pot taper and value change....help a dumb person please

[12AX7]

yeah, i know about the secret life of pots at geofex and tried to understand it and the math. But i'm a bit stuck and before i even go any further i wanted to ask so i don't waste my day and possibly destroy the pot with a tone of soldering. So i have a 250k pot which i need to use for my 5k requirement for the presence. The reason is it has a DPDT switch and i need that and it's the only pot like that i have left after my 5k pot/switch broke. So i added a 4.7k across the outer lugs and all is good as far as value now. But the taper is now such that it does nothing till it gets to about 9 and the full range is between 9 and 10. Question is, can i add a resistor between the wiper and one of the outer lugs to change it to linear taper and if so what value and would i then also have to change the 4.7k out thats there now to insure the 5k overall value stays the same?  My math skills are non existent so geofex didn't help. If anyone can help i'd appreicate it greatly, as i need the amp next saturday and don't have much time to find order and receive another one.

[pdf64]

Sorry, but no.
With some pot designs, it’s easy to swap tracks.
So eg a CTS 1M with pull switch can have a different track swapped in, provided it’s from a pot in the same product line.

[Williamblake]

I haven´t put this to use, yet but i guess 250k is too far away from 5k for a reasonable taper.

[12AX7]

Darn....oh well, i figured 250k would be too big but i hadda try. Thanks. As to swapping tracks, i've done that for years but these ones with the DPDT are really hard to disassemble and reassemble. I have in the past but it's a b*tch plus i don't think i have one thats lower to swap tracks with thats the same design.

[Williamblake]

Appearantly it isn`t for the feeble minded or there is some mistake in the PDF i postet regardig the last paragraph about using a linear pot in a tremolo circuit.

I can´t get the variable resitor to behave like an antilog pot, putting a resistor across input and output has the exact opposite effect. Which is a shame since high resistance antilog dual pots are hard to get these days.

I did compare it to an actual antilog pot in ciruit and measuring the resistance, i am almost sure i didnt get it wrong. Also there are not so many possipilities to wire a pot and a resistor the wrong way.

Maybe there is no way around using an antilog pot in a tremolo circuit or having an awkward travel?

[PRR]

....there is no way around using an antilog pot in a tremolo circuit...

The part is a pot but the circuit uses it as a rheostat. You can't bend a rheostat the way you want, as you have now proven.

[Williamblake]

Thanks PRR, now i can sleep at work again. With the current supply situation regarding dual antilog pots i will try to switch the "taper" in the pots, plan A being to use very large linears.

[sluckey]

What is an antilog pot?

[pdf64]

Bear in mind that Fender tended to use a 30% reverse audio taper for trem speed. Which isn’t that far off linear.

Antilog = reverse audio

The claim on that AMZ document

a linear pot used as a variable resistor can have its response modified into anti-log response by adding a resistor as shown below. The resistor chosen for this example is slightly larger than 20% of the pot value so that the parallel resistance of the pot with added resistor will be close to 100k. The anti-log response is useful in many variable resistance circuits, such as the frequency control of oscillators.

seems incorrect. An antog pot used as an oscillator frequency control would be wired to have max resistance at CCW, min resistance at CW, and the resistance at 50% rotation would be about 1/4 - 1/10 of the CCW resistance.
That seems impossible to achieve by adding resistors to a linear pot.

Omeg will build you a custom batch of pots with a min batch size of only 10 pieces, provided that component parts don’t need manufacturing specially. Unfortunately that condition may apply to 2M2 antilog tracks. 
https://www.omeg.co.uk/builder/

[tubeswell]

I made a do-dacky. Far from 'perfect' but good enough for a general idea.


Enter the resistance values in ohms in the yellow boxes, and be careful not to break it.


Dial goes up to 12 - so it works better on Peavy Amps LoL. I can make one for Fender Amps if you pay me a lot of money.

[tubeswell]

Dayum, something broke it already - converted to .xlsx file

[Williamblake]

Well, my apologies. I had the paper from muzique.com saved some time ago and now that i am actually using the information i don`t understand it or it has some mistakes.

Thank you, tubeswell for sharing your information in that other thread. https://el34world.com/Forum/index.php?topic=29815.0

My point today being, the muzique.com shows the resistor added to the pot the wrong way around, doesn`t it? In the attached drawing log and antilog is mixed up, no?

/ I just noticed tubeswells spreadsheet is bigger than i expected and he has it the same way as in the pdf. I will look at what i did again, i certainly messed it up. I have a 1M lin pot and a 220k resistor, shouldn`t be too hard to find out.

[pdf64]

I think the drawing is correct.
It shows a pot being used as a pot, potential divider, where the relevant parameter is Vo / Vin.

It also needs to be bourn in mind that the load seen by the preceding circuit would change significantly as the ‘resistor modified pot’ is rotated.

However, as PRR pointed out, using a pot as a variable resistor rheostat is a very different application, and these mods don’t apply to that.

It doesn’t seem feasible to get a log or antilog resistance taper by adding a resistor to a linear pot (being used as a variable resistor).

[Williamblake]

I am trying to use this as a volume pot and after cleaning it and wiring it as antilog like in the drawing it behaves antiloglike. I am measuring the resistance from output to ground.  I have a 1M grid leak following the volume pot anyway so i will leave the resistor out for now :-)

But looking at it so long, shouldn`t there be a resistance bigger than half the pot resistance in one scenario somewhere across the swipe?

[pdf64]

It depends what’s connected to the pot.
If in and out are DC isolated with coupling caps, the out resistance of the antilog should vary between 0 and 1M as the pot is rotated.

[tubeswell]

I am trying to use this as a volume pot and after cleaning it and wiring it as antilog like in the drawing it behaves antiloglike. I am measuring the resistance from output to ground.  I have a 1M grid leak following the volume pot anyway so i will leave the resistor out for now :-)

But looking at it so long, shouldn`t there be a resistance bigger than half the pot resistance in one scenario somewhere across the swipe?

Yeah the spreadsheet just works out the amount of ‘bulge’ of the pot taper curve, not the actual voltage divider function. I’m happy to add more columns to factor in the resistance on the other side of the pot wiper when I have time. Or someone else is welcome to play around with it.