PTB on a Push-pull pot (reimagining bass cut circuit part)

[ssstonelover]

I'm going to be wiring a rail type HHH guitar and the Fleor pickups are 4 wire type, so I want to use push-push 500K type pots and set them for series and parallel (the easy part).
My only concern really involves the bass cut portion of the PTB. I generally like this on the C1M pots (not available on a push-pull or push-push however).
I have seen drawings with A500K and reversed control (full tone at 0, cut tone at 10) and actually used such a set up once and hated it.

Therefore I'm looking for a work-around.
My idea was to use a B500K push-push so I could wire it like a C1M ("10" = no bass cut), but certainly the tone control could be drastic/ wonky and thus hard to control on a linear pot. I am thinking to add a resistor in series with the cap to diminish the effect until the pot has been turned substantially. However after reviewing other drawings, I'm not seeing that solution, meaning it may be a flawed approach.

Therefore my questions are twofold
-Could this idea work, and what might be the 'right' resistor value to get me in the ballpark?
-What are 'best' other alternatives, given the need to use a push-pull / push-push pot, want 'normal' rotation' and which combination might get me in the ballpark?

[JohnH]

I reckon the best way to use a linear pot for bass cut, would be a 500k one, as you suggest. Ive tried thinking before about adding resistors, but cant wrangle anything better by doing this.

This 500k B pot will give a reasonable spread, but not quite as much max bass cut as a 1M.

Here' s a plot



Its a Texas Special with 250k volume, and a no-load treble pot. Dashed blue is no bass cut, the lowest green curve is max bass cut with 500k and 2.2nF. The red one was to try to find where you get about half the effect, and its shown at 25% of the turn. So youd get no bass cut at 10, half the cut at 7.5 and all the cut at 0. Not too bad?

For comparison, a C1M pot would get to that same setting by turning down from 10 to about 5 or 6, and would have a bit more total cut at 0, compared to max cut on the 500k B

[ssstonelover]

JohnH

Cool, I'll get something usable, and that is the main thing.
(The HHH rails for this build are alnico and wound ~12K so may be a bit bass heavy. I will have a no load option on the treble control, to help the high end, but I may need more extra margin to sculpt the low end)

--Would there be any point to change out the cap from 2.2nF to chase a little more bass cut or will I lose at the other end of the spectrum when at "10"?

--As I understand it caps work the other way on bass cuts so I guess the next value available would be 1.5nF. What do you think?

[JohnH]

What seems to happen as you reduce the bass-cut cap is that you still get about the same max low cut, but it starts to eat more into the mids too. Probably best to try some values, but here are plots with a 500k pot at max cut, with 2.2(solid blue), 1.5(green) and 1.0nF (red) cap. There are also 1.8nF and 1.2nF values in between.




But no implications at max bass = 10. The great thing about all these bass cut circuits is that no matter what values they have, they all completely bypass themselves at 10.

[ssstonelover]

A good lesson for me about about the 'end points' and what happens in the 'mid frequencies' (depending on the cap). It cleared up a whole bunch of my ignorance. Thanks for the graphic illustration 0plus smart knowledgeable explanation!
I have alligator clips, etc., so can rig up some comparisons between caps when I finish the paint and begin the electronics assembly. I have 1nF and 2.2nF, but ideally should get the 1.5nF to do a comprehensive comparison.

[stevewf]

Sept 18, 2023 17:54:17 GMT -5 ssstonelover said:

-What are 'best' other alternatives, given the need to use a push-pull / push-push pot, want 'normal' rotation' and which combination might get me in the ballpark?

I've also not been able to find C1MΩ p/p pots. But there is the option of disassembling/reassembling pots in order to "flip" a wafer, which would turn an "A" curved pot into a "C" curved one. I've used this method on Bourns 18mm push-pull pots for making a bass cut control. The actual operation isn't very difficult.

Choosing pots for their parts might seem complicated; hopefully I can offer guidance. Unfortunately, two pots are needed.

