Guitar Tone Pot -How/Why does it work

[tjmicsak]

OK well I posted a short time ago about adding an input cap on my tonebenderMkII. I had originally used an incorrect .1uf instead of a .01uf which was "too" much for what I wanted, and then I eventually ended up at .02uf.
Now this dawned on me that maybe I could use a simple tone pot the same as a guitar pot and use the .1uf to have anything from o to the .1uf adjustability.
Which makes me think now how does the tone pot in a guitar work?
It is simply running the signal to ground through the cap at a varying reesistance of the pot dial right?
So is the total pot value just simply high enough that when it is set to 10 the resistance to the cap is so high that even though there is a value of say either 250K or 500K, it is for all intensive perposes, infinity?
I just am trying to figure how this resistance and cap going to ground has little to no effect on the tone when all the way up and farthest from the cap while still having some resistance value other than "open".
Which now makes me wonder how high a value does the pot actually have to be to have negligable effect on tone when all the way up?

[Mark Hammer]

OK well I posted a short time ago about adding an input cap on my tonebenderMkII. I had originally used an incorrect .1uf instead of a .01uf which was "too" much for what I wanted, and then I eventually ended up at .02uf.
Now this dawned on me that maybe I could use a simple tone pot the same as a guitar pot and use the .1uf to have anything from o to the .1uf adjustability.
Which makes me think now how does the tone pot in a guitar work?
It is simply running the signal to ground through the cap at a varying reesistance of the pot dial right?
So is the total pot value just simply high enough that when it is set to 10 the resistance to the cap is so high that even though there is a value of say either 250K or 500K, it is for all intensive perposes, infinity?
I just am trying to figure how this resistance and cap going to ground has little to no effect on the tone when all the way up and farthest from the cap while still having some resistance value other than "open".
Which now makes me wonder how high a value does the pot actually have to be to have negligable effect on tone when all the way up?

1) Not trying to be a jerk but the phrase is actually "all intents and purposes", not "intensive purposes".  Common error.  Just as common as people thinking that the contraction "could've" is actually "could of" and not "could have".

2) On to the question itself.  Your intuition is correct.  Signal leaks out through the cap to the extent that the pot resistance is low enough to permit it.  How much resistance is needed to prevent that signal leakage is partly a function of the pickup, the cap value, and the volume pot.  Of course, because we are generally limited to standard pot values (250k, 500k, 1M), our choices of "how much is enough" are constrained.  The tricky aspect is achieving a workable taper or gradient of treble cut.  A 1M pot does a better job of retaining highs than a 500k pot, but there is all that resistance to work your way through to get to a setting where you start audibly shaving off treble.  In a perfect world, I imagine most folks would like it if turning the tone down to 7 or 8 made a perceptible change in tone.  In reality, it rarely does, and for most of us, the only zone that really seems to make a difference is between 1 and 4, which isn't my personal definition of "dialability".

The way that some have taken on this challenge is by means of the so-called "no load" tone pot.  This would be a pot of a moderately high value - 250k-500k - with a "dead zone" at one extreme of the pot's rotation, where no contact is made by the wiper, and the pot is effectively open-circuit or infinite resistance.

Another way of taking on this challenge is by means of something like the Stellartone Tone-Styler, which is a multi-position rotary switch that simply selects among a variety of treble-cut caps to provide the same degree of rolloff but at different starting points.  On one of my guitars, I have a stripper down version of that, with s 3-position toggle that has no treble cut (middle position) and two different treble cuts in the outside positions. Works fine for me.

[Transmogrifox]

The actual wiring of the tone pot is different from guitar to guitar, but the idea is that it shorts high frequency signal to ground.

Usually, the tone pot is in series with the pot, where the wiper is connected to one of the outer lugs.  In other words, it's just a resistor in series with a capacitor.  The value of the resistance is adjusted by adjusting the pot.  When all the way counter-clockwise, the resistance is at its minimum value, and the network looks simply like a capacitor in parallel with the pickup.

As you begin to turn it clockwise, the resistor increases and two things happen:
1)  The frequency of the 3dB cut-off gets lower.
2) The amount of the high frequency component of the signal shorted to ground becomes less.

When it is all the way clockwise, the pickup sees it pretty much as a resistor to ground because the cutoff frequency is lower than what is contained in the guitar signal.  The resistor is also large enough at this point that it doesn't reduce the volume much.  For example, Guitar pickup output resistance may be 5k to 10k ohms while the tone pot is often 250k or 500k.

