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Volume sensitivity issue

Discussion in 'Shock Brother's DIY Amps' started by bryansbizaar, Apr 9, 2017.

  1. bryansbizaar

    bryansbizaar TDPRI Member

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    Hi there,

    I’ve been trying to chase down a volume issue with a new build and would appreciate any insights as to what may be going on.

    The amp is a Princeton Reverb clone (AA1164 circuit – see build pics here http://www.tdpri.com/threads/trouble-shooting-new-pr-clone-build.715687/). I also have an original Fender ’78 PR as well that I’m using for comparison.

    Here’s the problem: the volume doesn’t register until I hit 2/2.5 on the dial. It does get progressively louder, but it is vastly different than the ’78 PR. For example, the volume pot on the SF is quite sensitive and is rather loud by 2-3 on the dial.

    The volume pot in the clone was a mojotone CTS 1MA. I ended up pulling an Alpha 1MA pot from another amp for comparison. The Alpha was noticeably better, however it still wasn’t quite as sensitive as the ’78.

    I concluded that the CTS pot was bad, so I then swapped back in another new replacement mojotone CTS 1MA pot. Unfortunately it was no different than the first one.

    I double-checked the circuit and the voltages all seem to be in the ballpark with the schematic.

    What I want to know is if the issue is with the potentiometer or the circuit (or both). Any thoughts?
     
  2. jackinjax

    jackinjax Friend of Leo's

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    The CTS and the Alpha 1MA pots you used are both audio taper (the A in the part number) rather than linear taper.

    [​IMG]
     
  3. bryansbizaar

    bryansbizaar TDPRI Member

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    Yes - that's what was specified on the schematic.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Based on your description, personally I would prefer the action that you are getting with the build amp....if by "quite loud at 2-3 on the dial" you mean that the real Prin Rev doesn't have much more to offer above that. You do say that the clone gets "progressively louder" as you dial the pot towards 10.
     
  5. bryansbizaar

    bryansbizaar TDPRI Member

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    The Fender does have more headroom past 3, but it is quite full at this point. Overall, the volume knob is more sensitive. This model has the volume boost (w/push-pull switch), so may account for some of the difference.
    I just didn't expect such a difference between the pots. The Alpha is a bit closer in the ballpark, but it doesn't fit quite as well (the shaft is a little shorter).
    There is a significant difference at 3 on the dial between the clone w/ the CTS pot and the Fender. Has anyone else had this experience?
     
  6. clintj

    clintj Friend of Leo's

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    I've gotten a dead spot from overheating a casing during soldering before.

    Put the pot at mid travel and check from wiper to each outside lug. There's a couple of tapers out there, usually audio are 15% at mid travel, but you occasionally will find a 25 or 30%. 15 comes on a lot more gradually.
     
  7. tele_player

    tele_player Tele-Meister

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    Clintj - exactly right.

    I have a bunch of 30% taper CTS pots which are pretty close to what Fender used. Jelle Wellagen had CTS make a few thousand a year or two ago, and made them available. Some of them went to one of the usual amp parts shops, but I forgot which one.

    I'd sell up to 20 of them, $5 each shipped in the US, minimum 5.
     
  8. tubejockey

    tubejockey Tele-Holic

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    You may need to lower the gain in the preamp. A 5751 tune in the v1 slot or modify the cathode resistor. What you describe is quite common, some like it some don't.
     
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  9. bryansbizaar

    bryansbizaar TDPRI Member

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    So it sounds like this is fairly normal. If I understand correctly, it's possible that the '78 pot is a 30% taper and the newer CTS ones are 15%, which would account for the volume difference at the same point on the dial. Perhaps the Alpha is a little closer to 30%?
     
  10. jackinjax

    jackinjax Friend of Leo's

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    The action you're describing is consistent with what an audio taper potentiometer is supposed to do.
    The chart I copied shows the difference between an audio taper pot and a linear pot. The linear pot is the straight line across the center of the graph. So, if you want the volume control to be more linear throughout the travel range swap out your audio taper for a linear pot.
     
  11. bryansbizaar

    bryansbizaar TDPRI Member

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    Thanks for posting that - it was nice to have a visual to see the difference between the types of pots. Though I'm not sure that my reference was a linear pot. I just wanted to make sure the situation I was experiencing wasn't the result of a problem with my build since the volume response was so different from my reference point.
     
  12. D'tar

    D'tar Friend of Leo's

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    Does the 78 have the pull boost volume pot? I believe they are linear.
     
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  13. bryansbizaar

    bryansbizaar TDPRI Member

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    The '78 does have the pull-boost volume pot. I could only find a schematic for a Deluxe Reverb w/ volume boost - it looks like the volume pot is "999k HD".
     
  14. frankenxtein

    frankenxtein TDPRI Member

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    clintj mentioned a dead spot , I've had a similar effect when the set screw type knob is pushed up too close. Noticed a hump in the sound when the knob was turned. Could also feel the drag. Since then I leave a bit of slack so the knob is not riding on the bushing.
     
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