Effectively converting a pot from 1meg to 250K?

Discussion in 'Effects, Pedals, Strings & Things' started by leofenderbender, Jan 26, 2012.

  1. leofenderbender

    leofenderbender Member

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    How do I convert a volume pot from a value of 1meg to 250K? I have a strat set up with 1meg pots and I am looking to change to a 250K load at the pickups.

    Is there a resistor value that I can solder on the volume pot across the blades or between the ground that will get me there?

    Clorenzo?
     
  2. amp_surgeon

    amp_surgeon Member

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    The first article basically explains how John Spina discovered the formula for determining parallel resistance by experimenting with different value fixed resistors in parallel with a pot. Unfortunately, what John forgot to do was measure the effects of the parallel resistor at the wiper of the pot for a range of different knob angles. If he had done that he would have discovered that, although the resistance at full volume is what you'd expect from the pot's full resistance in parallel with the fixed resistor, the actual curve is dramatically different from what it originally was, and the resistance actually peaks well above this value somewhere between 2/3 and 3/4 volume setting. There's a point in the rotation where the pot he thought he'd converted from 1M to 500K will actually measure close to 700K on the wiper.

    In other words, as you rotate from full volume downward the volume is actually going to increase during the first part of the rotation until you hit the peak before it starts to drop again. Maybe not the sort of behavior you want if you fiddle with your volume knob a lot.

    In the second article, R.G. Keen does his usual masterful job of explaining how to create pseudo log and reverse log pots from linear pots by hanging a fixed resistor between the wiper and one of the outer lugs. R.G. Keen understands how the fixed resistor affects the linearity of the pot, and he exploits that to get what he wants, which is essentially to convert a linear taper pot into an audio taper pot. This method changes both the maximum resistance and the taper of the pot.

    The DIY Stomp Boxes site has some great calculators for modifying pots for different tasks. The linear pot calculator combines what R.G. Keen describes for log and anti-log pot conversions to create a pseudo linear pot of smaller value. As the description on the calculator page explains, the value can fluctuate. This is because, as R.G. Keen shows, the bump in the log taper mod is toward the bottom of the curve, while the bump in the anti-log taper mod is toward the top of the curve, so combining the two mods doesn't exactly nullify out the changes each mod makes to the curve, but it's a reasonable approximation for a lot of tasks. The only problem is that the volume pot on a Strat, as on most guitars, is audio taper - not linear taper.

    The audio taper pot calculator is a variation of the linear taper calculator, but designed to create a pseudo audio taper pot from a larger linear taper pot. Again, not what the OP is trying to do.

    Both calculators are great for their intended purpose, but neither one is intended to convert a larger audio taper pot into a smaller audio taper pot.

    The bottom line is that you can't reduce the total resistance of any sort of pot by adding fixed resistors without also changing the taper curve. This might not be an issue if you intend to keep that volume pot dimed all the time. On the other hand, if you're going to tweak that knob occasionally while you're playing then you might find it harder to find that sweet spot until you get used to the change in the taper curve. If the OP is willing to live with the bump in the curve then a 330K in parallel with the outer lugs on the pot will get him around 250K at full volume. This will peak near 350K when the pot resistance is at about 2/3 before it starts dropping again.

    The best solution is to break down and change the pot with one of the desired value.
     
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  3. leofenderbender

    leofenderbender Member

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    Thanks guys! Lots of good info here; lots of technical reading too. Hope I can get what I am looking for...
     
  4. jb4674

    jb4674 Member

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    Why not just get a new 250k pot and call it a day? Adding resistors to lower the value on the pot can be a headache sometimes.
     
  5. mhuxtable

    mhuxtable Member

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    My thoughts too.
     
  6. leofenderbender

    leofenderbender Member

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    The obvious solution.

    I am also going to install a switch that will give me access to a 1meg bright load and a 250K dark load across the pickups.
     
  7. chip46

    chip46 Member

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    You can add a parallel resistor to the pot to decrease the value, but it's going to affect the taper. You can test it by calculating the needed resistor value, and fixing it to the pot temporarily with alligator clips or something similar and see if you like it.

    Only other option I could think of is a dual concentric pot. Basically a concentric pot with two values, although I don't think I've ever seen them as high as 1M. You could then add a switch to toggle between the two pots and it would only take up one control space.

    Here's a an example of a dual concentric in 250k and 500k. Maybe someone else will know if a 1M exists. I've never used a dual concentric pot, but I don't see why it couldn't work.

    http://www.guitarelectronics.com/pr...m_medium=CPC&utm_content=&utm_campaign=CPCS25
     

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