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Gain Stage Topology

Discussion in 'Shock Brother's DIY Amps' started by jmp81sc, Dec 1, 2020.

  1. jmp81sc

    jmp81sc Tele-Meister

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    Hi Everyone,
    I am looking for advice on my project. I am pretty new to building amps and this is my first effort at modifying a design.

    I am in process of building a hybrid amp with a Brown Deluxe channel and a Brown Super channel. The Brown Super has an unused triode, so my idea was to use it as a switchable added gain stage to the Deluxe channel.

    I copied the topology of the Mesa Mark I and designed it to go after the first triode and before the tone stack, but after reading Kley's thread on his new build and looking at his schematic and the Merlin book I am having second thoughts.

    deluxe_6g3 input_schem.gif.png
    I am thinking of using this design instead. Which places it after the tone stack and recovery triode. Anyone have any ideas on the advantages or disadvantages of either design?

    Thanks
    John

    6g3 v2.png
     
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  2. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    You could try it both ways. There's hardly any loss to "recover" with this tone control, not like a TMB tone stack.

    If I were building it, I might even put another tone control *within* the extra gain stage, so that the extra distortion doesn't sound too bright compared to the low-gain mode.
     
  3. drneilmb

    drneilmb Tele-Meister

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    The general idea here is that a tone stack can only regulate the frequencies that are being fed into it. A triode gain stage driven into non-linearity produces additional frequencies that we hear as the characteristic distortion sound.

    A tone stack before the gain stage adjusts the input to the distortion process, but you take what you get out from it. A tone stack after the gain stage adjusts the frequency content of the distorted sound.

    For example, truly saturated flat-top distortion introduces high frequencies that are not in the original signal. Those high frequencies can make the distorted signal sound "fizzy". Adjusting the frequency content before the gain stage can't fix the fizz because the fizz is being created in the non-linear gain stage. A tone stack after the gain stage can remove some of the fizz by adjusting the tone of the distorted signal.

    Much of the individual character of various distortion circuits in amps and pedals comes from the different frequency filtering that is applied before and after the distorting stage.

    -Neil
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    During the post-silverface era they figured out that high gain amps need a late tone stage for reasons detailed by drneilmb above. Soldano put his tone stack after the last preamp gain stage but before the FX loop for those reasons. Without a late tone stack you'll need to add something late in the circuit such as a presence control or ice pick cap around the NFB resistor to tame the highs.
     
  5. mrriggs

    mrriggs Tele-Meister

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    I built my latest amp with the bass/mid before the distortion phase and the treble control after it. Cutting bass before the distortion really helps clean up the flubbyness. As mentioned previously, cutting treble after distortion helps tame the fizzy harmonics added by the distortion. You can still cut the treble going into it using the tone control on the guitar.
     
  6. Wally

    Wally Telefied Ad Free Member

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    yes, Soldano based his circuit in that 5F6A world and placed all,of the added gain stages prior to the tone stack. That said, the Hot Rod 50 I had around for a while had the harshest top end distortion I have experienced....until I put a 12AY7 in V1. That totally changed the amp into something usable and imho much more musical. I had a boutique Germino Classic 45, which is based in that 5F6A world as well right beside it that did the same thing; and a 12AY7 cured that harsh high end distortion as well.
     
    Last edited: Dec 2, 2020
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  7. solfege

    solfege TDPRI Member

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    For a pedal version of this approach, see Doug Tuttle's (midfi electronics) Magic "i." He has fun demos on his page for it showing exactly how the sound shaping can work.
     
  8. jmp81sc

    jmp81sc Tele-Meister

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    Thanks everyone for your input. After reading your replies I have been looking at schematics and trying to learn. I will have some tweaking to do, but I am thinking of copying the Marshall cold clipper gain stage as a starting point. Per usual Rob Robinette's web site has been a great resource, explaining the cold clipper and the Cascade mod section.
    thanks
    John
     
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