The simplest vacuum tube is the Diode in which two electrodes has been sealed within an evacuated glass tube. The electrode called the filament or the cathode emits electrons when heated by the passage of electric current. The electrons form a cloud called the space charge and move to the other electrode called the plate or anode being attracted by its positive potential with respect to the cathode. This results in a flow of electric current in the circuit from anode to cathode. Because the flow of electrons from cathode to anode inside the tube can not be reversed, the diode tube can act as a rectifier to convert a.c. to d.c. or as a demodulator of r.f. signals.

With the addition of another electrode between the anode and the cathode, the vacuum tube becomes a Triode. This third electrode is called the control grid and is in the form a fine mesh. The triode is a voltage-controlled device. By applying a voltage to the grid the electron flow from the cathode to the anode can be regulated. The relationship between the grid voltage and the anode current is a function called the transconductance (unit is mho). This function accounts for the use of the triode valve as an amplifier.

A fourth electrode, the screen grid takes care of oscillations due to anode-to-grid capacitance and the electron tube is named a Tetrode. More electrodes are required for special uses other than amplification. Tubes containing eight electrodes, an Octode have been manufactured. For the purpose of frequency conversion, the heptode or the pentagrid converter is suitable. There are multi-functional vacuum tubes such as triode-hexode, double diode-triode, etc.

During the middle of the 20th century, vacuum tubes became so commonplace that these were used in many DIY projects for building amplifiers and radios.

Some of the scientists contributing to the invention of the vacuum tube are Thomas Edison, Nikola Tesla, John Ambrose Fleming, Lee de Forest, Hull and Williams.

Vacuum tubes are making a comeback among High-Fidelity audio enthusiasts because of their ability to deliver superior sound quality and being less susceptible to power surges and transient overloads than solid-state devices.

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An additional electrode in the form of a small screen-like grid is sometimes contained in the tube, which is then called a triode, which is more efficient and able to amplify the voltage. As voltage is applied to the grid, the flow between the filament and plate can be varied. Rather than using a suppressor grid it used a new arrangement connected to the screen grid. This tube became so popular that it was later modified for RF applications by giving it a top cap for the anode. SED (C logo) sells the SV83, which is similar to the EL84 except for a lower screen-grid rating of 200 volts.

The screen-grid in the pentode disconnects this capacity, and makes the stage inherently stable. It is used as a switch, amplifier or display screen. Used as on/off switches, they allowed the first computers to perform digital computations.

Audio tubes are over a $200+ million dollar industry in the USA and possibly $500+ million dollars worldwide. The most popular tube made now is the 12AX7, followed by the 6L6GC and EL34. Audio tubes are designed for the 0V bias rule, horizontal output tubes (being essentially switches) aren't.

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