Pot#1 (what I call the pot that eventually ends up in the guitar) must be a factory 2-gang pot with a push-pull switch, because you need the special sweeper that comes in a 2-gang pot. You'll be replacing one of the wafers. To specify, Wafer#2 gets replaced; by "Wafer#2", I refer to the wafer that's furthest from the control knob. Wafer#1 can be any value you like, so if it's to be a PTB, choose the pot value that will be used for treble cut. Here are A500KΩ ones from Mouser (one knurled shaft, one plain).

Pot#2 must be valued at A1MΩ, since you'll be harvesting its wafer (and leaving the rest of the unit for parts). It's easier to harvest wafers from pots with push-pull switches, since they can be taken apart non-destructively. (Non-p/p pots often have the shaft deformed at the factory in order to hold the unit together, so the shaft must be destroyed in order to harvest the wafer, risking damage to the wafer. In contrast, the shaft of a p/p pot can simply be slipped out, and you'll have useable parts left over.) Here are options from Mouser.

How-to: By carefully bending metal tabs of the units' chassis (plural=chassises? ) they can be disassembled non-destructively. On both pots, 4 tabs for removing the switch block plus Wafer#2; on Pot#2, four more tabs for the pot chassis and its to-be-harvested Wafer#1. Swap out Pot#1's Wafer#2, and put in its place Pot#2's Wafer#1. Reassemble carefully and test.

Here's a photo of a partly-disassembled p/p pot.

Tips: Before pulling off the switch block, pull the shaft into the "up" position, and leave the switch in that position for easier reassembly. Keep fingers and dust off of the resistive tracks of the wafers. Notice the sweeper's original rotational position for reassembly, as the sweeper acts as part of the knob's detente.  Avoid handling the lubricated shaft, since this affects the spin friction of the knob. Test the modded unit's operation before bending the tabs back into place, in order to avoid fatiguing the tabs, should re-opening the pot be necessary; test for range of knob motion, correct resistance through the motion and for switch operation.

Lengthy tip for identifying wafers: Bourns push-pull wafers have two labels on them; the resistive value and curve shape are silkscreened on the outer face, and the original factory position in the unit is printed on the inside. Pot#2's Wafer#1 should read "A1MΩ" and "A", respectively. The tiny "A" means the wafer was originally in Wafer#1 position. Since that wafer came from a Wafer#1 position but will be placed into Wafer#2 position, its curve will be inverted. You should be able to verify this labelling convention by looking at the now-displaced Wafer#2 from Pot#1 - it should have a tiny "C" on it. Edit: this labelling convention does not apply to Bourns push-push pots, whose wafers are not interchangeable with Bourns push-pull pots.

In the end, you get a 2-gang push-pull pot whose Wafer#2 has an inverted curve, plus parts. This operation can be done in under 15 minutes, and in my opinion, the results have been reliable - no failure over 2 years of use, albeit light use.

[ssstonelover]

Sept 20, 2023 12:19:12 GMT -5 stevewf said:

....there is the option of disassembling/reassembling pots in order to "flip" a wafer, which would turn an "A" curved pot into a "C" curved one. I've used this method on Bourns 18mm push-pull pots for making a bass cut control. The actual operation isn't very difficult.
....two pots are needed.
...you get a 2-gang push-pull pot whose Wafer#2 has an inverted curve, plus parts. This operation can be done in under 15 minutes, and in my opinion, the results have been reliable - no failure over 2 years of use, albeit light use.

I'm interested. I guess I'd have to buy the 2 pots and plunge right in. I'm a bit confused as to why I can't just flip the A1M wafer on one pot rather than transplanting to the second pot and flipping it there, but I guess cracking the case open will answer the question

[stevewf]

Sept 20, 2023 15:52:17 GMT -5 ssstonelover said:

I'm a bit confused as to why I can't just flip the A1M wafer on one pot rather than transplanting to the second pot and flipping it there, but I guess cracking the case open will answer the question

A 2-ganger is needed because of its special sweeper; an A1M pot is needed for its wafer; a push-pull pot is needed for your particular application. I haven't been able to find a unit with all of those characteristics.

Edit: that special sweeper also has to come from a push-pull pot, as it is factory-formed to allow the push-pull shaft to slide through it.

[ssstonelover]

OK, no problem.

