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  • Low watt amp cathodyne nfb issue

    Hello guys,
    i've been desining a low watt amp as a PP of ecc99. It has a cathodyne PI and the NFB signal is injected in the stage before the cathodyne. The amp is working good, except that the presence and depth pots don't do nothing (almost). The values i took are for a marshall.soldano PI (long tail). I wonder why it does not work as well as in those amps. First the signal from the 16 ohms tap is quite weak, i had to reduce the nfb resistor to 1K5. If i put 4.7k i have almost no feedback. The depth pot is effective only in the 20 first percent and acts more as a loudness control, not only the low end is pushed. And the presence pot does nothing at all.

    Which values of pots and capacitor, or circuitry to get effective resonance controls?

    I post the preamp and power stage, the PSU is very common...

    Thanks for the help you can bring me...
    Attached Files

  • #2
    First of all the NFB series resistor needs to be before the presence circuit. But a value of 1.5k looks much too low for a presence effect.
    Problem is that your NFB loop uses a very low impedance cathode input.
    I think you will have to use lower resistance and higher cap values in the presence/depth network.

    Is this the circuit you want to copy?:
    https://www.ampbooks.com/mobile/amp-.../slo-feedback/
    http://diy-fever.com/wordpress/wp-co...CIAL-SCHEM.pdf
    Last edited by Helmholtz; 02-10-2021, 07:53 PM.
    - Own Opinions Only -

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    • #3
      Originally posted by Helmholtz View Post
      First of all the NFB series resistor needs to be before the presence circuit.
      In the soldano slo100 preamp, the 39k resistor is after the depth control though

      Originally posted by Helmholtz View Post
      But a value of 1.5k looks much too low for a presence effect.
      If i put more, i have no effect at all
      Originally posted by Helmholtz View Post
      Problem is that your NFB loop uses a very low impedance cathode input.
      ???
      Do you suggest to inject the nfb into the cathodyne directly? I inspire from https://chasingtone.com/meta-4/
      Originally posted by Helmholtz View Post
      I think you will have to use lower resistance and higher cap values in the presence/depth network.

      Is this the circuit you want to copy?:
      https://www.ampbooks.com/mobile/amp-.../slo-feedback/
      http://diy-fever.com/wordpress/wp-co...CIAL-SCHEM.pdf
      Yes i wanted to adapt it on my build
      Last edited by tepsamps; 02-10-2021, 09:43 PM.

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      • #4
        Originally posted by tepsamps View Post

        In the soldano slo100 preamp, the 39k resistor is after the depth control though-
        Doesn't mattter if it's before or after the depth control. But it needs to be before the presence control. Soldano has it between both.


        If i put more, i have no effect at all
        I understand that, but adapting the NFB network isn't as simple as copying the values of a different amp.
        You need to re-calculate (or simulate) filter time constants taking into account your open and closed loop gains in relation to the considerably lower NFB input impedance.
        Simply put, if your NFB series resistor needs to be lower by a factor 25, the presence cap value has to be increased by 25.But there's more to be changed.


        Do you suggest to inject the nfb into the cathodyne directly?
        No, that would mean too little loop gain, as the voltage gain of a cathodyne is lower than 1, while a LTPI is around 25.


        Last edited by Helmholtz; 02-10-2021, 10:05 PM.
        - Own Opinions Only -

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        • #5
          Thanks. More like this ?
          Attached Files

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          • #6
            Originally posted by tepsamps View Post
            Thanks. More like this ?
            Better but I think you need to lower the impedance of the depth circuit as well. Maybe try 47nF with a 100k pot. There is too much interdependence of component values to do a quick calculation.
            So you need to experiment.
            - Own Opinions Only -

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            • #7
              I've put 100kA/47n for the depth and 5kB/1F for the presence, i don't have 1k pot for the moment. It's fine for the depth, and for the presence it's just OK, i don't have the fizz of the LTPI tho. The NFB serie resistor is 1k5, it's located before the depth.

              Next one will be with a ltpi, but needs one tube more

              Thanks

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