Wiring tone pot question

[Dennis_Ajax]

Hello,

i;ve wired everything on my Stratocaster pickguard and i thought it was ok. Now with testing it comes out that the tone pot only cuts the high out when it is below 3... Any thought?


just 250 k CTS linear tone pots

 

[line6man]

You used the wrong taper. A linear taper stretches things out evenly, so that you don't get down to a usable range until close to the end of the rotation. For instance, the 50% point in the resistance range is at the half-way point in the rotation, rather than around four-fifths of the rotation.

 

[Cagey]

Pot taper doesn't matter as much with tone controls as it does with volume. Ideally, a linear taper would be best, but that's a whole 'nother discussion. In your case, you've probably got too small a cap. For guitars, you want a 500K pot in series with a .022µf cap. Smaller caps will cut off later, larger ones will cut off earlier.

Be aware that even if you put a .022µf part in there, it may not be .022µf. Capacitors and resistors are notoriously sloppy parts unless you buy parts binned for precision. 20% variance from marked is not unusual at all.

 

[line6man]

Pot taper doesn't matter as much with tone controls as it does with volume.

It makes a very big difference.

 

[Cagey]

I know how taper works and why you'd want to use variations of it. But, it has more to do with matching output levels to the response curve of human ears as it applies to sound pressure levels than it does with frequency sensitivity. That's why I say it matters less with tone controls, and a linear pot is theoretically the thing to use. I say "theoretically" because it's difficult to say where you've cut half the frequencies vs. a third, or a fifth, etc. There's a "knee" in the response curve of any filter that you can either sharpen or move (or both). So, you do your best to put it at a predictable and convenient place with a cutoff that softens the angle of the knee to give the appropriate control.

For a super-simple low-pass filter like we use in guitars, experience has shown the 500K/.022uf combo usually puts most of the action in the center of the pot. Part tolerances will move that around somewhat, but it's a good rule of thumb.

 

[Dan025]

its hard to say what the op's ear is picking up on. the character of the guitar changes in that last 1/3 as a new resonant peak forms. but if there really isn't an audible difference his pot impedance may be too high. especially if the tone pot is higher value than the volume.

 

[line6man]

I know how taper works and why you'd want to use variations of it. But, it has more to do with matching output levels to the response curve of human ears as it applies to sound pressure levels than it does with frequency sensitivity. That's why I say it matters less with tone controls, and a linear pot is theoretically the thing to use. I say "theoretically" because it's difficult to say where you've cut half the frequencies vs. a third, or a fifth, etc. There's a "knee" in the response curve of any filter that you can either sharpen or move (or both). So, you do your best to put it at a predictable and convenient place with a cutoff that softens the angle of the knee to give the appropriate control.

For a super-simple low-pass filter like we use in guitars, experience has shown the 500K/.022uf combo usually puts most of the action in the center of the pot. Part tolerances will move that around somewhat, but it's a good rule of thumb.

You have to look at it for what the tone control is supposed to be doing. A range of 0 to 250k Ohms is quite extreme for the average pickup impedance, and most of the range beyond a certain resistance point is largely useless, but you need to have a high resistance on one side for when you don't want the capacitor to be in the circuit. That being said, audio tapers tend to work better, because a good bit of that high resistance drops down when you roll it down a hair, so you can use the rest of the rotation to vary within a much more limited range of resistance. The use of audio tapers is analogous, in a way, to using a lower pot value, but retaining the "bright" option when turning the pot all the way up.

 

[Cagey]

That makes sense. Hadn't thought about it that way before.

 

[Dan025]

you can analyze the differences for hours. but both tapers are used successfully in guitar controls. typically on buffered controls for stereos with basic two band eq's I believe they use audio taper so that as you sweep the knob the -3db point falls in frequency on a log scale just like the musical scale...

but what you have to realize is the guitar circuit has a lot going on. you aren't using the pot as a voltage divider (though you could) you are using it as a variable resistor. the voltage divider is created with the series impedance of the pickup which changes with frequency and the pickup load from the volume control which is in parallel to the tone, and the cap which changes with frequency. the actual taper isn't gonna be easy to see. but I believe suhr uses linear pots half the value of the volume and ofcourse most factory guitars use audio taper same value as the volume. either tends to be acceptable. now they also add resistors so the tone never really reaches zero on fenders and they use smaller caps with two in series and one gets shunted as the knob is turned(grease bucket). you can analyze it to death and create all sorts of weird circuits that have some small improvement but I wouldn't really think too far into the subject.

just find a model you like and use it. if you like a guitar that's not too bright the linear taper at half the volume value works nice. if you want a jangly fender sound use pots with a greater value.

now if the controls are ineffective from 10-4 on the knob you may want a smaller value pot or a larger cap. what value components are you working with anyhow?

 

[line6man]

just find a model you like and use it.

+1.

People get really religious about tapers. I used to get into the debates, but I got tired of it, because there is no point to it. Sometimes one works better than the other, and sometimes people get stuck on theory instead of practice. Pots are so cheap, I usually say it's best to try both and figure out what works for you.

 

[Jumble Jumble]

The other thing is if you buy 10 audio taper pots and then actually chart the taper with measurements, none of them will have the same taper anyway.

 

[line6man]

The other thing is if you buy 10 audio taper pots and then actually chart the taper with measurements, none of them will have the same taper anyway.

The graph I posted above pretty much demonstrates the difference between a true logarithmic taper and the taper of many pots on the market. It is difficult/expensive, from a manufacturing standpoint, to create a proper logarithmic taper, so often, a psuedo-logarithmic taper is created by joining two linear segments in an approximation.

More concerning than taper, however, is the fact that many pots are manufactured to a 20% tolerance. People ask a lot of a mechanical component for the price they pay. If you want good parts, you usually have to pay up.