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Thoughts about tube output stages...

Discussion in 'Electronic Design' started by Steve, Apr 1, 2007.

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  1. Steve

    Steve Guest

    Hi all,
    Never worked much on tube circuits, though lately my interest has
    increased for this old technology.
    I thought some questions could clarify a few things:

    1. The bias current through the tubes, this is pretty much as the idle
    current in a solidstate design, right?
    2. How is the idle current set?
    3. As far as I can see, the idle current flows through the output
    transformer, why doesn't this saturate the transformer?
    4. What function does the screen grid have? Some design apply full HT
    to these grids while others apply HT through a resistor. Do these have
    an influence of the gain in the tube and/or the idle current?

    Thanks, I'm sure I'll come up with some more questions, hope it's ok
    to ask.

  2. Bruce Varley

    Bruce Varley Guest

    By the bias on the grids. Typically, there'll be a DC negative voltage set
    to the grids via resistors, or through the centre tap of the driver
    transformer. It's common to have a pot to adjust this. Normally there'll be
    a few specified conditions in the data for the tubes, typically they'll be
    'class AB', with some standing current, and 'class B' with significantly
    less. As you get closer to class B linearity becomes more of an issue.
    Not a problem for a pushpull stage, for single ended the tranny has to be
    designed for it. Single ended power amps are very inefficient, you'd rarely
    have one above a few watts IME.
    Generally the screen supply will be lower than the anode voltage. Once
    again, the specs provide the correct value. A screen supply above what's
    specified can damage the tube, because the grids are relatively fragile. A
    correctly set up tetrode or pentode will have higher gain than an equivalent

    There are unofficial variants. I remember driving a pair of 807s in a
    modulator with the input on the screen grids and the control grids fixed to
    control the standing current. It really smoked.

  3. Tim Wescott

    Tim Wescott Guest

    For a complete exposition on the function of the screen grid you need to
    find an old ARRL handbook, RCA tube data book, or other text from the day.

    Basically, a screen grid screens the cathode from the plate, so the
    cathode current isn't affected by the plate voltage. For most of the
    tube's operating region most of the electrons are captured by the plate
    even if its voltage is lower than the screen's, so the effect of the
    screen grid is to significantly raise the plate resistance. This also
    increases the "power sensitivity", or the ratio of power out to voltage
    in, significantly over a triode. The downside is that you must pay
    attention to the care and feeding of the screen.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

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    "Applied Control Theory for Embedded Systems" gives you just what it says.
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