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Vacuum Tubes

Discussion in 'General Electronics Discussion' started by 20849, Jan 2, 2017.

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  1. 20849

    20849

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    Jan 2, 2017
    I am trying to understand vacuum tubes. I have read that they are rectifiers and amplifiers. Do some pass an AC signal thru
    the tube?? I thought they were only for rectifying......is there a hobby kit that lets you set up tube circuits and gain an understanding of how they do the different things they do..??
     
  2. Minder

    Minder

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    Apr 24, 2015
    The first detection of the vacuum tube effect was when it was discovered that when a light bulb was lit a cloud of electrons boiled off from the element, it was then discovered that if a plate was introduced and a +ve voltage WRT the element (cathode) was introduced to the plate, the stream of electrons were attracted across the vacuum creating a DC current through the lamp or what became as the first tube rectifier.
    After that came the grid between the two in the current stream which then controlled the degree of current stream.
    It was known as a trans-conductance amplifier, a varying voltage controlled the current
    One of todays closest semiconductors to achieve a similar effect is the Mosfet.
    M.
    '
     
  3. 20849

    20849

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    Jan 2, 2017
    so the higher the voltage the bigger amp effect..??
     
  4. Minder

    Minder

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    Apr 24, 2015
    In a simple triode amplifier Cathode-Grid-Anode, the higher the grid voltage the higher the tube current.
    So in this respect it is amplifying.
    The vacuum tube demonstrates electron flow, negative to positive, which is in contrast to what is known as conventional flow in circuitry, of Positive to negative.
    M.
     
  5. Ratch

    Ratch

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    Mar 10, 2013
    Both FETs and BJTs function as a transconductance amplifiers.

    The charge carriers in vacuum tubes are electrons only (unipolar). The charge carriers in BJTs are both electrons and holes (bipolar). The charge carriers in FETs are either electrons or holes, but not both (unipolar). Conventional current is a mathematical protocol which assumes that all charges are positive and flow from positive to negative. The mathematical direction of flow may not agree with the real physical flow. Ammeters and semiconductor diodes are marked in accordance with the conventional mathematical protocol.

    Ratch
     
  6. Audioguru

    Audioguru

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    A vacuum tube and a transistor do not pass AC, instead they pass DC that is modulated with an AC signal.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I have the perfect answer that should answer the OP's every question.

    "Valves are transconductance devices."

    :)
     
  8. elebish

    elebish

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    Aug 16, 2013
    Actually, a tube will generate a signal that looks just like the signal applied to the control grid. It does this by varying the current drop across the plate load resistance which is the output signal. All this is dependent of the plate voltage, load resistance, cathode potential and the grid current. Conduction occurs when the filament is heated. Think of a tube as a 'valve'. Remember, an open grid will allow the tube to run wide open.
     
  9. davenn

    davenn Moderator

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    and there you have a rectifier
     
  10. Minder

    Minder

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    ONE of today's ....
    M.
     
    davenn likes this.
  11. Ratch

    Ratch

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    Mar 10, 2013
    Both FETs and BJTs are transconductance devices.

    Ratch
     
  12. BobK

    BobK

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    However, one of them draws a lot of current through the input, operates over a very limited range of input voltages and is far from an ideal transconductance device.

    Bob
     
  13. Ratch

    Ratch

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    Mar 10, 2013
    Yes, and the filament is an energy hog.

    Ratch
     
  14. BobK

    BobK

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    yep,nothing is perfect, but I think the JFET is the closest.

    Bob
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Had there been active development I'm sure this would have reduced. Some 1950's era valves had heaters which consumed in the low tens of mW. I wonder if there was ever research on "room temperature thermionic emitters"?

    However I can't imagine an integrated tube with a billion devices in a single envelope connected to a board using BGA!

    Edit: I want my vacuum tube iPhone.
     
  16. BobK

    BobK

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    Jan 5, 2010
    Me too. It would have a warmer, more expansive sound with greater depth and richness. Filaments are not a problem, my iphone is already warm enough to emit electrons when running some apps.

    Bob
     
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