The vacuum tube, more properly known as a thermionic valve, has been one of the most interesting and important devices in the last century. They are called tubes in the US and valves in Britain.

The simplest form of vacuum tube has three parts: the anode (or plate), cathode, and the grid. The anode is positively charged by the supply voltage and the cathode is negatively charged. The grid is used to control the current flow between the anode and cathode. A vacuum must be kept between the parts otherwise gasses would absorb the electrons emitted by the cathode.

The phenomenon that makes the vacuum tube possible is known as the Edison Effect. The Edison Effect describes the emission of electrons from a hot filament. Edison noticed this effect, but could not find any practical use for it. In 1904 a British scientist by the name John Ambrose Fleming used the Edison Effect in a device later known as the Fleming diode. This device could make alternating current into direct current. Later an American inventor named Lee de Forest made a further improvement by adding the gate to control the flow. The improvements made by de Forest allowed for the creation of amplification circuitry.

Ever since de Forest's invention of the vacuum tube there have been many further improvements. Extra parts were added inside the tubes to control the Miller Effect and other undesirable effects. The results of these additions were the tetrode, pentode, and beam tetrode tubes. Special shapes have also been added over the years to make them more suitable and more linear for use in special applications such as audio amplification.

Vacuum tubes have been used for many purposes. Almost all electronics since the first amplified radios up until the popularization of the transistor have been based on tubes. The first electronic computers were based on these valves along with mechanical relays.

Vacuum tubes are still used today. By far the most popular use of tubes now is in audio circuitry. There are two main reasons that these primitive electronic valves are so coveted in the audio world. The first of which is that they do not clip. Transistors that are given too much voltage will clip the excess off. A tube will slowly curve the signal off, rather than hard clipping. This gives tubes the mellow sound that makes them so valuable in guitar amplifiers and other classic (mostly 60's) musical equipment. The other redeeming factor of vacuum tubes in audio is the introduction of a great deal of second harmonic distortion. Second harmonic distortion adds a brightness or warmth to the sound that is desired by many listeners. The other common modern use for vacuum tubes is high power amplifiers. Some amplification circuits require so much power that any silicon transistors would be quickly vaporized by the current, so in these situations vacuum tubes (albeit very large ones) are used.

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