Simulated Coil Tap?

[asdaven]

I have a SG Jr with a Single P90 that is not wired up to coil tap. Wondering if I could use a push pull pot to run a cap or resistor in line to the output of the pickup but before the volume pot to either cut the output of the pickup or cut some of the bass making it brighter to simulate a coil tap? This would have to be an on/off feature because theres only two pots and holes for pots in the guitar. My goal is to further expand the sonic options I can get out of this guitar. My goal isnt to change the sound of the single pickup guitar. Its a pretty hot P90 like 9.5K. Goal is to be able to pull a pot up and make it sound brighter.

I was originally thinking a fixed resistor on the push pull switch to reduce output but thinking this will have the same effect as the volume pot and darken things. Someone mentioned to me running a .0047uf cap on the push pull switch and that would be a fixed bass cut and in a way cut output. I know people have created variable bass cut circuits in some guitar which I cant do here without putting another hole in the guitar. So what about a fixed value in line cap wired to a push pull switch (pot)?

[newey]

asdaven-

Hello and Welcome to G-Nutz2!

I won't have all the answers here, but we'll get the discussion started.

First off, you are correct that a fixed resistor will cut ouput across all frequencies, so a cap is what you want if the goal is to cut only part of the frequency range. However, a cap in parallel with your pickup will cut the highs, just as it does on your tone control. To cut the lows, the cap needs to be wired in series with the pickup's "hot" lead.

The cap value that works best will vary with the pickup, there isn't going to be any "one size fits all" solution. So, some experimentation is in order. Fortunately, this can all be done external to your guitar, so no dismemberment will be required unless/until you decide whether to wire a particular one in permanently.

To do so, you will need to buy a variety of caps of various likely values to test. Others here may have some suggestions as to appropriate values. But, basically, you would make up a test rig out of a old guitar cable that you're willing to sacrifice. Cut through the insulation to expose a good length of the conductors. Leave the shield/ground intact, and cut into the "hot" conductor. Attach 2 alligator clips to the ends of the hot conductor. Separate the shield/ground so that it doesn't toch the clips (maybe some electrical tape to keep them separated). Then, clip in a capacitor, plug into your guitar and amp, and see if you like the sound. Then try a different cap value. "Season to taste".

Your test rig may be a bit noisy, but you should still be able to tell whether you like the amount of cut you get.

There are other ways to achieve this as well- you could add some brightness with a no-load tone pot, or you could use the push/pull to bypass both pots entirely. Or, you could do that and add the cap to boot.

But, others may have better ideas, so let's let them chime in here.

[asdaven]

Yes in series. And want to put it on a push /pull so its on/off. Surprised this isnt more common because theres only so much you can do with 1 pickup guitars.

[ashcatlt]

The big thing that happens when a coil is tapped is that the inductance of the pickup is reduced. This causes the cutoff frequency of the pickup’s LPF to increase. You don’t just (or even at all, really) get more of what high frequencies were there, you get higher frequencies. A cap in series can’t help with that. An inductor in parallel might.

[asdaven]

I know it wont be the same. I cant coil tap this pickup. But best I can do is I can cut/filter some of the bass frequencies with what I have with a cap. On and off with a switch of course. Thats what im going for. And going with a conservative enough cap, in this context higher value is less bass cut ...that it wont be super thin or quiet.

I guess a resistor in series and in line with the output will just have the same effect as the volume pot and darken the tone and have a opposite effect. I think I need to go with a cap. Ive seen some people combine a resistor in parallel with this to simulate a setting on a pot but I would think you could just keep it simple and go with a more conservative bass cut cap value

[asdaven]

Got a testing rig with a old push pull pot to try some caps. Lowest cap I have right now is .015uf. But this is such a subtle bass cut that it could be wired in full time. Brings more clarity to the guitar and notes are clearer...the tone control barely gets muddy with it on like a greasebucket. But I got lower caps coming. Probably will end up going with .0047uf. This is cap alone no resistor in parallel.

