Single knob: volume with push-pull fixed tone circuit

[evan]

Hello everyone, I hope you and your families are safe and well. I just bought a guitar with a single knob and I plan to install pickups which can at times be too bright.

Here is what I want: a push-pull volume knob (with a treble bleed circuit) where, pushed in, the tone is cut as if there were a tone knob attached to a .022uf cap and dialed to 7. Pulled out, it would sound like a no-load tone pot dialed to 10.

Is this possible? Any guidance or advice would be greatly appreciated. This is my first wiring project.

[newey]

evan-

Hello and Welcome to G-Nutz2!

This is certainly possible, although it might take some experimentation to get the right value for the capacitor so as to give you "tone pot at 7" sounds. If you simply use a .o22µf cap, it will sound like the tone control is at "0".

There are (as with most things), two ways to approach this. The first is the scientific, "mathy" way- this would involve measuring the resistance of a tone pot at 7, calculating how much filtering will take place with a .022 cap at that level, etc. Someone else here can, I'm sure, explain to you much better than I exactly how to do that.

Or, if we get really lucky, someone will have done just what you want, and can give you a suggested value. But the specific pickups enter into the equation here as well, so someone else's value for their particular pickups might not be your cup of tea.

The second way is the "brute force method". Build yourself a "cap substitution box" and try different capacitor values until you find the one you like. This can all be done external to the guitar. You can get fancy, and use in/out jacks like you were building a stomp box, or you can just sacrifice a guitar cable by exposing the two wires so that you can wire a cap across the cable. (Note that the substitution "box" does not have to be an actual box, that's more a figure of speech).

[JohnH]

Just to run with the maths, given a clear spec of 'tone pot at 7, 0.022uf cap', we could estimate as follows:

The first thing to decide is what pot values are we basing it on? let say its a guitar which would normally have two 500k pots.

Tone at 7 is really about the tone pot resistance, not much about the cap, so I think it needs a resistor with a cap in series

A 500k tone pot has about 10-15% of its resistance at mid turn, lets use 15% which is 75k. From mid turn to max it adds the remaining 425k and its often a bilinear taper. If the knob goes 0 to 10, then from 5 to 10 adds 2/5 of the remaining 425 to add to the 75 = 245k. So using the nearest common values youd be looking to switch in probably a 220k in series with the 0.022uF cap. Lets say it was a fender knob that goes 1 to 10 instead of 0-10, and a 10% taper, then the resistor for 'knob at 7' is 200k, ie, that 220k is still a good value.

If you are basing all this on 250k pots, then everything is halved and id reckon 120k is a good estimate.


However given this is the one and only tone-fix to be provided, there's scope for experimenting first and fine tuning, and you might actually like the sound of just a cap with no resistor, in which case a very small cap like 0.0022uF or 0.0033uF might be be good - its a different tone to anywhere on a normal tone pot, with a sharper smoother roll off of the very high treble but keeping all mids. Youd have to experiment and listen though.

[Yogi B]

Nov 27, 2020 13:32:06 GMT -5 JohnH said:

A 500k tone pot has about 10-15% of its resistance at mid turn, lets use 15% which is 75k. From mid turn to max it adds the remaining 425k and its often a bilinear taper. If the knob goes 0 to 10, then from 5 to 10 adds 2/5 of the remaining 425 to add to the 75 = 245k. So using the nearest common values youd be looking to switch in probably a 220k in series with the 0.022uF cap. Lets say it was a fender knob that goes 1 to 10 instead of 0-10, and a 10% taper, then the resistor for 'knob at 7' is 200k, ie, that 220k is still a good value.

Comparing these values to the various tapers log taper measurements, they're definitely in the ballpark, but maybe a little on the high side -- mainly because CTS appears to use an (at least) trilinear taper, so it also depends on what make of pot evan is basing the "dialed to 7" on.

Below is a graph of the various 10-15% tapers scaled to 500k. Note that: at '7' (0.7) the difference between the tapers is about at its largest; the highest values in the region 0.55 to 1.0 (orange line) were actually recorded from a 10% taper pot by Bourns, because it exhibits almost full value at only 0.9 thereby 'squeezing' the rest of the taper towards the lower range of rotation; finally the vertical line between 0.6 & 0.7 is at 0.666 i.e. (7 - 1)/9 or '7' on a Fender knob.

However given this is the one and only tone-fix to be provided, there's scope for experimenting first and fine tuning

To that end, it might also be an idea to use a 250kΩ or 150kΩ trimmer potentiometer (rather than a fixed resistor) so that the exact amount of treble-cut can be dialled in, without needing to audition various individual resistors.

