Triode Valve / Vacuum Tube

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Thermionic valve or vacuum tube technology came into its own when it was discovered that the triode valve could be used to amplify electronic signals.

As the name indicates, the triode valve or triode vacuum tube uses three electrodes, the cathode and anode as in the diode and a third electrode called a grid placed between the anode and the cathode.

The addition of the third electrode in the triode vacuum tube opens up its operation making it a considerably more functional device.

Triode valve basics

The triode valve or triode vacuum tube takes the basic concept of the diode and moves it on a significant stage further. A third electrode called a grid or more correctly a control grid is placed between the cathode and anode of the basic diode and by applying a potential to the grid, it is possible to repel or attract the electrons being emitted from the cathode and in this way affect the flow between cathode and anode of the triode vacuum tube.

When there is no voltage on the grid of the triode valve or triode vacuum tube, the current between the cathode and anode will be at its maximum. Placing a negative voltage onto the grid will have the effect of repelling some electrons back to the cathode and thereby reducing the number of electrons travelling to the anode.

Varying the negative bias on the grid will modulate the current flowing towards the anode.

When a triode valve is used within in a circuit, a resistor is placed in the anode circuit. The modulated audio current between anode and cathode will appear across this resistor as a large amplified replica signal, but 180° out of phase with the input.

Triode control grid

The control grid typically consists of a helical wire that is placed between the cathode and the anode. In some instances a mesh may be used. The actual construction and ‘density’ of the control grid being dependent upon the performance and characteristics that are required from the triode vacuum tube.

The control grid in the triode valve or triode tube serves as an imperfect electrostatic shield allowing some, but not all of the electrostatic flux from the anode to leak through to the cathode.

The triode valve / tube will normally operate under space charge limited conditions. Under these circumstances the number of electrons that reach the anode, i.e. the anode current, is almost solely determined by the electrostatic field in the cathode grid space.

Once the electrons pass through the grid, they travel so rapidly towards anode that any space charge effects in the grid anode region can be virtually neglected.

In this way the negative voltage on the grid acts as a controlling voltage that controls the amount of current that flows in the anode circuit.

Typical triode valve / tube circuit

The circuit of a very simple valve or tube amplifier using a triode is shown below.

In this circuit, the resistor R3 serves to keep the grid at ground potential. Typical values for this may be around 100kΩ. The resistor, R2 in the cathode will develop a voltage across it as a result of the current flowing in the cathode - anode circuit. As the grid is at ground potential, the voltage across R2 will be the amount by which the grid is negative with respect to the cathode.

The resistor in the anode circuit, R1 develops a varying voltage across it as the current through the triode valve varies.

Capacitor C1 provides coupling on the input and the capacitor C2 provides coupling for the AC signal on the output, blocking the DC which is likely to be high.

The capacitor C3 acts as an AC bypass capacitor across the cathode resistor R2. This increases the AC gain of the circuit whilst retaining he required DC bias conditions.

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