder enclosed the anode with metal gauze over the end of the cylinder pointing to the grid. The double-ended S625 was both technically successful and commercially expensive to employ because of its two base caps. The first single ended UK screened tetrode was the S215 the fifth connection was made by taking the anode to the top cap.

When this screen is maintained at a steady positive voltage, it is found that the amplification factor of the valve, as compared with the triode, is very much higher, the impedance is also greatly increased.

The reason for this increased amplification lies in the fact that the anode current in the tetrode valve is far less dependent on the anode voltage than it is in the triode. In any amplifier circuit, of course, the voltage on the anode must be expected to vary since the varying anode current produces a varying voltage-drop across the load in the anode circuit. A triode amplifier suffers from the disadvantage that when, for instance, the anode current begins to rise due to a positive half-cycle of grid voltage swing, the anode voltage falls (by an amount equal to the voltage developed across the load) and the effect of the reduction in anode voltage is to diminish the amount by which the anode current would otherwise increase. Conversely, when the grid voltage swings negatively the anode current falls and the anode voltage rises. Because of this increased anode voltage the anode current is not so low as it would have been if it were independent of anode voltage. This means that the full amplification of the triode cannot be achieved. The introduction of the screen grid, however, almost entirely eliminates the effect of the anode voltage on the anode current, and the amplification obtainable is thus much greater.

A screen is found to function best when its voltage is below the mean value of the anode voltage. Most of the electrons from the cathode are thereby accelerated towards the anode, but some of them are unavoidably caught by the screen. The resulting screen current serves no useful purpose, and if it becomes excessive it may cause overheating of the screen. The total cathode current is equal to the sum of the screen and anode currents.

Another important effect occurs when a screen grid is introduced into a triode: provided that the screen is kept at a constant voltage (not varying with the signal) it reduces the capacitive coupling between the control grid and the anode and therefore helps to eliminate unwanted feedback in amplifier circuits especially at radio frequency. To take full advantage of this feature the screen grid is made with a finer pitch or smaller mesh size than would be necessary merely to obtain greater amplification, and auxiliary electrostatic shields are built into the structure in an attempt to reduce the grid-to-anode capacitance to the lowest practicable value. If the size of the apertures in the screen is made too small the electron flow to the anode will be seriously impeded, but with a reasonable compromise the residual capacitance between control grid and anode can be made 1,000 times smaller than in a triode. The improved stability in a radio frequency amplifier depends on the constancy of the screen voltage, and it is for this reason that thorough capacitive by-passing of the screen to earth (ie decoupling) is so important.

For radio frequency amplification with a resonant anode load the screen tetrode was effective, but for audio use the tetrode suffers from the disadvantage that, when the anode voltage swing is so great that on downward peaks it falls below the screen voltage, there is a flow of secondary electrons from the anode to the screen, the effect of this secondary emission is to cause a drop in anode current and a rise in screen current, which in ordinary amplifier circuits results in serious distortion and a reduction in useful power output. The search began for ways of keeping the benefits of the tetrode but to eliminate the kink.

There is another type of tetrode, known as a space-charge grid tetrode, in which a positively charged grid is interposed between the control grid and the cathode. The purpose of this positive grid is to overcome the limiting effect of the negative space charge and thus enable the valve to operate efficiently with very low anode voltage (for example, a 12 volt supply as used for mobile equipment).

See also Diode Triode Beam Tetrode Pentode Hexode Heptode and Octode.
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