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After the diode invention, Lee de Forest had the idea to put a grid between the cathode and the plate. By applying a variable negative voltage, he managed to vary the current flowing through the tube.
Electric charges of the same polarity repel each other thus a negative charge, with its excess of electrons, applied to the grid push back the electrons coming from the cathode.
The electronic era was starting ...

A highly negative grid can stop completely the current flowing in the tube.

A less negative grid lets a small current flowing.

A grid at the same voltage than the cathode lets a high current flowing.

A positive grid not only drives the tube to saturation (the current will no more increase after a certain level) but a grid current appears also.

the variations of the grid voltage provoques a variation in the tube and plate resistor current and thus a voltage variation at each side of this resistor (U=R.I).

With a constant plate voltage, a variation of the grid voltage gives a variation of the plate current.

With a constant grid voltage, a variation of the plate voltage gives a variation of the plate current.

Amplification Factor Calculation.

In this example, a change of 2V in the grid voltage gives the same current change as a plate change of 40V.

= Ri.S

where Up = plate voltage
          Ug = grid voltage
          Ri = internal resistance of the tube
          S = tube slope

The amplification factor of a tube is called "miu" and, in this example, equals 20.

Internal Resistance Calculation

With a constant grid voltage.

where Up = plate voltage
          Ip = plate current
          Ri = internal resistance of the tube in Ohms

Static Slope Calculation

where Ri = internal resistance of the tube
          Ug = grid voltage
          Ip = plate current
          S = tube slope in A/V or mA/V

Gain of a plate-loaded stage

The gain of a tube stage, whatever the tube type, is computed by:

where Ri = internal resistance of the tube
          Rp = plate resistor

Triode Curves

This curve indicates the plate current variation when applying a variable grid voltage and a constant plate voltage.
The linear part indicates the grid voltages where the tube could operate with low distorsion.
in this example, the cut-off voltage is around -14V.

Its a typical figure of an audio amp.

This curve indicates the variation in the plate current when applying a variable plate voltage and a constant grid voltage.

To obtain big variations in the plate current, it's much more effective to apply a variable voltage to the grid than to the plate.

This curve indicates the variations of the plate voltage when changing the grid voltage.

You can see that a lower grid voltage gives a higher plate voltage. The signal is inversed.

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