Pot values and taper, and capacitors too

Amigo · September 2021

So, there is this HH guitar, with a toggle switch and only two pots: volume and tone. New humbucker pickups will go in, ~7.5kΩ in the neck, ~8.5kΩ in the bridge. I don't know caps and pots inside, but they might go anyway. I have two questions, if someone can answer (no names mentioned... @ICBM)

1. Values for pots and cap
Humbuckers are usually recommended to have 500kΩ pots for both volume and tone, and 0.022µF caps for tone. In the case of only 1 volume and 1 tone with one toggle switch, the same capacitor value will apply irrespective if there is one humbucker connected, or both humbuckers in parallel. The same applies to the pots values. In the case of both pickups active, the output impedance of the pickups together will be half that of a single pickup. Therefore, would not it be more appropriate to concoct somehow a circuit that switches to 250kΩ pots when the switch is in the middle position, and the cap switched to 0.047µF? Has anyone tried that? Have you found this to be an issue? Any blue-sky experiences or ideas?

2. Pot taper
I did some calculations and it seems that a logarithmic taper is the best choice for both volume and tone pots. I also remember finding a while ago that the so-called audio taper is in fact close to a logarithmic taper, but with a more acute angle, more like two regions of linearity. The taper graphs that I found for a few pots manufacturers were all slightly different, but the same general shape. Do you know which taper is the most appropriate in perception—i.e. linearly perceived volume and cut-off frequency—in the real world? The taper of which pot companies you prefer, and for which types of pickups?

tanihhiavlt · September 2021

Personally for two humbuckers - I like linear 500k pots for volume and audio pots for tone using a 50s wiring.

This is because I use the volume control as a gain control for a timmy that's always on and somewhat cranked so I can switch from polite and twangy to driven using the volume.

I like the 50s wiring because I like the options it provides more than the individual effect on tone.

I also use an EQ pedal a lot

ICBM · September 2021

You could use extra switching to simulate 250K in the middle position but it will be complicated even with a lever switch (you'll need a more expensive one) and not possible with a conventional toggle switch. Or you could use dual-gang pots and wire it exactly like a normal 2V2T setup but with only two visible controls.

Personally I can't see the point and I would just use standard 500Ks and a .022uF cap, or possibly a 250K volume since those pickups are not overly hot anyway so all the positions will be fairly bright - and especially if it's a Fender-type guitar rather than a Gibson-type one. Almost all factory-made guitars with two humbuckers and VT wiring (eg PRS) simply use 500Ks and .022uF.

I prefer linear or audio taper for volume - linear is better as a true volume control for clean sounds or only-just-breaking up overdrive, audio is better for cleaning up a more heavily overdriven sound. (Audio is mostly 30% taper nowadays.) Tone controls should be audio or if possible a true (10%) Log - unless you want all the range right at the bottom of the turn, for hand-wah effects and don't use the tone control normally.

normula1 · September 2021

I simulated an additional vol & tone in my PRS for the mix position. The difference is very subtle indeed.

Amigo · September 2021

ICBM said:

You could use extra switching to simulate 250K in the middle position but it will be complicated even with a lever switch (you'll need a more expensive one) and not possible with a conventional toggle switch. Or you could use dual-gang pots and wire it exactly like a normal 2V2T setup but with only two visible controls.

Personally I can't see the point and I would just use standard 500Ks and a .022uF cap, or possibly a 250K volume since those pickups are not overly hot anyway so all the positions will be fairly bright - and especially if it's a Fender-type guitar rather than a Gibson-type one. Almost all factory-made guitars with two humbuckers and VT wiring (eg PRS) simply use 500Ks and .022uF.

Excellent suggestions, thank you, although I take it that it might not be worth bothering? The guitar is a PRS SE Hollowbody Standard.

normula1 said:

I simulated an additional vol & tone in my PRS for the mix position. The difference is very subtle indeed.

Or maybe it is worth bothering? Is the difference so subtle as to not be noticed unless listened for?

ICBM said:

I prefer linear or audio taper for volume - linear is better as a true volume control for clean sounds or only-just-breaking up overdrive, audio is better for cleaning up a more heavily overdriven sound. (Audio is mostly 30% taper nowadays.) Tone controls should be audio or if possible a true (10%) Log - unless you want all the range right at the bottom of the turn, for hand-wah effects and don't use the tone control normally.

This is the golden information I was after, and @ICBM delivered. Excellent and many thanks!

normula1 · September 2021

It's very subtle to the point I have to actually listen carefully. I've only left it in as I don't need the switch pole for anything else.

Amigo · September 2021

I had a look at the audio tapers for the Alpha, Bourns and CTS pots, and I thought I'd share them there:

Alpha Pots:

Bourns Pots:

CTS Pots:

ICBM said:

I prefer linear or audio taper for volume - linear is better as a true volume control for clean sounds or only-just-breaking up overdrive, audio is better for cleaning up a more heavily overdriven sound. (Audio is mostly 30% taper nowadays.) Tone controls should be audio or if possible a true (10%) Log - unless you want all the range right at the bottom of the turn, for hand-wah effects and don't use the tone control normally.

For the Tone pot I will go for a logarithmic pot, as I want to be able to control the corner frequency better, and it confirms my calculations as the best option for that. I happen to have a Bourns pot with the taper A2 above, which seems to fit the bill.

For the Volume, @ICBM, which of the tapers above would provide a better middle ground for achieving both good control of cleans and of overdriven? Or just go linear?

Bourns seems out of the question, as the only other option from the two audio tapers is a pure linear.

