### Author Topic: Potentiometers and values  (Read 4026 times)

#### BruceJustice

##### Potentiometers and values
« on: January 31, 2015, 08:12:48 PM »
Hey there So, my question is very straight forward but any explanaition of how these things work would be appreciated as I am very new to all that. Basically, I want to build "Magnavibe" pedal, kind of mix of tremolo and vibrato effect. Schematic says that it requires linear potentiometers with value of 25k. And it is not a problem, however, I have loads of 10k pots as I got them quite cheap and had to buy so will there be a big difference if I will use these 10k pots instead of 25k? Also, maybe there is a way to change the potentiometer value? Thanks!

#### slacker

##### Re: Potentiometers and values
« Reply #1 on: February 01, 2015, 06:14:11 AM »
Going by this schematic http://i605.photobucket.com/albums/tt137/pinkjimiphoton/MagnavibeSchemsmall.jpg both of the potentiometers are being used a variable resistors, when you turn them all way in one direction lugs 1 and 3 are connected and the resistance is zero Ohms, turned the other way the resistance is the total resistance of the pot. Therefore if you use a 10k pot instead the maximum resistance is about half what the circuit wants.
The depth pot controls the maximum brightness of D1 which then controls the resistance of PH1, the higher the value of the depth pot the dimmer D1 will be, so using a 10k pot you won't get the full range of depth control. In this case the way the circuit responds will be different anyway depending on the exact components you use for D1 and PH1, so a 10k might be fine or a 25k might not be enough.

The speed pot is a bit different because it is in parallel with a 27k resistor, so if you know your Ohms law stuff, the total maximum resistance is about 12k, which is pretty close to 10k so if you remove the 27k resistor a 10k pot will work about the same. https://en.wikipedia.org/wiki/Parallel_resistors#Ohm.27s_law.

You can change the value of pots but you can only make them smaller and change the taper, have a look here for more info http://www.geofex.com/Article_Folders/potsecrets/potscret.htm