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Adding Low Impedance Overdrive to a Plexi-type build

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  • Adding Low Impedance Overdrive to a Plexi-type build

    After re-reading Guitar Amplifier Overdrive I was wondering if it is possible to modify the AX84 October (http://www.ax84.com/october/October_Club_101010.pdf) to include a Low Impedance Overdrive (LIO) stage. Something like the following:

    - Add an AC coupled LND150 follower after the paralleled / jumpered 1st stages
    - The output of the LND150 feeds the 2nd tube stage + CF with the grid stopper and leak resistors selected as per Ulrich's site: https://sites.google.com/site/string...fier-overdrive

    My reading of the book leads me to think that it could be an interesting way to get differing clipping behaviour with a simple addition of some cheap silicon.

    No idea is this is advisable? E.g., can the LND150 take the signal coming off dimed 1st stages? If it can, then the following stage would show some of the potential tonal benefits discussed in the book and the above link.

    Thoughts? Has anyone tried this?

  • #2
    Try it and you tell us. Serious

    That said, you *might* have a problem: once you are above 1 or 2 V RMS, next grid retification will not only clip the signal but also shift bias, both effects shown in your link.
    It does not happen *all the time, everywhere* , in part because driving stage can not feed very much current into the next grid.

    Hidden in plain sight
    is a grid stopper .... driving stage internal impedance, which in a typical 12AX7 stage is around 40k.

    Yes, in 2 cascaded triodes the second one has an "invisible" but real 40k resistor in series with its grid ... which of course limits peak current into said grid.

    Now you add a buffer stage which is low impedance and can feed higher current ... you very well *might* drive next stage into grid blocking.

    But in any case experiment and report
    Juan Manuel Fahey

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    • #3
      Thanks Juan, I'll knock up a schematic as it would be good to get another set of eyes checking for mistakes!

      I'm finally getting round to assembling my 'modular' PCB build after a year of other distractions. The idea was that I can swap out generic preamp PCB stages to try differing approaches - the starting point being the AX84 October. My plan is for two builds following this approach - one with 6V6s and a second with a 12AU7 following the Club and Stage variants of the October.

      The modular approach is exactly to try out these differing ideas without requiring huge rewiring / desoldering.

      T
      Click image for larger version

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      Last edited by tristanc; 06-25-2018, 12:12 PM.

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      • #4
        AX84 is a killer project and a veritable "Tube amp building School".

        Best is that you can start with a simpler (but tasty) Champ tyupe one, and throwing nothing away, just ading different gain blocks and EQ you can reach a high gain monster.

        And all usable at home.
        Juan Manuel Fahey

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        • #5
          Originally posted by J M Fahey View Post
          AX84 is a killer project and a veritable "Tube amp building School".
          Completely agree. That's why this is my 5th build! Going from a P1-style to a High Octane to a Single Ended Lead, with a Firefly in there too. I've converted them to be Merlin's High Gain preamp - so this October will be a step down in 'gain' terms. But hopefully more 'mature' being push-pull.

          My favourite build, as far as sound goes, is Merlin's High Gain preamp with a Bone Ray TS coupled to the Firefly output stage - the self-split 12AU7. Perfect for the living room in a small chassis.

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          • #6
            tristanc, the site seems to be down.
            How is the structure of that amp? Like a JCM800?

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            • #7
              The October is, basically, a jumpered 6V6 plexi, tuned carefully. My interpretation of that is here: https://www.tristancollins.me/ax84/M...-style_V03.pdf

              Some tweaks here and there from Merlin's books.

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              • #8
                How do you think to rearrange the gain pots?

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                • #9
                  Originally posted by Roberto View Post
                  How do you think to rearrange the gain pots?
                  I'm hoping I can just feed any MOSFET from the two 470k mixing resistors. So no rearrangement needed. I might need an additional blocking cap to prevent DC getting to the pots from any MOSFET bias. Will get a schematic drawn up soon to show the modifications. This all, of course, might not work or sound terrible!

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