cathode follower vs. plate fed tone stack?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rocknrolldaniel, Feb 8, 2006.

  1. I'm trying to get a better inderstanding about the differences between these two styles and how they actually work. From what i've read a cathode follower has less insertion loss(which I assume means it sends less signal to ground when the controls are on 0) which means it sends a stronger signal(gain) to the output section. Technically speaking, why does it work this way? A plate fed tone stack has more insertion loss because it sends more signal to ground, in effect decreasing the gain. This is why when you put the tone controls on 0 in a blackface fender you don't get much sound at all. Why does this happen? Do these different style tone controls affect the frequency response at all or simply the insertion loss?
     
  2. Blue Strat

    Blue Strat Member

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    A cathode follower has a very low output impedance, plate output has a 100K impedance. The load of the tone stack is fixed for any particular setting.

    Think of the output impedance of the preceding stage, and the impedance of the tone stack as a voltage divider. The lower the source impedance, the more signal transfers to (and beyond) the tone stack. Hence, less loss with a low impedance source.
     
  3. Shea

    Shea Member

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    Actually, a lower output impedance means that less signal is lost to ground when the controls are maxed.

    If you download the tone stack calculator at www.duncanamps.com, you can have some fun messing with different values in the tone circuit, including the output impedance of the signal source, to see what effect they have on insertion loss and eq.

    Shea
     
  4. Matt H

    Matt H Guest

    plate output has a 100k impedance? i don't think so- higher than a CF output, but it is NOT equal to the "standard plate resistor value for your average 12ax7 preamp stage" (which is where i'm guessing you're getting that number).

    i don't have the equation in front of me, and annoying enough- aiken's site is down (suspended, wtf?!) but it's something like the plate resistor in parallel with the internal plate resistance-


    an average fender 12ax7 preamp's tonestack (think blackface circuit where TS is anode driven) will have a source impedance of around 40k (60somethingk in parallel with 100k should yield around that)

    a far cry from 100k.
     
  5. Blue Strat

    Blue Strat Member

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    You're probably right, but it's still orders of magnatude higher than the a cathode follower, hence, the explanation remains unchanged.
     
  6. Matt H

    Matt H Guest

    basic principle? sure- i'm just saying it's not an issue of comparing 3k and 100k- it's a lot closer- and there isn't necessarily a "huge" night and day difference.


    the real gain difference gets made up because the CF acts as a buffer, preventing the preceding gain stage from being loaded down by the tone stack and volume pots... so it's running "full bore".
     
  7. scottl

    scottl Member

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    Many Dumbles and D-style amps use a 9.8k tail to ground from the bass pot. So, you still get plenty of sound with the knobs off. The stack is plate driven fwiw.
     
  8. Chiba

    Chiba Gold Supporting Member

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    Moved to proper area & duplicate thread deleted. Please don't cross-post your questions in the future.

    --chiba
     
  9. hasserl

    hasserl Member

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    Good stuff!

    Thanks Matt and John.
     

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