Slope resistor

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by bluetweed, Dec 7, 2006.

  1. bluetweed

    bluetweed Supporting Member

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    What happens when I go lower than the 100K value slope resistor in a blackface circuit?
     
  2. ChickenLover

    ChickenLover Member

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    Try this and see for yourself. :AOK
     
  3. Blue Strat

    Blue Strat Member

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    Better yet, install a 33K resistor with a 100K pot in series and HEAR for yourself.
     
  4. brad347

    brad347 Member

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    Will someone be a dear and tell me finally what a slope resistor actually does? Educate me!
     
  5. donnyjaguar

    donnyjaguar Member

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    Tell me more about this 33k & 100k pot, BlueStrat. Are you swapping the Bass or Treble control? What's the net result? Will it get rid of some of the "mid-scoop" (which is what I'd like to get rid of in my one amp) ?

    DJ
     
  6. Blue Strat

    Blue Strat Member

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    Check out the link above to be sure. It sorta sets the center frequency that the tone controls operate over. Changing the value slides the frequencies at which the tone controls operate, higher or lower in the spectrum.
     
  7. bob-i

    bob-i Member

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    Lowering the slope resistor will add bottom, take away highs and reduce the effectivness of the tone controls. Raising it will have the opposite effect.
     
  8. brad347

    brad347 Member

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    Thanks!!
     
  9. brad347

    brad347 Member

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    Bummer. I'm on a mac and mac isn't supported with the tone stack calculator.
     
  10. Blue Strat

    Blue Strat Member

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    Damn! Seen any of the recent Mac TV ads? ;)
     
  11. brad347

    brad347 Member

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    nope. I don't have a TV. :)

    What are they like?
     
  12. Blue Strat

    Blue Strat Member

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    The Mac guy tells the PC guy about all the fun things he can do on his Mac while the PC guy berates him for not being able to do any "serious" work.
    :p
     
  13. mbratch

    mbratch Member

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    Yeah, by "serious" work he actually means spending loads of time installing and running programs to try and rid your PC from ad-ware and viruses. ;)

    I'm running Linux and, happily, the TSC is supported, and I get the benefit of not having those pesky email viruses distributed by the Microsoft Virus Propagation Tool... er... I mean... Outlook. ;)
     
  14. vibroverbus

    vibroverbus Member

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    TV's? well, they're kinda like computers, except you just turn it on and wait for stuff to go by. and you can't talk back to it. well you can, but only during sporting events and political debates.
     
  15. ChickenLover

    ChickenLover Member

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    Computers are just fancy TVs...in fact, at my job we call the computers 'TVs'. "Turn off your TV Joe...it's time to go home".:D
     
  16. donnyjaguar

    donnyjaguar Member

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    Watching TV cuts into my practice time too much. :)
     
  17. Blue Strat

    Blue Strat Member

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    Not if you're practicing while you're watching:)
     
  18. tjs

    tjs Senior Member

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    In what way does it reduce the effectiveness of the tone controls?:confused:
     
  19. Blue Strat

    Blue Strat Member

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    A 7, for instance, is "the new 4". :)

    You'll have to turn them to a higher setting to get the same effect you did with the old slope resistor.
     
  20. Wakarusa

    Wakarusa Member

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    BF Fender tone stack, controls centered, screenshot from Duncan's software. Red = 100K, yellow = 56K, purple = 33K.

    [​IMG]


    So it becomes pretty obvious that it's called a slope resistor because it changes the slope of the curve as it descends into and rises out of the "mids scoop". There are also some other side-effects --
    - the range of adjustment for any of the tone controls (the effectiveness referred to above) changes with the slope resistor.
    - the insertion losses (how much the tone stack attenuates the signal overall) change with the slope resistor.
    - the center frequency of the mids control changes with the slope resistor.
    - the width of the "mids scoop" changes with the slope resistor.

    In more literal terms, the insertion loss change is due to the change in the load seen by the driving stage because the slope resistor is in parallel with the previous stage's plate resistor. Also, because the tone stack is effectively a collection of RC filters you're changing the "R" in RC which changes the center frequency of the filters and the filter's Q (the narrowing or widening of the mids scoop).

    One other tidbit to keep in mind if you try Mike's idea re: a potentiometer in place of or in addition to the slope resistor is that the slope resistor in this tone stack sees full plate voltage from the previous stage -- so if you make it a front panel control it's likely to be scratchy from the large DC voltage it sees.
     

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