For an input cap, you can use the same concept, but the pot should be many times larger than the input impedance of the circuit.  You would put a .01uF cap in parallel with a .01 uF cap in series with a very large pot.

I think you would get better results simply using a switch, or maybe two switches with intermediate values.  The varying cuttoff frequency vs. MKII input impedance would give possibly interesting results, but most likely undesirable.

I'm uncertain whether that makes sense.  It seems a sketch would be appropriate for explaining this, but it takes a while to draw it, scan it & post it.

[tjmicsak]

1) Not trying to be a jerk but the phrase is actually "all intents and purposes", not "intensive purposes".  Common error.  Just as common as people thinking that the contraction "could've" is actually "could of" and not "could have".

Thanks for the info and the grammar lesson, but it was a pun of dual meaning, although I am lost without spellcheck at times. 
Intensive-
"of, pertaining to, or characterized by intensity"

In this case the amount of resistance applied with the pot.
Keep your eye on the ball
Eye, Ball, Eyeball
It's a joke son!
Actually I did end up with the cap on a switch but my whole idea was to have the ability to choose any capacitence between 0 and .1uf. Short of using a huge air gap variable cap I was trying to figure out how to use the guitar type tone control application as a variable capacitor.
So what I am really after is a simple variable cap in the same manner as a alpha pot is a variable resistor. Is such a small thing made?

[bluesdevil]

 You can go to the General Guitar Gadgets site and look up Joe Gagan's EZ face circuit for another way to do it.
   

[JKowalski]

So what I am really after is a simple variable cap in the same manner as a alpha pot is a variable resistor. Is such a small thing made?

Not that I know of. It takes a very very thin dielectric and/or a very large surface area to create 0.1uF.

The huge air variable capacitors have spacing between the plates of something like 0.4mm, with about 10-20 plates of a square inch area.... and they only get up to something like 5nF. It's a practical impossibility to make one up to 0.1uF.

Sorry.

[Mark Hammer]

Well you could probably install one in a Martin dreadnaught!

The Stellartone unit probably somes as close to what tjmicsak wants as anything.  I haven't tried one, so I can't comment on the degree to which the control feels like a continuous pot or a stepped rotary switch.

[Mark Hammer]

OK well I posted a short time ago about adding an input cap on my tonebenderMkII. I had originally used an incorrect .1uf instead of a .01uf which was "too" much for what I wanted, and then I eventually ended up at .02uf.
Now this dawned on me that maybe I could use a simple tone pot the same as a guitar pot and use the .1uf to have anything from o to the .1uf adjustability.
Which makes me think now how does the tone pot in a guitar work?
It is simply running the signal to ground through the cap at a varying reesistance of the pot dial right?
So is the total pot value just simply high enough that when it is set to 10 the resistance to the cap is so high that even though there is a value of say either 250K or 500K, it is for all intensive perposes, infinity?
I just am trying to figure how this resistance and cap going to ground has little to no effect on the tone when all the way up and farthest from the cap while still having some resistance value other than "open".
Which now makes me wonder how high a value does the pot actually have to be to have negligable effect on tone when all the way up?

Rereading this now - and ignoring the subject heading of the thread - what you propose is actually already in use in Reverend and some G&L guitars.  A fixed highs-only path and a variable-resistance full-bandwidth path that can serve as a bass-cut control, operating in tandem with the standard treble cut pot.

The tone pot found in a number of early tweed Fender amps provides another nifty option.  Here, a 1M tone pot has its wiper tied to the input of the volume pot.  One outside lug goes to a small value cap which is tied to the wiper of the volume pot.  The other outside lug goes to a larger-value cap going to ground.  Rotate in one direction and you reduce treble cut while simultaneously providing treble bypass (i.e., variable bright switch function).  Rotate the other way and you cancel treble bypass while bleeding off more treble to ground.  Works great.

These days, I'm a fan of the bi-directional tone control.  I use a 1M linear pot with the wiper tied to the volume pot input. Each outside lug goes to a different value cap then to ground.  Mid-position puts 500k in series with each cap for least treble bleed overall.  Rotating in one direction provides "rounding off", while rotating in the other provides standard dulling/muting.  Best of all, it compresses the full range of each tonal shift into half the number of degrees of rotation to permit "pinky wah".