[gckelloch]

Dec 29, 2022 10:01:36 GMT -5 asdaven said:

I know it wont be the same. I cant coil tap this pickup. But best I can do is I can cut/filter some of the bass frequencies with what I have with a cap. On and off with a switch of course. Thats what im going for. And going with a conservative enough cap, in this context higher value is less bass cut ...that it wont be super thin or quiet.

I guess a resistor in series and in line with the output will just have the same effect as the volume pot and darken the tone and have a opposite effect. I think I need to go with a cap. Ive seen some people combine a resistor in parallel with this to simulate a setting on a pot but I would think you could just keep it simple and go with a more conservative bass cut cap value

You can actually lower the inductance substantially if you're willing to spend ~$40. The Wilde Q-Filter is a very low impedance 1.4H inductor that doesn't add any significant noise or roll off the high end (as would a high impedance inductor). You can get a 4kHz+ peak with some bass reduction with it configured as an L-filter. Just wire it on a pot as you would a cap, leaving the outer pot lug ungrounded to increase the peak level. You can wire a 5~10k R in series so the bass doesn't drop so much when it's turned way down. You could also PM Frets to see what she suggests for sourcing your own inductor for less money, but it must have a very low wind count for the high-end not to be rolled off or add noise. The Q-Filter is only a few hundred Ohms. The inductance value is achieved via the coil being under very high pressure. I agree it's pricey for what it is, but the custom parts may cost them a substantial %.

[JohnH]

An inductor might be good. But here's an idea that could get close, using a resistor in parallel with a small cap, like a bass cut but with even smaller cap:




(EDIT 5/1/2023 - my mistake, the charts above noted as 680k were actually from 320 or 330k - was reading my spreadsheet incorrectly!)

This is a GuitarFreak plot based on analysis models derived from physical tests by Antigua

The light blue line is a typical P90, feeding into a 500k pot and a 10' cord. You can see it has high output, with a low frequency resonant peak

The red and dashed dark blue are a couple of typical single coils, with a 250k pot. I picked a Fat50 and a CS69. You can see the lower output and higher frequency peaks

The green curve returns to the same P90 and 500k pot, feeding the P90 in series with a parallel combo of a (EDIT )330k resistor and a 0.56nF cap. You can see that it brings the level right down, to about that of the CS69, and creates a higher-frequency peak. Its not quite the same as the two real SCs but it in the ball-park and its very simple to add. Id suggest to try a few different values in this range.


Note: One of the keys to this is, in 'single coil' mode, use a 500k pot and no treble pot (or a no-load one)

[gckelloch]

The LP P90 in Guitarfreak 6 is listed at 8.4H, and his SG LP is a little hotter. I'd assume it's ~9H. With the internal coil C or ~100pF and maybe the 50pF or so a 12ax7 in the first preamp stage adds, you'd need a very low C cable to get a peak up near 3kHz with one 500k pot. A typical 12' 350pF cable would give it a ~2.2kHz peak. That should work very well in general, and famous guitar players get a peak around there with SC pickups and the high C cables they use on stage. A very low C cable might be worth trying with John H's recommendation. You may notice more gritty harmonic complexity that the lower peak in his circuit will smooth off. Might be worth a try. A good very low C Sommer Spirit LLX cable can be found cheap here:

store.haveinc.com/p-62046-sommer-cable-300-0091-sc-spirit-llx-low-loss-instrument-cable-per-foot.aspx

I got 10' with the HiCon noise-free connectors on the page, and they work great.

[JohnH]

I agree about getting as low a capacitance cable as possible, and no longer than you need. That always helps add to the 'zing'

Note the RC wiring I was suggesting is making a peak, not really as a resonance but more by sculpting the response by an RC filter.

[gckelloch]

Dec 31, 2022 6:29:42 GMT -5 JohnH said:

I agree about getting as low a capacitance cable as possible, and no longer than you need. That always helps add to the 'zing'

Note the RC wiring I was suggesting is making a peak, not really as a resonance but more by sculpting the response by an RC filter.

With the Low C cable, your RC filter and a 500k pot, his P90 might get a peak up to ~3.5kHz. For more contrast, it might be better to include the 500k tone pot on the other switch option without the RC filter. The peak would then be more like 2.5kHz, and rolling the tone knob down to ~7 would bring it down to the smoother 2kHz range. You might prefer to leave it there for most things. That would be a very useful simple setup.