I actually wrote Mouser (and also their US operation) about making a special version with all the requirements you listed some days ago. No answer, and after some days of waiting, I expect none now.
In the meantime It's as you say, cannibalize and rebuild.

-If I were to go any further on 'ideal requirements' it would be to ask for a push-push instead of a push-pull (I find it annoying to grab onto slippery knobs and pull).
-Another possibility is the S-1 pot (not slippery), but then again it becomes an issue of finding and transplanting C1M (or upside down A1M) wafers, plus S-1's are pretty expensive).

[JohnH]

Another avenue, not yet explored but easy to try:

Take a standard linear 500k pot, open it up and remove the track and do some careful scraping at the edges of one half of the track to 'stretch' the resistance. I know this works in principle and I've done this to take low-specced log pots that measure at around 400k and get them up to 500k. But can this idea go further?

I reckon you could maybe add a couple of hundred k by working on a linear 500k pot. The trick will be to keep the centre of the track consistent and do the scraping at the edges. Also, to pre-figure which half of the track length to work on so it tends toward being the anti-log pot that you need.

Could end the process with a full no-load cut across the track that will be at 0 on the knob, for max bass cut. Then, when wired up, could put say a 1M resistor across the resulting contraption so its net 1M at 0? This will also smooth out the response and make the 10 to 5 range a slightly smoother 200k instead of 250k

This is speculative, but the starting point is a $10 pot so not much to lose. What I don't know is how much scaping is feasible but still maintain the reliability of the pot.

A nice aspect of this pot-brain surgery is that it can be extremely precise. You can have a meter clipped to each end of the track, measuring the track resistance in real time and you can watch the k's increase scape by scrape!

What could be done with a 250k linear pot?

(let me know if this is interesting but unclear and then I'll sketch something)

[ssstonelover]

Sept 22, 2023 14:44:15 GMT -5 JohnH said:

Then, when wired up, could put say a 1M resistor across the resulting contraption so its net 1M at 0? This will also smooth out the response and make the 10 to 5 range a slightly smoother 200k instead of 250k
(let me know if this is interesting but unclear and then I'll sketch something)

It is interesting as I have some 'extra' 500K pots (albeit not the push pull type) I could experiment on (plus the multimeter and scraping tools), to check feasibility and practice with. 
The 1M resistor would be on which lugs?

[JohnH]

If its used, the 1M resistor would be across the same two lugs that the cap uses. One pot lug is not involved in bass cut.

[ssstonelover]

Sept 23, 2023 1:10:13 GMT -5 JohnH said:

If its used, the 1M resistor would be across the same two lugs that the cap uses.

I just did a test with alligator clips using parts lying around.
I used a 504K (measured) pot along with a 1M resistor (measured ~987K), but omitted the cap to get a speedier 'proof of concept' result.
The final peak resistance was 334K, meaning the pot resistance and resistor are in parallel with each other, and the resistor was attached just like in the drawing below (though the cap was not in the test) so unless I am misunderstanding something, that idea will have to be eliminated, as resistance is worse than with the pot alone, meaning just working on the 'track' inside the pot will be the only way forward to possibly increasing pot value using the basic idea.
Let me know if I got some detail wrong.

[JohnH]

The 1M idea was to be in addition to the track scraping. So scrape one half of the track length to increase resistance, and also scrape a full disconnect at 0, and then also wire the 1M.

This should offer zero resistance = full bass at 10, 200k at mid turn (250 x 1000/(250+1000) =200), rising to say 700k (or what ever can be achieved) at just before 0, then 1M at 0

[stevewf]

Sept 22, 2023 12:53:51 GMT -5 ssstonelover said:

I actually wrote Mouser (and also their US operation) about making a special version with all the requirements you listed some days ago. No answer, and after some days of waiting, I expect none now.
In the meantime It's as you say, cannibalize and rebuild.

-If I were to go any further on 'ideal requirements' it would be to ask for a push-push instead of a push-pull (I find it annoying to grab onto slippery knobs and pull).
-Another possibility is the S-1 pot (not slippery), but then again it becomes an issue of finding and transplanting C1M (or upside down A1M) wafers, plus S-1's are pretty expensive).

