When a vacuum tube is used as a signal amplifier, it is so configured that an input signal is provided to the grid of the vacuum tube and the amplified output signal is obtained at the plate. Such a conventional vacuum tube amplifier is described in Japanese laid-open patent publication No. Incidentally, the terminal that collects the electrons in a vacuum tube is called the anode. The part of a vacuum tube which is heated, to "boil off", or emit electrons, is called the cathode. The hardest thing to control making a vacuum tube is the metal to glass seal- depending on the type of glass and ESPECIALLY your choice of metal, a good seal can be hard to achieve. Many Nixies have gone bad due to their seal.

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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.