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Thread: Tube amps.

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    Lightbulb Tube amps.

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    I have a Marshall JCM2000 DSL50 tube amp with 1960 4x12 cab. I was wondering if anyone knew exactly what the presence knob does. I know it changes the sound and makes it sound better, but does turning up the presence knob drive the tubes more for a smoother warmer sound or is it something else?

    http://www.marshallamps.com/images/p...dsl/dsl50.html

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    Quote Originally Posted by ecktronic
    I have a Marshall JCM2000 DSL50 tube amp with 1960 4x12 cab. I was wondering if anyone knew exactly what the presence knob does. I know it changes the sound and makes it sound better, but does turning up the presence knob drive the tubes more for a smoother warmer sound or is it something else?

    http://www.marshallamps.com/images/p...dsl/dsl50.html
    The presence control adds "presence". The higher you turn it, the more "in you face" the guitar sound is. The lower you turn this control the more the guitar tone softens and blends in with the other instruments.
    I was country before they took the O out.

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    I have a Fender Bandmaster with a presence control. It seems to be an eq boost that accentuates the mid frequencies (when I listen), and is in the feedback loop of the last amplifier stage, including the output tubes and the output transformer, so it may have some effect on the tube drive at those frequencies.

    http://www.kbapps.com/audio/schemati...master5e7.html

    I don't know squat about tube circuits - maybe somebody can explain its action.
    I'm going through a difficult period - it's called life

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    yeah, i think crazydoc is right; it affects the high end response on the output section of the amp.

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    It is a feedback loop. It takes the signal after the power tubes, filters out the low end, and sends it back through the power tubes again.
    Jay Walsh
    Farview Recording. I am also the forum spokesmodel for Terasyne Amplification

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    Quote Originally Posted by Farview
    It is a feedback loop. It takes the signal after the power tubes, filters out the low end, and sends it back through the power tubes again.
    This is probably just splitting hairs, but it looks like it variably shunts the high frequencies to ground. Assuming this is a negative feedback loop, this would have the effect of increasing the gain at those frequencies. I don't understand why it is done at the output stage of the circuit, rather than in the preamp section. Does it produce a substantialy different sound or effect if done this way, or is it just a matter of convenience or circuit simplicity?
    I'm going through a difficult period - it's called life

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    In Fenders, the loop feeds back into the preamp, from after the output trannie. The loop is there to keep the output from running away or oscillating in a poorly designed amp, so that is why it comes from the output stage. In a good one, it keeps the gain in the design limits. You are right, it is a negative loop, in Fenders anyway, and selectively dumps the highs from the feedback loop to ground.
    It comes after the trannie because.....I don't know.

    I would imagine the presence control is jumped on that loop like you said, for convenience.
    Last edited by boingoman; 01-11-2005 at 17:00.

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    i'm pretty sure that in my marshall (JCM 800) the presence is just a high end filter, that is, higher than the treble. am i wrong?
    A postmodern theory of audio engineering: Polyvocality of incommensurable mic-positions as a method of Behringer emancipation.

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    Quote Originally Posted by boingoman
    In Fenders, the loop feeds back into the preamp, from after the output trannie. The loop is there to keep the output from running away or oscillating, so that is why it comes from the output stage. You are right, it is a negative loop and selectively dumps the highs from the feedback loop to ground.
    I understand why the neg feedback is there - I don't understand why the presence control is part of it. What is the sonic difference in this, or simply having a high frequency boost in the tone controls of the preceding stage?
    I'm going through a difficult period - it's called life

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    Quote Originally Posted by mcolling
    i'm pretty sure that in my marshall (JCM 800) the presence is just a high end filter, that is, higher than the treble. am i wrong?
    That is the result, but it isn't part of the tone stack.
    Jay Walsh
    Farview Recording. I am also the forum spokesmodel for Terasyne Amplification

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