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The Pentode

Author: N.H. Crowhurst

In a triode, the plate current is controlled by the combined electric field at the cathode due to both the grid and the plate. Because of the triode's open grid structure, the plate voltage influences the plate current. If we could decrease this dependence of plate current on the voltage at the plate we could boost the amplification of the tube.

This is what the pentode type tube does. Two extra grids are inserted between the number 1 or control grid and the plate. The first of these, the screen grid, is maintained at a constant positive potential and, from the viewpoint of plate current, is responsible for providing the electric field at the cathode. This means the control-grid voltage controls the electron flow from the cathode in conjunction with a constant field derived from the screen grid, not a fluctuating one from the plate. Because the screen grid is an open mesh and not a solid plate, most of the electrons reaching it go through the spaces instead of hitting the wires.

Fields and electron paths in a pentode

How secondary electroncs behave without a third grid

The second extra (i.e., third) grid is inserted to stop electrons from bouncing back from the plate to the screen grid when the plate potential is lower than the screen potential. If the second grid has the highest voltage of all and there were no third grid, electrons would hit the plate, cause further electrons to bounce off it, and be attracted by the more positive voltage at the screen grid. This secondary emission could even result in negative plate current.

The third (suppressor) grid prevents secondary emission by providing another electric field (between the suppressor grid and the plate) that discourages the bouncing off of any electrons. The third grid does not collect any electrons, because its voltage is so much more negative than that of both the screen grid and the plate. It merely serves to insure that all the electrons that come through the spaces in the screen grid arrive at the plate and stay there.

With third grid at same voltage as cathode, secondary electrons are always sent back to plate

This results in a tube in which the plate current is almost completely independent of the plate voltage, and is determined by the voltages on the control and screen grids. (Usually the potential on the screen grid is kept constant, and the audio fluctuations are applied to the control grid.) The plate resistance of this type of tube runs to very high values - perhaps several megohms (millions of ohms). A pentode with a transconductance of, say, 2 milliamperes per volt, and a plate resistance of 1 megohm has an amplification factor of .002 X 1,000,000 = 2000.

Obtaining pentode plate curves

This much amplification cannot be achieved in practice because a very high plate load resistor would be necessary. Supposing that we use a 1-megohm coupling resistor, using the formula we should achieve a practical amplification of 1000. As we shall see presently, even 1 megohm is rather a high value for a coupling resistor, unless we will be content with poor-quality amplification. A resistor of about 220K, however, is quite normal and will give an amplification in the region of 360, using a tube with a transconductance of 2 milliamperes per volt.

Last Update: 2010-11-03