Replace resisitors with pots?

[Vince]

Is there a rule of thumb when replacing a resistor with a pot? I mean, obviously two wires come from board but is there a standard way of linking it the the pot. i.e across say lugs 1&2 and then 3 to ground? or vice vera?

Cheers.

[Madferret]

It depends on what you're trying to do with the pot I think, The main uses seem to be mixing two signals, so lugs 1 and 3 are the inputs and lug 2 is the mixed output, dumping some of the signal to ground, like a vol pot and your example, or just providing a variable resistor, like a lot of gain pots where only lugs 1 and 2 or 2 and 3 are used.  If you're just trying to replace a single resistor I'd connect a wire from the board where one side of the resistor was to lug 1 and another wire from where the other side of the resistor would be to lug 2 and leave lug 3 unconnected, if the pot seems to work backwards change to using lugs 3 and 2.  Hope this helps and isn't too far off the mark.

[dodido]

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[Vince]

Cheers guys!

I'll have a tinker and see how it turns out. I was just thinking about small fuzz circuits and wondering what effects it would have so I'll have a little experiment.

[IvIark]

If you connect one wire to 1 and 2, and the other wire to 3, in the clockwise position the pot value will be essentially bypassed, 1 and 2 will be directly connected to 3.  And so of course in the counter clockwise position the full pot value will be between pins 2 and 3.

So if you connect one wire to 2 and 3, and the other wire to 1, the reverse happens.  So it just depends if you want the full pot value in the clockwise or counter clockwise position.

[Vince]

Nice one Mark.

I feel a fuzz with about 10 pots coming on!... not to mention capacitor switches!

[IvIark]

You can call your company VZex coz that's basically what Zack did with the Fuzz Factory!

[Vince]

Haha... Good thinking!!

[dodido]

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[tonecut]

Newbie question: how do you know what value pot to use? Or is it just whatever hits your fancy for different sounds? Any issues with messing up the circuit with an improper value?

[Geiri]

I'm guessing you base it around the resistor that you want to replace with a pot. If it's a 10k then you use at least a 10k pot. If I were to do it, I'd try a higher one like 20k just to mess around and see what the different values do to the sound.

[IvIark]

There are so many different examples and so it's hard to cover all bases with an overview.  With something like the Tubescreamer Drive pot for instance, it acts as a variable resistor in the feedback loop of the opamp (between inverting input and output), and this determines the amount of gain from the opamp with other surrounding components.  The pot is 500K and has a 51K resistor in series with it which sets minimum gain when the pot is fully counter clockwise.  Those values combined determine the gain of the opamp when divided by the 4K7 resistor to ground (or vbias) from the same inverting input (4.7K).  So it means the actual gain you get from the stage is a minimum of 51K/4.7K and a maximum of 551K/4.7K or between 10.8 and 117.2 depending on the position of the pot.  Some drive/gain pots work differently, the Big Muff for instance acts as a voltage divider which dumps some of the signal to ground to reduce gain.

For a tone control there are lots of different ways they are used, but looking at something simple like the Treble pot in a Timmy, the pot is again used as a simple variable resistor in the signal path, which creates a variable low pass filter with the 10n cap to ground following it.  Bearing in mind the Timmy's Treble pot works backwards to what you would expect from a usual tone control, but the filter results are the same, just at the opposite side to what you would usually expect, so there is more treble in the counter clockwise position and less clockwise.  In the Timmy the Treble pot is 50K with a 1.5K resistor in series with it, so the possible values from that combination at either end of the pot rotation is 1.5K to 51.5K, and with the following 10n cap it means the variable filter will start cutting highs from 309.039hz - 10.6khz depending on the position of the pot.

Volume pots are usually voltage dividers (like the Big Muff gain controls but at the end of the circuit), which dumps some of the signal to ground to attenuate the signal.  More often than not they also create a high pass filter with the circuits's output capacitor and so an effect with a 47n output cap followed by a 100K volume pot will also create a variable high pass filter which will let through all frequencies above 32hz at full volume, but then roll off low frequencies as well as dump more signal to ground as you rotate the pot counter clockwise.

Some pots like the Filter pot on the DAM Drag'n'Fly takes the input signal and directs it through one of two different input caps depending on the rotation of the pot which acts as a rudimentary tone control at the input.  These pots will again create a series filter with the following input caps, and also determine the input impedance and so are selected based on the desired results.

There are lots of different ways they are used but that covers what you will probably see most frequently in many of these effects.