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Marshall Circuits 101 - Non-MV Models 1959,1987,...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by KLB, Feb 7, 2006.

  1. KLB

    KLB Member

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    In case you haven't seen it, this is a highly informative page about vintage Marshall circuits:

    http://marshall.redpt.com/clay/marshall_circuits_101.html

    It can help you in choosing/understanding the variations of the basic Marshall/Bassman circuit and the effect of different component values used in a given section of the amp.

    I originally said the following, but Matt H pointed out in a post below that the online text is technically correct.

    It is pretty accurate, with an exception or two...

    In the Slope Resistor section, it says:
    "You can even go to 16k for more bass and mid gain (and the overall frequency response flattens some)."

    With a 16K slope resistor, the frequency response does not get flatter -- it gets even more of a bass/mid-range hump. If you want flatter frequency response, go to 56K or higher.
     
  2. Matt H

    Matt H Guest

    i think you're wrong in saying they're wrong-

    marshall used two basic values, 33k and 56k in that spot.

    with 56k, you're going to get less mids- more of a scoop (closer to fender territory, no surprise given the tweed bassmans' 56k in that spot and the ultimate evolution of that amp)

    33k will give more mids... still a mid scoop- just not as severe.

    16k will still give a mid scoop, just a very slight one.


    i think you're forgetting that virtually every popular tonestack used in guitar amps, even with "all knobs to ten" gives a mid-scoop. with mids all the way up, adn treble and bass to zero- in a fender amp that approaches "flat". the notable exceptions being stacks based on the baxandall/james.

    if you'd like to see this in action- download duncan's tsc (tone stack calculator) and play with it... you can see how changing the controls works.


    it's not wrong, you're not wrong- it's just a matter of perspective... one against "zero"- and one against what our ears have been trained to think when we say "flat" with a guitar amp (which is really "frickin' scooped")
     
  3. KLB

    KLB Member

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    Matt H,
    Thanks for the correction. I was referring to my perception of the effect on the tone. In a guitar amp, true "flat" doesn't sound good to most folks.

    My Heritage Colonial has a "UK/US" tone switch to go from a 33K slope / 500p treble cap to a 56K slope / 250p treble cap. This is a very useful tone-shaping option. I generally prefer the US mode with a Strat and/or when using OD pedals -- which tend to boost midrange.

    Cheers,
    Ken
     
  4. KLB

    KLB Member

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    I clipped a .68mf cap in parallel with the stock .1mf cap on my presence control, creating a ~.78mf cap. I like having less NFB in the upper mids. It adds some "hair" to the note while keeping the tightness in the bass. Just reducing NFB alone would make the bass looser.

    With the larger cap, I'm not sure what frequency the NFB HF rolloff occurs at. Does anyone know how to calculate this?

    The amp is a Germino Club 40 with a 56K/250p tone stack, and fixed-tap 8ohm-->56K NFB loop.
     
  5. bluesaxe

    bluesaxe Member

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    I played with this stuff ENDLESSLY on my 76 JMP 1987, and finally settled on the 56k. It seems to work nicely with singles and humbuckers, and makes for a much more useable tonestack.

    bluesaxe
     
  6. bluesaxe

    bluesaxe Member

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    I played with this stuff ENDLESSLY on my 76 JMP 1987, and finally settled on the 56k. It seems to work nicely with singles and humbuckers, and makes for a much more useable tonestack.

    bluesaxe
     

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