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Tube preamp stage

Discussion in 'Electronic Design' started by powerampfreak, Mar 19, 2007.

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  1. Hi all,
    Me and a friend at work argue about the behaviour of a simple tube
    preamp stage found in most guitar amp front ends. I know I'm right but
    he says he's right. Well, here we go:
    Imagine a standard circuit with 100kohms anode resistor and 1.5kohms
    cathode resistor. The tube is a standard ECC83 (12AX7). Across the
    1.5k resistor a 1u cap is in parallell.
    In my eyes this will introduce a slope at around 100hz (1/2piRC)
    simply limiting low frequencies.
    My friend (former...) says that changing the 1U cap to a 50u cap will
    increase gain. This is true, but this also affects the low frequency
    gain of the stage. Instead of 100Hz, it will go down to around 2Hz.
    But my opponent says this is not true.
    I'm not used to tube circuits, just transistors stages, but I know the
    above applies to transistor stages.
    So, would anyone just tell me I'm right in my point?
  2. Jim Thompson

    Jim Thompson Guest

    You be right.

    ...Jim Thompson
  3. Why don't you build it and measure it?

  4. Eeyore

    Eeyore Guest

    At low frequencies only. Midband gain will be the same.

    Not exactly, see above.

    Basically yes.

  5. Bob Eld

    Bob Eld Guest

    You are right, sort of. But, in the bass region below 106Hz, the break point
    of the 1u, 1.5k resistor, the gain is falling. Using a larger cap will
    increase the gain in this region by moving the break point lower. It won't
    increase the gain at high frequencies. However, the kicker is what happens
    to the gain in the critical region from 100 to 300 Hz where the fundamental
    frequencies of notes around middle C are. Here the gain will increase one to
    three dB depending on the exact frequency. This increase coupled with the
    increase in the bass will give the sound added umph or the perception of
    more "body" and power. Depending on the speakers and other factors, you'll
    probably like the sound of the added cap better than without it.
    If there is feedback around this tube stage, all bets are off.
  6. Thanks for all replies. I kind of love the situation when telling my
    opponent he's out bicycling (that's how we say in Sweden
    when somebody is not right.. :)
    BUT - I thought that bigger cap would increase the gain over the
    entire spectrum. Not only pushing the breakover point lower.

    Have I missed something?
  7. Eeyore

    Eeyore Guest

    The larger cap will indeed increase the gain over the entire spectrum
    *minutely*. The effect will simply be too small to notice though and only
    measurable with the finest equipment with digital readouts that can measure
    differences of 0.01 dB.

  8. Sorry, BUT - the capacitors impedance decreases with increasing
    Why doesn't the gain increase more than in the 0.01dB range?
  9. Eeyore

    Eeyore Guest

    Because, once the cap becomes a 'virtual' short to AC, the characteristics of the
    tube itself determine the gain.

  10. Oh, so you mean when caps Z is low enough the tube will provide
    maximum gain?
    I see....hmmm

    Thanks for your always informative replies!

  11. Eeyore

    Eeyore Guest

    Yes, I found that an ECC83's mutual conductance at 1mA for example is ~ 1.2mA/V. This
    is a bit like having a 'perfect' tube with a cathode resistance of 830 ohms. Once the
    caps' impedance becomes small compared to that 830 ohms, the gain won't increase
    significantly. I do think 1uF is rather on the small side though.

    You're welcome.

  12. Some guitar amp manufacturers seem to be using limited gain in each
    stage and instead use multiple stages to achieve the necessary gain.
    Some designs completely omit the cap. I think it boils down to
    personal taste of each designer, and what makes two amps sound
    different in the end.

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