Approximating a 1KB pot w/ tapering resistor

[insomniac2295]

Hey guys, I've poured over several threads/websites trying to figure this out, but the technical aspect is just outside my grasp. I'm trying to approximate a 1KB pot for a Rat build. The reason why I can't just buy one is because I'm using clear shaft pots for this build and the lowest value they come in is 5k. If someone could explain to me exactly how I could approximate a 1K Linear pot with my limited values, that would be amazing!
Here's a link to the clear shaft pots I have a selection from:
http://www.smallbearelec.com/servlet/Detail?no=1186

[GGBB]

Short answer - 1KB is possible, perfectly linear is not possible.  How you go about it is dependent on how the pot is being used.  Is it used a divider or as a simple variable resistor?  For the variable resistor, just parallel the 2 adjacent terminals of the 5K pot that are being used with a 1.2K resistor.  This will give you a 968 ohm pot with either a log (using pins 2 & 3) or an antilog (pins 1 & 2) taper.  If it's a divider, try Linear Pot here: http://www.diystompboxes.com/analogalchemy/emh/emh.html.

[insomniac2295]

I should share that I have read the Secret Life of Pots several times and am still at a wall. I'm fairly certain that the application I'm using my pot for is as a variable resistor. I'm using the pots for the Ruetz Mod depicted here:


So, I know that as a variable resistor, I need to hook the resistor up to the CCW lug and the middle lug, with the CW lug tied to the middle. I just need some help setting up the equation, just to make sure I get things right.

[Govmnt_Lacky]

Metal can LM386 

Typo??

[insomniac2295]

Metal can LM386 

Typo??

Ha! I've looked at this image a thousand times for various Rat builds and have never noticed that typo!

[GGBB]

Metal can LM386  

Typo??

Ha! I've looked at this image a thousand times for various Rat builds and have never noticed that typo!

Nope - not a typo - the LM386H LM308H.  I have two but have yet to get around trying them out in my RATs.  It's one of those perpetual To Do items.  

Never mind - it's morning - I even made the same typo myself.   

[GGBB]

I should share that I have read the Secret Life of Pots several times and am still at a wall. I'm fairly certain that the application I'm using my pot for is as a variable resistor. I'm using the pots for the Ruetz Mod depicted here:

So, I know that as a variable resistor, I need to hook the resistor up to the CCW lug and the middle lug, with the CW lug tied to the middle. I just need some help setting up the equation, just to make sure I get things right.

Yes - variable resistor.  add a 1.2K resistor across the 5K pot's lugs 2 & 3 in the diagram (middle and CW - remember they are showing the back of the pot).  It will work just as well if you use 1 & 2 but the effect of adjusting the pot will be reversed and the pot taper will be opposite.

[insomniac2295]

I should share that I have read the Secret Life of Pots several times and am still at a wall. I'm fairly certain that the application I'm using my pot for is as a variable resistor. I'm using the pots for the Ruetz Mod depicted here:

So, I know that as a variable resistor, I need to hook the resistor up to the CCW lug and the middle lug, with the CW lug tied to the middle. I just need some help setting up the equation, just to make sure I get things right.

Yes - variable resistor.  add a 1.2K resistor across the 5K pot's lugs 2 & 3 in the diagram (middle and CW - remember they are showing the back of the pot).  It will work just as well if you use 1 & 2 but the effect of adjusting the pot will be reversed and the pot taper will be opposite.

Awesome! I like I have it now. Thanks a bunch. I'm going to crack open the pedal and give it a whirl

[insomniac2295]

Yeah, the proper IC should have been an LM301AH...
Anyways, the 1.2k resistor worked perfectly! I checked the pot out with my DMM after installing the resistor and it worked just as you said it would! A thousand thank-yous!

[Govmnt_Lacky]

Yeah, the proper IC should have been an LM301AH...

LM301AH... 

Typo??

LM308AH maybe.

Sorry... not trying to be a dillweed. Just want to make sure the right stuff is posted for future reference.

[insomniac2295]

F this thread's drunkenness! Yes 308AH

now, to put this thread to bed...

[PRR]

The "natural value" of that resistor is 47 ohms.

With a 1K *reverse taper* pot, 100 ohms happens at half-way, so the "natural" setting is 25%, one-tenth effect is at 75% , a useful span.

With a 1K *linear* pot, 100 ohms happens at 10%, so the "natural" setting is 5%, one-tenth natural is 50%. Half the dial is "hardly any" effect. Not ideal, because most of the action is down in the 0-2 part of the dial.

There is NO way, in this circuit, that a 5K (100 times too big!) can be loaded to give useful action. It all happens in the first 10% of the dial, 1-10 are pretty much all the same and too mellow-action to be interesting.

It is possible that a radical re-scaling of all resistors around that opamp could come out with 5K as a happy value. But I'd need a full schematic to see what-all else is involved, if any values scale to "impractical" numbers.