can any one explain how to change pot values using resistors ?

[Subzero]

thnx in advance

Itsik

[Mark Hammer]

Change how?  Taper?  Range?  Total resistance?  You need to be more specific.  But maybe first you need to read "The Secret Life of Pots" at www.geofex.com.  That might even answer all your questions.

[Subzero]

Thnx for the link Mark.

I'm interested in total resistance and now that I've read the artical, taper change as well.
My problem is that the only log pots I can get here are 10K & 100K. If these methods are reliable and effective, I suppose it would be cheaper then having to order pots overseas ...

Thanks
Itsik

[Mark Hammer]

There are many cases where a pot is being used simply as a variable resistance (where the resistance between the wiper and one outside lug is all that matters), rather than as a voltage divider (where the ratio of one resistance between the wiper and outside lug to the overall resistance matters).  In these cases, you can often simply add a series resistance with a toggle switch.  Foir example, many modulation effects (tremolo, chorus, phaser, etc) may use a 500k pot to set the speed.  If you only have a 100k pot, you can simply add a 100k, 220k or 330k resistor in series with the pot to achieve other speed ranges.

Sometimes, a schematic may show a pot value for changing the gain of something, and that value is used only to make a very wide range of gains possible, many of which you are not particularly interested in.  For me, the textbook case is the MXR Distortion+.  The schematic stipulates a 1meg reverse-log pot.  For those interested in authenticity, such pots are available, but when you work out the math, you realize that nothing interesting actually happens until the pot's resistance drops from 1meg to about 50k, and that the exotic taper is simply to get you past the uninteresting parts more quickly.  A 100k pot is really more than enough for that circuit.

I am not saying that if you have access to 10k and 100k pots that will be sufficient for everything you want to build, but it will prpvode more possibilities than you might think.

[Subzero]

Thnx for the explenstion Mark.
But here's a question. I've built a SRV Special which requiers a 500K - 1M log pot for the gain. In this case, a 100k pot just can't give sufficiant gain, and a 1M lin pot goes from 0 gain to 100% in about 15% of it's turn.. So how can I turn the A100K pot into a 500k/1M?, To change resistance, it seems to me I need to add a resistor of around 900K betwin the viper and leg 3 to achieve a 1M pot, or a resistor of around 400K to get a 500K pot. Is this correct ?
Or maybe I should  mod the 1M lin to a 1M Log ? I have absolutly no understanding what so ever in this.

Itsik

[frankclarke]

http://www.diystompboxes.com/analogalchemy/
http://www.diystompboxes.com/analogalchemy/emh/emh.html

[Subzero]

Thanks for the link

Another couple of questions if I may.

1. There's a lin pot calculator. is it correct to assume that if I use it for log pots the results will be the same? will the responce curve of a log pot remain the same if I pot the same value resistors on both ends ?
2. In the calculator for turning lin pots to log pots, I need to enter the taper precentage. What is the correct precentage for a regular log taper pot?

Thnx
Itsik