Of the CTS and Alpha, it is interesting that the tapers are quite different, in that on the Alpha there seems to be linearity in the last third of the turn, whereas the CTS pots seems to have the linearity in the first half of the turn. Any thoughts? Even if it is purely theoretical, as the actual real tapers may differ from one pot to the next, just as the resistance values.

In terms of using the three pots, I found that the Bourns pots have the best perceived quality, smooth and precise, while not stiff, then the CTSs, that were moving a bit more loosely, and then the Alphas, that felt very cheap. Would that be the general consensus as well?

Amigo · October 2021

I have just played a Gibson SG which is reported to have linear 300k pots for the volume. The 490 pickups seem rather hot, and the volume pot does not clean the signal too well. Maybe an audio taper (30%) should do the trick?

Funkfingers · October 2021

The usual rule of thumb is 250k for single coils and 500k for humbuckers. American guitar enthusiast forums are awash with intense debates about the relative merits of pots of between 440 and 550k used with PAF-inspired moderate output humbuckers.

Pay close attention to the pickup manufacturers' installation recommendations.

DiMarzio Tech Talk recommendations may come as a surprise. Several of their replacement pickups for Stratocaster and Telecaster are intended to be run through 500k pots.

Conversely, I have found that running some Duncan rails and/or li'l 'bucker pickups through any pots other than 250k ruins their tone.

gavin_axecaster · October 2021

Amigo said:

In terms of using the three pots, I found that the Bourns pots have the best perceived quality, smooth and precise, while not stiff, then the CTSs, that were moving a bit more loosely, and then the Alphas, that felt very cheap. Would that be the general consensus as well?

CTS and Alpha have different levels of quality/construction available for their pots so you have to compare like for like.

For example, CTS have a couple of different torque values so the shaft may turn freely or need more effort. Alphas tend to be on the firmer side for twidleability except the mini pots and push/push pots which are freer turning. Your perception of which is better quality will be influenced by personal preference.

There are also different constructions available with both - CTS can have brass or aluminium shafts and threaded bush or a mixture.

Same for Alpha.

CTS has the option of a circlip on the top of the shaft to help prevent accidental shaft-ectomies removing stubborn push-fit knobs.

CTS also has different tolerance levels available. Not sure about Bournes or Alpha but industry standard is 20%, which is why suppliers make a big deal about 10% "or under" tolerance on their custom CTS pots.

When you've determined which taper you prefer, work out which feel/construction you like and find a supplier that ticks all the boxes.

Amigo · October 2021

gavin_axecaster said:

When you've determined which taper you prefer, work out which feel/construction you like and find a supplier that ticks all the boxes.

That is the question: there are many tapers, especially in CTS pots, but the websites seem only to specify "Audio" for the taper, with no part numbers for them to be able to identify the exact taper. Which websites where you can purchase such pots do specify the exact taper?

tanihhiavlt · October 2021

https://www.axecaster.co.uk/index.php?main_page=index&cPath=11&sort=20a&filter_id=12&alpha_filter_id=0 mentions them, I've not seen others talk about vintage tapers apart from in wah pots

Amigo · October 2021

For example this page states Log/audio taper:
https://www.axecaster.co.uk/index.php?main_page=product_info&cPath=11&products_id=593
Model number is EP-5586, but I couldn't find anywhere the taper of it, i.e. 10%, 20%, 30%? I couldn't find on CTS's website any information about such part numbers.

Amigo · October 2021

I tried to make sense of the model number EP-5586 by looking at the CTS guitar pot catalogue with push/pull:
https://www.ctscorp.com/wp-content/uploads/Series-18VRG-Data-Sheet-Rev-A.pdf
If I check the the part number decoder there against the pot available at the link in the previous message, based on the description there, and it is not clear as to which taper is that pot actually.

gavin_axecaster · October 2021

CTS will engrave whatever you want on the pots when you order directly from them. EP-5586 is an Allparts product code that has nothing to do with CTS part numbers.

Allparts don't state the taper of the p/p pot but after measuring a few and taking tolerance into account I think it's 15 or 20%

Amigo · October 2021

That makes sense. Looking at Allparts website, the photos look different to what CTS provide in their catalogue. I suspect the Allparts pots are matching the CTS part number 18VRGT324K504NA3, which is a 15% Log.

CasperCaster · October 2021

CTS don't sell pots directly, but rather through various supply houses. In the world of guitars the various supply houses pick from CTS' various options and order 10,000 plus of each pot. As @gavin_axecaster has said, CTS will stamp the pots with whatever the purchaser requires, hence the full factory codes are rarely available on the pot itself. Rather the supply house gives the pot, or a range of pots a particular name such as the CrazyParts True Vintage Taper TVT pots (and subsequent soft vintage taper and burst taper pots). For better or worse I think the CrazyParts TVT pots probably started the interest in different log taper pots for most guitarists. Anyway, @sixstringsupplies provided some info about tapers, and who supplies what here:
https://www.thefretboard.co.uk/discussion/214765/new-pots#latest

Amigo · October 2021

You can sometimes buy Bourns guitar pots from the usual suspects: RS, Farnell, Mouser, Digikey. I have my Bourns pots, log taper (10%) from RS, bought a good while ago, both regular and Push-Pull. On these websites they appear with their manufacturer's part number and the datasheets that describe exactly what you get. I much prefer that, to satisfy my obsession

I love the feel of Bourns pots, but they do not do audio taper apart from 10% and 15%—and the latter do not seem to be available on the sites above. CTS and Alpha do do more tapers, as per the plots earlier in this thread, but, as you said, the names tend to say very little about their taper.

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Pot values and taper, and capacitors too

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