50kB to 25kC Pot Mod?

[Bio77]

Hi all,

I need a 25kC pot for my next build, but I'm having trouble finding one.  Can I solder a 50k resistor across two legs of a 50kC pot as a work around?   

[cooder]

That should work afaik.
Here's a bit of discussion about it:
https://www.diystompboxes.com/smfforum/index.php?topic=90931.0

[Bio77]

Cool, thanks cooder. 

[Rockhorst]

Some info on the particular circuit would be helpful.

[Bio77]

Some info on the particular circuit would be helpful.

Here is a shot of the part of the circuit in question, I thought that would be more illustrative.  I can upload the whole schematic if necessary. 

I plan to place the resistor across pin 1 and 3 of the pot.

[Bio77]

I did some searching on pot tapers and learned that most audio pots are not truly logarithmic, they are a combo of two linear tapers.  I'm thinking that placing a resistor in parallel will actually give a more anti-log response.  I'm excited to give this a try. 

Here is a picture of different pot tapers and also some plots I made for tapers expected from different combos of pots and resistors. 

[cooder]

Very cool, do you have a software to calculate the taper changes or a link to a calculator?
And yes the proof is in the pudding. Give it a try.

[Bio77]

I made the taper plots in excel, I can send you the file if you want, PM me.

[cooder]

I made the taper plots in excel, I can send you the file if you want, PM me.

Very kind and cool, will send you PM! Cheers and Merry Xmas!

[bryanrichie]

This is very interesting! Would it be possible to simulate a 50kC pot out of a 250kB pot?

[EBK]

If you can find a 50kC dual gang pot, you can wire both halves in parallel and not have the taper change.  I did something somewhat similar on a build once.

[Bio77]

This is very interesting! Would it be possible to simulate a 50kC pot out of a 250kB pot?

I haven't tested my idea out yet, but the math works.  Actually, you just made me realize that the OP was for converting a C pot to a C pot, which isn't nearly as cool as converting a B to a C.  That's what is modeled above. 

Here is a few plots of the R tapers 50kCs made with a 100kB, 250kB, and a 500kB.

 

[cooder]

That's very usefull stuff, thanks! Also thanks for sending me those excel files, top notch!

[Bio77]

So, I tried this and it didn't work.  Does any one have any ideas where my logic is flawed?

I used a 100kB pot and ran a 33k resistor in parallel with lugs 1 and 3.  So R14 should always see 25k from the pot (100*33/(100+33)=24..  The voltage divider at lug 2 should work the same no mater the value of the resistors used.

Does that seem right?

[Bio77]

So, I switched out the work-around (100kB with 33k) for a 25kB pot, the mids control is still not appreciable.  Maybe the work-around was working after all. Any thoughts?