Vacuum tubes or thermionic valves, and low-pressure gas-filled tubes, or discharge tubes were used before the advent of semiconductor devices. Thousands of tube types were used in consumer electronics. Many industrial, military or otherwise professional tubes were also produced. Only a few types are still used today, mainly in high-power, high-frequency applications.

The basic components of a vacuum tube are a glass tube (containing a vacuum), an emitter of electrons, called a cathode and a collector of electrons, called an anode.

A vacuum tube with two electrodes is called a diode (a di-ode) and acts as a rectifierI have no idea what this means because current (electron flow) can only pass from cathode to anode, not the other way around.


By adding a control grid between the anode and cathode produces a triode (tri-ode) where, varying the current between the grid and the cathode, the flow of electrons to the anode can be controlled.


Applying a positive voltage to the grid accelerates electrons towards the anode (most of them miss the grid and pass right through) and increases the current flow between cathode and anode; applying a negative voltage to the grid repels the electrons and, if it is high enough, can stop the current flow all together.

So, by controlling the current on the grid voltage, you can control the current passing from cathode to anode - a primitive electronic switch. Also, if the grid voltage is quite small, but the main voltage is high, a small variation in the grid voltage can cause a large change in the main voltage and the vacuum tube can act as a crude amplifier.