[asdaven]

What does the 680K resistor in parallel do versus just a conservative cap value on its own? What would be the closest combo with just a cap alone? I can get resistors but only have caps at the moment. I assume the resistor lets some signal bypass the cap? Thats a really low value cap at .56nf which is a lot of bass cut. Im looking at values around 4.7nf using it as a lone cap. Which higher in this circumstance is less bass cut.

I cant include an extra tone pot for this because theres only 2 pots in the guitar for the volume and standard tone control. Im swapping the tone out for a push-pull and this is just going to be a fixed on-off wired to the push pull part of the pot not the tone pot itself. Tone pot is standard 500k tone pot with .022uf Orange Drop cap.

[JohnH]

The resistor in my diagram lets some lows and mids through, similar to a single col. The cap just lets some extra highs through to make the high peak. Id suggest trying different values, the cap and resistor are a few cents each I doubt that youd like just a cap though but its best determined by testing. A larger cap on its own will probably sound like a P90 with less bass, but same high end roll-off

[asdaven]

Jan 3, 2023 15:32:38 GMT -5 JohnH said:

The resistor in my diagram lets some lows and mids through, similar to a single col. The cap just lets some extra highs through to make the high peak. Id suggest trying different values, the cap and resistor are a few cents each I doubt that youd like just a cap though but its best determined by testing. A larger cap on its own will probably sound like a P90 with less bass, but same high end roll-off

So the resistor balances out the cut? My P90 is on the hotter side like 9.5K resistance I believe. Just would like to take the output down to say a 5K-6K pickup sound like on a Strat or maybe Tele on the push pull switch of course. To get some of those tones. Do you know what a lone capacitor would do to the line on that graph? Just trying to understand how the cap values and adding a parallel resistor and its values shift that line up, down, forward, back.
Thanks-

[JohnH]

I'll plot again with some variations so you can see.

Can you confirm that you can use a 500k volume pot?

[asdaven]

It already has a 500K volume pot and 500K Tone pot but not no-load. And I use about a 10 foot cable.

Thanks-

[gckelloch]

Jan 4, 2023 11:12:47 GMT -5 asdaven said:

It already has a 500K volume pot and 500K Tone pot but not no-load. And I use about a 10 foot cable.

Thanks-

Cable capacitance values vary widely. Your 10' cable could be 2~4x the C of the one I recommended.

[JohnH]

Here are some more plots FYI

First - in my previous post, I have a curve labelled for 680k in parallel with 0.56nF. My error, its actually 320k, or 330k being the nearest fixed value. (i was misreading my spreadsheet)


New plots:





This time, red is the full P90 with 500k volume pot and no tone. (I only have one set of values loaded up for a P90 but its near enough to show the effects)

Dashed blue as a reference, is one example of a true single coil - a CS69, into a 250k Volume pot - also no tone pot, eg like a traditional Strat bridge pickup.

There are four curves representing 0.56nF or 1nF, combined in parallel with 330k or 180k. The 180k curves let some more output through, and the 1nf allows a higher treble peak, but at lower frequency.

Out of these, I think the purple curve looks to be nearest to the SC, 330k in parallel with 0.56nF (as I plotted before, though initially labelled incorrectly!)


Effect of cable capacitance

All of the curves described above are based on the guitar feeding into a load of 0.5nF and 1M. This is intended to represent a decent (but not special) 10' cord, feeding a typical amp. (eg 40pF per foot plus 100pF for the amp input C)

But the green curve takes the purple values (330k//0.56nF) and puts them into reduced capacitance, 0.3nF and 1M. This could be an ultra-low capacitance cord (20pF per foot) and an amp. You can see how it has shifted across closer to the reference dashed curve for the real single.


No load tone

If you want to have a normal tone control as usual for the full P90, then change to the SC tone, you could use 1/2 of a two-pole switch (push-pull or mini-toggle) to cut out the tone control in SC mode, at the same time as changing the 330k//0.56nF combo from being bypassed (full P 90 mode) to not bypassed ('SC' mode). This will let you set your tone for your P90 sound and leave it there while you switch to your clearest brightest SC sound with the one switch. I think the lack of a tone control in SC mode would be ok, given that the point of that mode is to get a brighter tone.

