Wah potentiometers vs. regular potentiometers

[wayfaerer]

Hello everyone!

To what extent is a wah potentiometer found in a Dunlop or Vox wah interchangeable with a regular potentiometer used in pedals, e.g., an Alpha pot? More specifically, here's what I'm wondering about:

• Are they (generally) rated for longer life without getting scratchy?
• Does the diameter of the "body" of the pot matter? Wah pots look bigger than Alpha pots, though maybe not as big as the ones used inside guitars.
• Are wah pots made to rotate fewer degrees than a regular pot? For example, I can usually rotate an alpha pot roughly 270 degrees, but when I watch the pot in my Vox wah turn as I move the treadle up and down, it only rotates about half that, give or take. This is the most confusing point for me. For example, if a wah pot is 10k, does that mean that it goes from 0 to 10k over that reduced span, and stops there? Or is it actually a 20k pot (for example) with a full 270 degrees worth of rotation, but the wah mechanism only rotates it halfway? Or is it a 10k, 270 degree pot, and the wah mechanism only rotates it halfway, thus as far as the wah circuit is concerned it's actually a 5k pot?
• Can I take the gear off a wah pot and put it on a spline shaft pot and use that?

What all this comes down to is: Can I skimp out and buy a $0.50 pot for my wah, or do I have to get a nice one?

[Marcos - Munky]

A wah pot is rated for longer life, since you'll be rotating it a lot. You can use a regular pot instead, but it'll gone bad way sooner than a wah pot. Also, usually the treadle doesn't do the full pot rotation, it is limited to a partial rotation and you'll be missing the other part. It's a pretty common "mod" to remove the pot and set it as you like, I mean, some people like to use the beginning part of the rotation, some people like to use the ending part and some people like to stick with the middle.

[Mark Hammer]

With the mechanical advantage the gear mechanism provides your foot, if the mechanism permitted the pot to fully rotate, the chances are very good you would break the pot, by pushing it hard enough into the stopper that halts rotation at the end point on each side, that you'd break or otherwise deform the stopper.  So rack-and-pinion foot treadle systems are deliberately designed to rotate less than the full 270 or whatever degrees.  What that does permit, however, is the ability to move the pot a couple of "teeth" in one direction or the other, to adjust the range of sweep.  You still have to be sure that full heel or toe-down doesn't push the wiper past the brink.

Of course, rack-and-pinion is not the only type of mechanism for rotating pots in a foot pedal.  Some use a cam.  Some use a string.

One thing to be mindful of is that the "dirt" inside of a pot that makes it scratchy-sounding only partly comes from outside.  A fair amount is actually generated within the pot itself, and comes from the constant rubbing of the wiper against the resistive strip, wearing off particles of the resistive strip, that accumulate over time.  Remember, the only reason pots can work is because of the pressure applied by the wiper against the resistive strip.  My own experience - and I certainly haven't sampled the universe of pots of different sizes - is that the smaller the pot diameter, usually the poorer designed the wiper.  Take apart a 50-cent Alpha pot, and they are well-made (or well-enough made) but the wiper looks like an iddy-bitty snow shovel, or even fork with slightly curved tines.  It scrapes the resistive strip.  Take apart a pricier 24mm pot and you're likely to find a wiper that is gently rounded, and able to apply just the right amount of pressure to maintain contact without scraping.  So, all other things being equal, larger-diameter pots tend to have an internal structure/mechanism that holds up nicely over many many rotations.  In contrast, 50-cent pots are designed to hold up for plenty of rotations, but not nearly as many.  Over the entire duration of ownership, you will probably not adjust the Tone control on your favorite overdrive nearly as much as you would like adjust/rotate a wah pot for just a single playing of the Theme From Shaft.  50-cent pots are more than adequate enough for the former, but not the latter.

So, the larger wah/volume pot is going to have a better-designed wiper.  Naturally, residue generated by scraping depends not only on the scraper, but the material being scraped.  And here, the composition of the resistive strip on wah pots seems to be hardier; one more factor that contributes to their longer life

Finally, wah and volume-pedal pots tend to have longer threaded collars around the shaft, similar to the difference between guitar pots for installation into Fender guitars, where they attach to a control plate, and those for installation into a Les Paul, there the pot has to be longer to have enough thread sticking up through the hole in the body that you can still fit a washer and nut on.  Wah pots have a longer threaded area than smaller Alpha pots so they can be attached to the mounting portion of the pedal and stick have enough shaft sticking out to reach the gear mechanism.  They almost always have a solid rather than split shaft, to provide a snug and stable fit of the gear.