And I wrote to Bourns a couple years ago, offering advice that there's a growing market of guitar models who would benefit from having parts made available. I didn't have much hope for a response, and I didn't get any.

I did take apart one push-push pot from Bourns, and found that the wafers are not the same shape as the ones used in their push-pull pots.To me, that's a head-scratcher, as I didn't notice anything special about the push-push chassis that would necessitate having to retool a production line just for the new shape of wafers. Nevertheless, I found that Bourns push-push and push-pull wafers are not interchangeable; what's more, their range of push-push offerings is narrower than that of the push-pulls. Still scratching.

I haven't yet taken apart a CTS-style pot, which I believe are the ones in the Fender S-1. Needless to say, I also haven't taken apart an S-1. Nothing to add there, sorry.

[ssstonelover]

Sept 23, 2023 3:05:53 GMT -5 JohnH said:

The 1M idea was to be in addition to the track scraping. So scrape one half of the track length to increase resistance, and also scrape a full disconnect at 0, and then also wire the 1M.

OK, 1st experiment done. The track, unfortunately, could not be directly accessed at all angles as the mechanism holding it on covered most of the track, meaning scraping was from an angle and at best quite imprecise. I would need a pot with better track access than what I had available.
In any case your thinking is correct but for the reason above I could not achieve 700K nor, even reliably, 600K. On the other hand 500K and below, and also 1M were easy to get.
Any idea which pots might allow me to really get in there with a scrapper and refine the 'track' and chisel in a taper shape?

-I suppose I have to try this finally on a push pull pot (though I'd like to test it on something else first to refine my technique) as that is what is really needed for the build, and those are the smaller ~16mm size.... ah
-Then there is the idea Stevewf gave, and though it is costly, it is practical, at least compared to my imprecision in this 1st attempt at track scaping.

However it is too early to call it quits on this idea, with the right stuff and technique, it should be possible to get up to about 700K before leaping to 1M after all!

Here is the pictorial

[JohnH]

Thanks for trying it, and maybe it'll work!. I guess the idea is a lot easier on a full-size simple single pot where the track lifts right out

[ssstonelover]

The theory is sound enough at least!
Assuming the right pot, proper technique, etc, I should then have a reasonable spread of bass cutting, and even get some substantial cutting at 1M ("0" on the knob) it would appear.

[aikiharp]

stevewf Sorry to resurrect this oldish thread, but I joined Guitar Nuts to ask you a question:

I am trying to wire a PTB circuit on a single pot also. I successfully disassembled and reassembled two Bourns push/pull pots to build a dual-gang push/pull pot with A500K in the upper gang and C1M on the lower gang using the method you described. The guitar in question is a Yamaha Revstar with standard 1 volume and 1 tone, 3-way switch, and non-original Duncan Phat Cats in neck and bridge position.

I'm having some difficulty getting my head around the wiring and how this would work. On a standard two-knob PTB circuit, the signal from the pickups travels through both the passive treble and passive bass pots before passing through the volume pot on its way to the jack. With the dual gang, DPDT push/pull pot, I can wire it so pull is running the signal through the A500K wafer (treble cut), and push runs the signal through the C1M wafer (bass cut). 

The part I'm struggling with is when the DPDT switch is in one of those positions, the signal passes ONLY through the indicated wafer, right? Therefore, if I pull the pot switch to adjust treble cut, when I push it to adjust bass cut, I've lost the effect of the treble cut. Or, that's how the logic seems to me. I bought and rebuilt the pots before I really thought through the wiring, so now I'm a bit stuck. Is there a way to wire the two gangs of the pot and/or the switch so both wafers are active, and the pickup signal is affected by both the treble cut and the bass cut? Also, since both gangs of the pot share the same shaft, wouldn't rotating the knob when the switch is up (for treble cut/top wafer) also set the resistance on the bottom wafer (bass cut) to wherever the wiper ends up after adjusting the top gang? 

It seems to me that a dual concentric pot would allow independent control of the treble and bass cut, but a dual gang pot with or without switch would cause treble and bass to become interdependent with each other since they share the same shaft and the wipers move together? 