[gckelloch]

So John, it looks like the P90 with 330k//0.56nF & the 200pF cable has a stronger peak than the P90 with 330k//0.56nF & 400pF cable? That doesn't seem right, but I don't fully understand how the RC affects the peak. I might opt for the ~180k//0.56nF in either case. The smoother peak will be less strident in the bridge. There should also be more eddy current loss at higher freq's with a P90. You might also consider going down to ~8' of the cable I recommended to make the peak that much smoother and higher. You could order 20' and then try an 8' and 12' length to hear the difference.

I'd actually look for a lower wound cheap P90 to get an even smoother 4kHz+ peak. A GFS 8k neck P90 might have the same spacing as the bridge version. You could then use a little parallel C on the default P/P switch setting to achieve a ~2kHz peak, and/or just leave the tone knob down a bit. I think the GFS P90's come with AlNiCo V magnets. Some people prefer lower or higher power magnets, but it's a good average power level for a Steel core pickup. Magnets can actually be swapped in a P90 if you want to experiment at some point. I have a GFS neck P90 with an AV bar and it sounds great. I think the short fat coil makes the wire insulation thickness have that much less effect on the sound than with a tall thin Fender SC bobbin, and the high-end loss from the Steel core should even make the high-end loss typically caused by cracked PE insulation wire less significant. In short, I don't think there is any advantage in expensive boutique P90s. Any wire of the same gauge should sound essentially alike with the same magnet, assuming the wind tension is similar and doesn't exceed the spec.

[JohnH]

In the plots I've put up, the P90 is based on a model of a Tonerider Hot90, which matches these Bode plots by Antigua:

guitarnuts2.proboards.com/thread/7909/tonerider-vintage-hot-analysis-review

[asdaven]

Jan 4, 2023 19:02:08 GMT -5 JohnH said:

Here are some more plots FYI

First - in my previous post, I have a curve labelled for 680k in parallel with 0.56nF. My error, its actually 320k, or 330k being the nearest fixed value. (i was misreading my spreadsheet)


New plots:





This time, red is the full P90 with 500k volume pot and no tone. (I only have one set of values loaded up for a P90 but its near enough to show the effects)

Dashed blue as a reference, is one example of a true single coil - a CS69, into a 250k Volume pot - also no tone pot, eg like a traditional Strat bridge pickup.

There are four curves representing 0.56nF or 1nF, combined in parallel with 330k or 180k. The 180k curves let some more output through, and the 1nf allows a higher treble peak, but at lower frequency.

Out of these, I think the purple curve looks to be nearest to the SC, 330k in parallel with 0.56nF (as I plotted before, though initially labelled incorrectly!)


Effect of cable capacitance

All of the curves described above are based on the guitar feeding into a load of 0.5nF and 1M. This is intended to represent a decent (but not special) 10' cord, feeding a typical amp. (eg 40pF per foot plus 100pF for the amp input C)

But the green curve takes the purple values (330k//0.56nF) and puts them into reduced capacitance, 0.3nF and 1M. This could be an ultra-low capacitance cord (20pF per foot) and an amp. You can see how it has shifted across closer to the reference dashed curve for the real single.


No load tone

If you want to have a normal tone control as usual for the full P90, then change to the SC tone, you could use 1/2 of a two-pole switch (push-pull or mini-toggle) to cut out the tone control in SC mode, at the same time as changing the 330k//0.56nF combo from being bypassed (full P 90 mode) to not bypassed ('SC' mode). This will let you set your tone for your P90 sound and leave it there while you switch to your clearest brightest SC sound with the one switch. I think the lack of a tone control in SC mode would be ok, given that the point of that mode is to get a brighter tone.

What if I leave it as a standard 500K tone control? Ive played with no load tone pots and the different is very small to negligible to my ears from a no load to a standard pot on 10. With 500K, the difference is smaller.

So what would a lone higher value capacitor look like on the graph in leau of the extra resistor? Wondering what value capacitor when used alone with no resistor, achieves the closest to your result there? 4.7 nf seems to sound good but not a huge difference when engaged. Kinda the same sound as turning the volume down with 50s wiring.