Can you use a cheap Alpha pot for a wah?  Yes.  Will it function reliably?  No.  Will it last as long as a proper wah pot?  No.  But of course "reliable" and "lifespan" will depend on how and how often you use it.  FWIW, I wade myself a Craig Anderton EPFM Super Tone Control, built into an old wah shell.  The circuit requires a dual-ganged pot, which is not readily available as a wah-pedal type, only in smaller form factors.  The threaded collar is not as long as I needed but I was able to get it to work.  Works fine, but I would NEVER, however, use it for gigging.

[wayfaerer]

Thanks for the replies. It's clear now that an Alpha pot or similar will not suffice!

At this point, what I don't understand is the limited rotation. I checked my Vox wah just now, and moving the pedal rotates the pot about 160 degrees, up to about 180 when the switch is being actuated. It appears that I could push down a little farther if I used a switch with a shorter "neck". I'd like to get as close to a full range of rotation as possible.

It makes me wonder why wah pots don't have a reduced range of mechanical rotation in order to more closely match that of wah pedals, of course keeping in mind what Mark Hammer said about the pot needing a bit of extra room so it doesn't break. In my case, the pot presumably (I haven't taken it out yet) has about a hundred degrees of wiggle room.

[mozz]

I think Allen Bradley type J pots were used at one point. Regular pots but a mil spec part, sealed.

[Mark Hammer]

Thanks for the replies. It's clear now that an Alpha pot or similar will not suffice!

At this point, what I don't understand is the limited rotation. I checked my Vox wah just now, and moving the pedal rotates the pot about 160 degrees, up to about 180 when the switch is being actuated. It appears that I could push down a little farther if I used a switch with a shorter "neck". I'd like to get as close to a full range of rotation as possible.

It makes me wonder why wah pots don't have a reduced range of mechanical rotation in order to more closely match that of wah pedals, of course keeping in mind what Mark Hammer said about the pot needing a bit of extra room so it doesn't break. In my case, the pot presumably (I haven't taken it out yet) has about a hundred degrees of wiggle room.

But like I say, that additional potential range of movement allows one to "tune" the wah to achieve the specific range of emphasis you want.  Let me clarify.  If the full range of a pot goes from 1-100, and the pot is only allowed to move from 35-85, one can get inside, lift the "plunger", and move it over a gear tooth or two, such that the range might now be set to 25-75, or 40-90, etc.  That is, the full range of sweep is not changed, but where in the spectrum it is made to occur is changed.

As the saying goes: it's not a flaw - it's a feature!

[mozz]

So a higher value pot would give you more range in the same foot sweep. Just ordered some of the 125k from amplified parts, because it seemed like a good idea, we shall see.

[strungout]

So a higher value pot would give you more range in the same foot sweep. Just ordered some of the 125k from amplified parts, because it seemed like a good idea, we shall see.

Yes, BUT the sweep would go up faster since every degree of rotation of the pot would increase by a bigger resistance %. Let's say you have a 10k pot and moving 90 degrees increases the resistance by 25%, that means the pot goes through 2.5k in that 90 degrees. If you have a 100k pot, moving 90 degrees still moves 25%, but now where talking about going through 25k of resistance instead. Which depending on the circuit can have a big or small effect. Hope my math is right...
The general idea is that the sweep will move faster with a bigger pot value than a smaller value.

[Mark Hammer]

It would depend on the taper of the pot.  The taper doesn't determine what the range of peak frequencies is, but does determine how easy it is to move the filter to the peak frequency you want within that range, with easy ankle movements.  I don't know that there is any "ideal" taper that works for everyone.  Preferred taper depends on what you like and how you like to move your foot.

[strungout]

Good point, Mark.

EDIT: A point I'm curious about: A larger pot means less travel at the point of the wiper per degree, right? The wiper contact is further from the axis, so longer travel. And more precision. How does that relate when connected to a rocker?

I built a Tycobrahe Parapedal clone with a dual-gang alpha pot. It works but it really feels like it won't stand up to heavy use.

EDIT 2: Nevermind, re-read and it answered the question. I guess it relates to that ankle movement, and what you preference is, in the end.