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Usable range, Frondelli master volume

Discussion in 'Shock Brother's DIY Amps' started by Diverted, Jan 28, 2021.

  1. Diverted

    Diverted Tele-Meister

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    Hi all,

    A friend just had me build him a Fender Harvard 5F10, and asked that I put in a master volume. I was reading on Rob Robinette's page about the Frondelli. It seemed better due to not having to use the fail-safe resistors.
    I put it in and I'm not so sure I like it. The pot has pretty much zero functionality until you turn it down to about 1.5. From there to 0, the pot kicks in. This requires extremely slight adjustments of the pot to get it to where I want it.
    Does this have to do with the audio taper of the pot? I'm using a dual gang 250KA pot. I wonder if a linear pot would be better?

    So my question: Is this just a characteristic of Frondelli that has to be lived with? Or is there some way to make the sweep more effective and spread out from 1-10, and not have to make minute adjustments of it?

    And do different MVs, such as Lar-Mar, offer greater sweep? If so I may end up switching it out.

    Thanks,

    Ted
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Hey Ted, a linear pot will make it even worse. Did you add the "MV AC Ground cap" shown in gold below? If not give that a try, it should help with the volume taper.
    [​IMG]
     
  3. Diverted

    Diverted Tele-Meister

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    No, I didn't put that in but will try that. Thanks!
     
  4. sds1

    sds1 Tele-Afflicted

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    I thought of this too but pulled out the 5F11 schematic, seems that bias circuit already has a quick short to ground for signal.

    Still curious if it helps him. My guess is no but hope I'm wrong.
     
  5. Diverted

    Diverted Tele-Meister

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    Just dropped in a 10uf/100V non-polarized cap between the bias voltage and ground. It helped a little bit, but a step in the right direction. Now I'm getting discernable difference in volume at anything under 3. Still not a lot of range, but better.
     
  6. Diverted

    Diverted Tele-Meister

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    Would a different MV type be more usable in this amp? Say, a Lar-Mar or across the line? Switching to LarMar would be 10 minutes of work. While I could do an across the line, it would involve a little more work as I built the board without the grid leak resistors or eyelet for the negative voltage. I could do it no problem, it would just be a little shoehorning.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    It might and since it's fairly easy to re-wire to Lar-Mar I'd give that a try.

    You are right on the 5F11 having the bias cap in the right place to make the "MC AC cap" unnecessary. I missed that when I reviewed the schematic.
     
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  8. Diverted

    Diverted Tele-Meister

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    I'm going to try that one this afternoon. I'm assuming there's no issue with just leaving that 10uf to ground in place? Also the amp's the 5F10 not 11, but that really doesn't matter since it's the same bias circuit. Thanks for your advice! Will advise on how it worked out.
     
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  9. printer2

    printer2 Poster Extraordinaire

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    It sounds like you have a linear taper pot or the pot wired up backward if it really is an audio taper. I had an audio taper labeled pot that really was a linear. With the amp off set the pot to noon and checl the resistance on either side of the pot. if they are the same then it is a linear. If audio it should have more in the inverter side.
     
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  10. Diverted

    Diverted Tele-Meister

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    WOW. Lar-Mar all the way. Just rewired the existing pot and what a difference. Smooth decrease in volume from 10 all the way down to 1. It's like night and day. Thank you all for the help! IMG_6617.jpg
     
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  11. Ten Over

    Ten Over Tele-Holic

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    The problem isn't in the components used. The problem is that the Frondelli MV is bogus.

    The Frondelli MV works by loading down the triode outputs. It turns out that a cathodyne can drive incredibly heavy loads as long as both outputs are loaded the same, so there is no significant change in gain until the MV pot is turned way down.

    Loading down the outputs also decreases the peak voltage at which the signal clips.

    The low frequencies become more attenuated as the MV pot is turned down in accordance with the decreased R in the 2*pi*R*C figure of the corner frequency equation.
     
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