Sorry, it all makes my head hurt. I've recently wired a 4-knob Gretsch-style wired Epiphone Wildkat with 2xP90s, using a 4PDT on/on/on switch and a dual gang blend pot that's only active in the middle position of the switch, as well as adding 2 knob PTB and master volume (happy to share that wiring diagram if desired). But I'm still stuck on the mechanics of the this dual gang DPDT pot...


Any advice or thoughts are appreciated!

[stevewf]

Jan 14, 2024 15:46:35 GMT -5 aikiharp said:

On a standard two-knob PTB circuit, the signal from the pickups travels through both the passive treble and passive bass pots before passing through the volume pot on its way to the jack.

I usually think of it more as "the signal passes by the treble cut, then through the bass cut before it finally passes through the volume." But I don't think that matters right here.

The part I'm struggling with is when the DPDT switch is in one of those positions, the signal passes ONLY through the indicated wafer, right? Therefore, if I pull the pot switch to adjust treble cut, when I push it to adjust bass cut, I've lost the effect of the treble cut. Or, that's how the logic seems to me.

Yes, if that's the way it's wired. The pre-made PTB's I've run across do this. With a home-made one, I think there'll be other ways to wire it, e.g. so that the signal always goes though one wafer, and the push/pull includes/bypasses the other wafer. Not sure if that helps.

Also, since both gangs of the pot share the same shaft, wouldn't rotating the knob when the switch is up (for treble cut/top wafer) also set the resistance on the bottom wafer (bass cut) to wherever the wiper ends up after adjusting the top gang? It seems to me that a dual concentric pot would allow independent control of the treble and bass cut, but a dual gang pot with or without switch would cause treble and bass to become interdependent with each other since they share the same shaft and the wipers move together?

On the dual-gang, the wipers move simultaneously. Though they're electrically separate, they're mechanically joined. So moving the knob while using one wafer will also affect the setting of the other wafer, waiting to get switched into the circuit. The dual-concentric is definitely a way to get independent controls. It'll sacrifice a stealth approach, but that might be a small cost. I, for one, have never taken apart a dual-gang CTS-style pot, let alone a concentric one. From what I can see, they're like two normal pots, stacked (without the mirroring that's in a Bourns dual-gang).

Sorry, it all makes my head hurt. I've recently wired a 4-knob Gretsch-style wired Epiphone Wildkat with 2xP90s, using a 4PDT on/on/on switch and a dual gang blend pot that's only active in the middle position of the switch, as well as adding 2 knob PTB and master volume (happy to share that wiring diagram if desired). But I'm still stuck on the mechanics of the this dual gang DPDT pot...

Any advice or thoughts are appreciated!

I don't know the context that led you to ask me about dual gang pots. I recall making one that acted as (A500K) a Series Humbucker spin-a-split that dumped one coil's signal to ground or as (C1M) a Parallel Humbucker spin-a-split that choked one coil's signal. I don't remember that pot having also a push-pull, but if it did, it might be a series/parallel switch.

By all means, post the Wildkat, I think the best spot might be in the Gallery. [edit: see sumgai's better suggestions, next post]

[sumgai]

aikiharp ,


First, to the NutzHouse!

Next, a slight correction to what Steve just said about where to post.... The Gallery is for images of your guitar itself, either as built by a manufacturer, or as you built it, or as a "work-in-progress" on its way to being finished, etc. It's not where we publish circuit diagrams, such as you've offered. (Quoting you: (happy to share that wiring diagram if desired).) 

Generally speaking, the place for diagrams is in the Guitar Wiring forum, but there is at least one exception. A completed, tested, and working diagram can go in the Guitar Schematics forum. Pretty much, a poster in that forum is not asking questions, they're simply presenting something that others can use. However, other posters with questions can start discussions in those threads just as easily as if the circuit diagram had been posted in Guitar Wiring.

For questions about a diagram, completed or otherwise, it's probably best to use the Guitar Wiring forum. If one wishes to discuss tone controls specifically, then a partial diagram in this forum (Tone Control Discussions) is also a good place to get answers. As you've no doubt seen already, there is a good mixture of both completed tone circuits that are thrown out there for use by other modders, and extensive discussions stemming from questions about such-and-such a circuit.

HTH





sumgai