Triode Constants
The families of triode characteristics, which show the characteristic performance of each type of tube are not a result of accident. Rather they represent the outcome of purposeful design to make each tube behave in a curtain manner. The design factors of the tube are summarized by a series of numbers, called the tube constants. The three most important constants are the amplification factor, the ac plate resistance and the transconductance.
The amplification factor of a triode is a measure of the relative effectiveness of the control grid in overcoming the electrostatic field produced by the plate. To determine the amplification factor it is necessary to change the plate voltage by a certain amount, record the change in plate current and then change the grid voltage by an amount just sufficient to restore the previous plate-current value. By comparing the plate-voltage change to the grid voltage change for the same change in plate current we can determine their relative effectiveness, which is the amplification factor.
The plate resistance describes the internal resistance of the tube for the flow of alternating plate current, when varying voltage is applied to the electrodes. The ac plate resistance is defined as the radio of a small change in plate voltage to the change in plate current produced thereby, when the grid voltage is kept at a constant value. A third constant used in describing the properties of electron tubes is the control grid-to-plate transconductance designated by the symbol S. The transconductance is the most important of the tube constants, since it reveals the effectiveness of the control grid in securing changes in the plate current.
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