Additionally depending on the type of treble bleed you're using, we might think about utilising the both poles of the push-pull: one to switch the tone cap & resistor to ground; the other to modify the treble-bleed to achieve a more consistent tone-cut operation at any volume setting (by adding some extra resistance in series with the TB's capacitor -- or maybe part of it, by swapping it for two parallel capacitors).

[evan]

Thank you newey for the warm welcome and thank you all for your thoughtful responses. You've helped me understand the task much better than I did.

To that end, it might also be an idea to use a 250kΩ or 150kΩ trimmer potentiometer (rather than a fixed resistor) so that the exact amount of treble-cut can be dialled in, without needing to audition various individual resistors.

This is exactly what I'm going to do.

Additionally depending on the type of treble bleed you're using, we might think about utilising the both poles of the push-pull: one to switch the tone cap & resistor to ground; the other to modify the treble-bleed to achieve a more consistent tone-cut operation at any volume setting (by adding some extra resistance in series with the TB's capacitor -- or maybe part of it, by swapping it for two parallel capacitors).

Please let me know, Yogi B, if I'm understanding you correctly:

When the pot is pushed in and the circuit is passing through the trimmer pot, the treble-bleed circuit will interfere with the tone cut provided by the trimmer pot by bleeding excess treble at lower volume settings. This would be ameliorated by wiring the pot so that when it's pushed in, there is extra resistance in series with the treble bleed capacitor. And I'm definitely confused about this part: this could be achieved by either (a) setting up extra resistance in series with the TB circuit capacitor which was only active when the pot is pushed in, or (b) by swapping the TB capacitor for two capacitors, one of which is wired in series with extra resistance and is used when the pot is pushed in.

Are there any kinds of treble bleed circuits where this wouldn't be necessary? What about a TB circuit with a parallel resistor or JohnH's improved TB circuit?

[Yogi B]

Dec 2, 2020 16:59:08 GMT -5 evan said:

Are there any kinds of treble bleed circuits where this wouldn't be necessary? What about a TB circuit with a parallel resistor or JohnH's improved TB circuit?

The parallel treble bleed arrangement was already assumed on my part (I don't 'get' the Kinman/Series TB). An inherent 'problem' of treble-bleeds is that they tend to give too much treble in the lower volume range, JohnH's dual-gang design tries to balance that by decreasing the resistance in parallel with the TB cap as the volume gets turned down, (reducing the overall impedance of the 'upper' half of the potential divider that is the volume control, thereby providing less volume reduction to the full-range signal, and thus proportionally reducing the amount of treble). However when compared to a normal parallel treble-bleed at low volumes it's actually quite similar (where it differs most is at near full volume -- giving proportionally more treble than the regular parallel TB arrangement). Overall it is a clever solution, but isn't any better at maintaining consistent tone across the whole volume range when paired with an actual (or in this case simulated) partially rolled-off tone control.

I wouldn't go as far to say as what I'm proposing is a necessity, as the difference will only be a handful decibels -- plenty of people have managed fine with how treble bleeds and tone controls interact, but the exception is that they have a tone knob they can tweak at their leisure. Here where you don't have that luxury, it might just be worth it to include this refinement.

That being said, even for those with both knobs there already exists a mod to specifically combat the 'issue' by rewiring the tone control & treble bleed similar to the arrangement of the bright cap in Fender's Princeton amps. I can't recall whose name is associated with this on the internet, but the concept itself is at least half a century older, and my suggestion is essentially a version of it.

I've been trying to think of the best way to show a comparison between a regular treble-bleed and my suggestions, and I think it's clearest if we think about your 'dulled' setting as the main tone & pulling the push-pull as a kind of treble boost. The below graph tries to illustrate this effect across the sweep of the volume pot, each is based on having a resistance of 180kΩ in series with a 22n tone cap for the dulled setting. The red lines are identical and represent the effect at full volume, proceeding at 20% decrements in spectrum order (i.e. 100% - red; 80% - orange; 60% - yellow; 40% - green; 20% - blue; and, what would theoretically happen at 0% if there were any output - purple).

The top graph is a parallel 150kΩ & 820pF treble-bleed, second is JohnH's improved treble bleed with 120kΩ & 820pF, third is my single capacitor variant as shown in the upper schematic on the right, and finally is my two capacitor variant as shown in the lower schematic.

In the traditional parallel set up as well as JohnH's you can see that as the volume is lowered the amount of 'extra' treble that can be had is reduced -- in other words, your 'fixed at 7 tone pot' is cutting less and less treble. Whereas my 'improved' versions somewhat overshoot, giving a little more difference between the two settings with the volume in the range of 90% to 60%, are overall more consistent.

(These were all calculated using JohnH's 5-part SD59 pickup model (which is the humbucker with the highest amplitude resonant peak of those in GuitarFreak) in addition to a 500kΩ volume pot, 500pF cable capacitance, and 1MΩ input impedance)