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Tube preamp

Discussion in 'Electronic Design' started by Jeff Johnson, Jan 22, 2011.

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  1. Jeff Johnson

    Jeff Johnson Guest

    I'm playing around with my amp's tube preamp(12AX7). I have set to pots for
    the bias for both degeneration and cathode, i.e.,

    R1
    |
    +-C-Gnd
    |
    R2
    |
    Gnd

    R1 is the degeneration resistance and R2 + C is the cathode bias.

    What I don't understand is that when R1 and R2 are almost 0(< 100 ohms) I
    still get a rather decent signal even with the grid has a signal of at least
    1V p2p. I would expect a significant amount of clipping on the positive half
    cycle. I have checked the voltage on the grid and it is 0 and the cathode at
    around 200mV or lower.

    I get the input signal and output signal on and they are rather very close.
    Changing the bias from around 0 ohms to 2.5k ohms essentially just starts
    clipping the peaks. This is fine except I would expect at around 0 that the
    positive part of the signal would be completely clipped.


    What am I missing? (I've checked everything and all the measurments are
    correct). All I can think of is that the grid is being driven positive but
    the tube's plate resistance isn't dropping a significant amount to create
    clipping(the tube could be bad).
     
  2. Pimpom

    Pimpom Guest

    Except at very low plate currents (relative to normal operating
    levels), triodes have quite a linear transfer with a positive
    grid. With zero bias, a high source resistance could result in a
    distorted waveform at the grid input due to grid current on the
    positive half cycle. Otherwise, the input-output characteristics
    should not show serious clipping just because the grid goes
    positive.
     
  3. Guest

    Are you sure that the R1 potentiometer is actually working properly
    when in the extreme position and is not erratically open ?

    Indirectly heated small signal tubes have such strong electron
    emission from the cathode that it will drive a floating grid slightly
    negative.

    Look at various small signal RF/IF/audio (especially pentode) stages
    from previous decades, they often have the cathode directly grounded
    and a few megohms resistor from grid to ground, with the signal AC
    coupled to the grid.

    The electron charge cloud will bias the grid to several volts negative
    respect to ground if completely floating. The resistor just leaks
    excessive charge in order to make the bias more predictable.,
    resulting typically to -1 .. -2 V, which is enough for small signal
    operation.

    This grid to ground potential can be measured with a voltage meter
    with sufficiently high input impedance ( > 10Mohm).
     
  4. Rich Grise

    Rich Grise Guest

    Grid-Leak Bias.
    Hope This Helps!
    Rich
     
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