Dont know what a tapped P90 would like on this graph with half the coils running? Thats what im trying to emulate here but a lower output Strat pickup is probably a good target. I know it wont be the same as its passive but just trying to get as close as possible with caps and resistors.

[JohnH]

Hi asdaven

The arrangements I was describing above are my best estimate of how to get what you describe. The no-load tone (or switch off the tone in SC mode) make quite a big difference in this case, see below, showing the real SC dashed, the P90 with0.56nF//330k in series, with just a 500k volume (red), and then with 500k tone (at max treble) as well (green). You would definitely notice that, more so than for other cases with no load. The tone pot is cutting down on the high peak by several dB:





Here is the P90 with just a series cap of different values. The bigger caps start with just a slight cut in the low bass, then as the cap is reduced, this moves up the spectrum until it becomes a general reduction of all frequencies, sloping down towards the bass. IMO, this doesn't look like any tapped or SC pickup.



Now I think you can see all the options (that I can think of) based on just R and C parts, and the next stage is to try some. All these parts cost almost nothing. You could temporarily wire your pickup with flying leads outside the guitar, then experiment. If you have some capacitors, but not resistors, maybe you have a spare pot and could wire that up in parallel to capacitors to try different settings? If any are good, then you could measure and obtain a fixed value resistor to match

Thats about all I've got on this, good luck!

[asdaven]

Jan 5, 2023 18:23:25 GMT -5 JohnH said:

Hi asdaven

The arrangements I was describing above are my best estimate of how to get what you describe. The no-load tone (or switch off the tone in SC mode) make quite a big difference in this case, see below, showing the real SC dashed, the P90 with0.56nF//330k in series, with just a 500k volume (red), and then with 500k tone (at max treble) as well (green). You would definitely notice that, more so than for other cases with no load. The tone pot is cutting down on the high peak by several dB:





Here is the P90 with just a series cap of different values. The bigger caps start with just a slight cut in the low bass, then as the cap is reduced, this moves up the spectrum until it becomes a general reduction of all frequencies, sloping down towards the bass. IMO, this doesn't look like any tapped or SC pickup.



Now I think you can see all the options (that I can think of) based on just R and C parts, and the next stage is to try some. All these parts cost almost nothing. You could temporarily wire your pickup with flying leads outside the guitar, then experiment. If you have some capacitors, but not resistors, maybe you have a spare pot and could wire that up in parallel to capacitors to try different settings? If any are good, then you could measure and obtain a fixed value resistor to match

Thats about all I've got on this, good luck!

Awesome I can see where the resistor makes a difference. If I dont use a no load pot, could I tweak the values on the resistor and cap to compensate for the normal 500K tone pot? Like using the 1nf vs the .56nf which the 1nf had a higher peak?

[JohnH]

You should try whatever you think might work

But here is my suggestion (red) vs one per your idea above with 1nF//330k and vol and tone pots both connected (green). You can see it is a flatter, lower-frequency peak, not as close to the dashed reference. But, if it turned out that you like how it sounds, that's all that matters.

BTW, I'm not actually suggesting that you need a no-load tone pot, I think it'd be better to keep your tone pot, but just disconnect it in SC mode.

[gckelloch]

I'm tellin' ya, man, a strong peak in the 3~3.5kHz range on the bridge pickup is the harshest thing you can have. I experimented in Guitarfreak with a 9.5k/9H pickup (with 100pF internal C). It got a 3.95kHz peak with 120pF C in series and 170pF cable C & 500k into a 1M Ohm load. That peak is 4dB lower than with 5n6F(560pF) C. Going down to 100pF losses 1dB off the peak, but doesn't increase the peak freq. Going up to 150pF adds 1dB, but drops the peak freq to 3.73kHz. 120pF seems like the sweet spot. Compared to the 7dB peak John H got with a low C cable and the 330k//560pF load, that would still create a 3dB peak (if adding a parallel 330k R has the same result as his). That should be more than enough to get a nearly flat response at ~4kHz (after the eddy current loss from the Steel screw P90 core). I have several guitars with SC bridge pickups configured to be essentially flat out to 4kHZ or so, and it sounds great-- not harsh, but nice chime and articulation. I'd try 330k//120pF and no tone pot (if John H concurs). Adding the 500k tone pot would drop the peak another 5dB, entirely defeating the purpose.

