Where to place the bigger gain stage?

Discussion in 'Amp Tech Center' started by jsnwhite619, Feb 24, 2020.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I'm working on a fixed bias 6v6 Tweed-type build that I'm trying to piece together and was wondering about the effects of gain stage placement. It's originally a Harvard-type circuit with several changes, one being stealing the Princeton Reverb NFB circuit and adding a bypass cap gain stage on V2 instead of the single V1 cap of the original. I'm looking for something mean and good overdrive, but not just turn to mush. What are the pros/cons/expectations to

    1. Full Fender 25uf bypass cap on V1, then a smaller .68uf-2uf boost on V2
    2. Keep V1 more low key with a smaller .68uf-2uf bypass cap, then go big in the second stage.
    My current inclination is staying smaller on V1, then boost it harder later on, but I often end up looking at things bass ackwards and doing the exact opposite thing that I should. My reasoning - in my mind - is that keeping it cleaner in the early stage would lend itself to a tighter & more articulate sound than just going full throttle from the very beginning.

    Thanks
     
  2. Phrygian77

    Phrygian77 Tele-Afflicted

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    1uF matches up well with a 1.5k. kind of gives you that Marshall-esque .68uF roll off with a lower cathode resistor. Playing with a few vintage '40s Fenders lately, I think vintage output pickups make a big difference.
     
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  3. Nickfl

    Nickfl Tele-Afflicted

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    I think you might be a little off base in the way you are picturing the bypass cap value affecting the amps behavior. Remember the value of the cap doesn't so much affect the degree to which the signal is boosted so much as it affects the frequency range that is boosted. So a smaller bypass cap isn't going to really increase headroom or cause later onset of breakup, it is just going to create more of a treble boost effect and less flabby bass response.

    Personally, I would go with the smaller values in both positions if its its going to be a higher gain amp. But really, as much as you are changing the circuit you are going to have to experiment with both those cap values and the level of NFB.
     
  4. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    I agree. You may want to increase the cathode resistors value as well.

    You should consider the loss of gain thru the tone stack. A "Raw switch" or pot will help keep some of the gain of the first stage.

    The coupling caps throughout should be reduced to get rid of mud. Use a high pass filter calculator to adjust the coupling caps to adequate high gain levels.

    Usually bass freqs are reduced in the early stages and brought back up in the later stages. (This is done with cathode bypass caps and with coupling caps). The goal is not to distort bass freqs.

    Increasing the preamp plate resistors is commonly used to get a grittier gain.

    Usually high gain amps have larger filter caps.
     
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  5. radiocaster

    radiocaster Poster Extraordinaire

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    You can experiment by clipping on a cap without soldering it on. Do it in both places you want to try, with the chassis open and connected to a speaker.
     
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