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lynx

Introduction to Electron Tubes

Vacuum tube, electron tube, or thermionic valve is a type of an electronic component mainly used on

  1. lynx
    Vacuum tube, electron tube, or thermionic valve is a type of an electronic component mainly used on radios and televisions since mid 20s and in terms of use it's analogous to the modern transistor. Thomas Edison, Eugen Goldstein, Johann Wilhelm Hittorf, Joseph Swan, Nikola Tesla, were among the key people involved in the development of the electron tube so later RCA to be the first company that made vacuum tubes available for commercial products.

    in more detail it's a device usually enclosed in a thin glass "envelope" almost in total vacuum and it is able to control the flow of the electric current, a simple form of a vacuum tube which is close related to the incandescent light bulbs is the diode vacuum tube.

    [​IMG]

    A diode vacuum tube consists of two electrodes named anode (plate) and cathode and a bulb filament used to heat the cathode so sufficient electron emission can be possible to the anode. This one way movement of electrons make the diode vacuum tube act as rectifier, very similar to the silicon rectifiers that we can find on modern electronics.

    Other than that, many other different types has been created, the most common types are going to be described later, such as the triode, tetrode, pentode, but the list is way too big and i will refer to the rest of them only by their names at the end of this article.

    Some precaution details before we start examining the above refered electron tubes. First of all depending the type of the electron tube, it can be hazardous to health if it is mishandled, mostly because of their operating voltages which can be hundreds of volts few milliamperes up to thousands of volts some amperes. although it is known that many types of electron tubes can also be used within safe voltage range, in fact that is not common and also it may not provide the performance rated in their specifications. Another issue that can pose a danger is the X-ray emission during the operation of some very high power electron tubes used in certain applications such as high radio signal transmission, another thing is the materials that some electron tubes are made of which can be toxic or radioactive (thorium-232, beryllium oxide) and in that case we should avoid to disassemble or brake the electron tube.


    Previously i briefly described the simplest form of an electron tube which is the diode electron tube, now let's see another type of tube which is called triode:

    [​IMG]

    The triode electron tube is just a diode electron tube with the addition of a third electrode called control grid. The voltage applied to it causes the anode current to vary in similar way a transistor can vary its output, thus a triode tube can also be used as a switch or as an amplifier.

    Another type of electron tube is the tetrode electron tube, the addition of one more electrode called Screen Grid makes it have improved characteristics and a better function on certain applications. Screen Grid provides an isolation of the control grid from the anode, reducing the parasitic capacitance between the two which helps to reduce an unwanted effect of triode tubes called the Miller effect. The screen grid is connected to a positive voltage.

    Finally a pentode electron tube has one more electrode called Suppressor Grid, a Suppressor Grid is used to suppress secondary emission, the electrode is negatively charged relative to both the anode and the screen grid and it repels any electrons trying to escape from the anode and going to the screen grid which otherwise could possibly causing negative resistance.

    Electron tubes also need a power supply for their bulb filament that is were the characteristic glow comes from and heat is created so electron emission can start. The power supply is usualy 6.3 or 12.6 Volts (few hundreds milliamperes). This is the only part of the electron tube that can efficiently operate with so low and safe voltages while the rest voltages has to be high enough so the electrons can jump the gap between cathode and anode.

    Close looking at an electron tube having a glass enclosure, other than the inner parts that consists it, we might notice a shiny black-metallic area usually at the top, that silver paint it is called the Getter, an alloy such as zirconium - vanadium that is placed inside a vacuum tube to absorb any gasses so the vacuum can be maintained.

    Other than the above mentioned electron tubes there can be tubes containing two of them in a single glass envelope and they are called double triodes etc, electron tubes called "magic eye" can function as signal strength meter, indicating the level by a special glow that they emit, also there are hexodes, heptodes, Octodes, Nonodes which are less common in use.




    Sources:
    http://inventors.about.com/od/uvstartinventions/a/Vacuum_tube.htm
    http://inventors.about.com/od/tstartinventions/a/Television.htm
    http://en.wikipedia.org/wiki/Vacuum_tube
    http://en.wikipedia.org/wiki/Incandescent_bulb
    http://en.wikipedia.org/wiki/Johann_Wilhelm_Hittorf
    http://en.wikipedia.org/wiki/Nicola_Tesla
    http://en.wikipedia.org/wiki/Thomas_Edison
    http://en.wikipedia.org/wiki/Joseph_Swan
    http://en.wikipedia.org/wiki/Getter
    http://en.wikipedia.org/wiki/Magic_eye_tube
    http://en.wikipedia.org/wiki/Pentagrid_converter
    http://blog.kotarak.net/2009/02/gettering-gu74b-4cx800a.html