[asdaven]

Jan 6, 2023 1:40:36 GMT -5 JohnH said:

You should try whatever you think might work

But here is my suggestion (red) vs one per your idea above with 1nF//330k and vol and tone pots both connected (green). You can see it is a flatter, lower-frequency peak, not as close to the dashed reference. But, if it turned out that you like how it sounds, that's all that matters.

BTW, I'm not actually suggesting that you need a no-load tone pot, I think it'd be better to keep your tone pot, but just disconnect it in SC mode.

Well heres the thing. Im using a push-pull pot on the tone....they dont make a no load pot in the Push pull pots. I cant use it in the volume spot because im using a vintage taper volume that I like and they dont make a push-pull version of that.

Something I maybe able to do because I think this is only using one side of the push pull switch. Its a DPDT switch. So I maybe able to wire it to bypass the tone pot when this is engaged.

[JohnH]

yes exactly right!

One side of your switch connects the tone control to the volume pot, for normal use. When you pull the switch, it disconnects it.

You have the 330k and 0.56nF cap in parallel, all I'm series with the hot lead of the pickup. The second side of the switch bypasses the R and C to get the full P90 when pushed in, but disconnects when pulled out.

I'll sketch a diagram.

[asdaven]

Jan 6, 2023 14:38:25 GMT -5 JohnH said:

yes exactly right!

One side of your switch connects the tone control to the volume pot, for normal use. When you pull the switch, it disconnects it.

You have the 330k and 0.56nF cap in parallel, all I'm series with the hot lead of the pickup. The second side of the switch bypasses the R and C to get the full P90 when pushed in, but disconnects when pulled out.

I'll sketch a diagram.

Thats an idea.


It would be nice to have a tone control on the single coil mode to tweak the sound though so its not a wide open bright sound.

I wonder how going up to a 680K resistor would affect things?

And I guess putting the resistor in series with capacitor would just have the same affect as the volume pot?

[unreg]

Jan 6, 2023 15:10:08 GMT -5 asdaven said:

And I guess putting the resistor in series with capacitor would just have the same affect as the volume pot?

Hmmm… resistance and capacitance are in calculated in opposite ways for series AND parallel. So, my tiny opinion, a resistor in series with a cap will NOT be the same as volume pot… but, I’ll be quiet; sry, I haven’t a clue.

Here’s ChrisK:

Sept 9, 2009 21:21:22 GMT -5 ChrisK said:

Resistance

Resistances in series add.

Rt = R1 + R2 + ... Rn


Resistances in parallel are calculated this way.

Rt = 1/[1/R1 + 1/R2 + ... 1/Rn]

For two resistors;

Rt = [R1 * R2]/[R1 + R2]


Inductance

Inductances in series add.

Lt = L1 + L2 + ... Ln


Inductances in parallel are calculated this way.

Lt = 1/[1/L1 + 1/L2 + ... 1/Ln]

For two inductors;

Lt = [L1 * L2]/[L1 + L2]


Capacitance

Capacitors in parallel add.

Ct = C1 + C2 + ... Cn


Capacitors in series are calculated this way.

Ct = 1/[1/C1 + 1/C2 + ... 1/Cn]

For two capacitors;

Ct = [C1 * C2]/[C1 + C2]

EDIT: Maybe, since resistors are calculated opposite of caps, a resistor in series with a cap wouldn’t do anything. Obviously, there would be more components that the signal moves through, but maybe they would cancel each other out? 🤔

[unreg]

Sigh, maybe this similar thread can help?

guitarnuts2.proboards.com/thread/3876/capacitor-bass-cut


That’s just a thread about cutting bass at a specific freq… but, maybe ChrisK’s wisdom in there will help answer your question.


I crossed out my edit in my previous post bc resistors are used with caps frequently. Learned about them being used together in the midst of ChrisK’s text.