guitar pots

[rudutch]

I need enlightenment

The Gibson Les Paul (2001) is supposed to have 300k pots, I hear that changing these out to 500k pots is supposed to make a big difference.

IF I understand, a pot goes from near zero ohms to ground (off) up to "X" when all the way up. So, is 200k ohms gonna make s dramatic change? I realize we are dealing with very low voltage.

2nd question, is it then necessary to also change the tone pots?

too much marketing and misinformation out there for me to figure out

thx in advance

[Glass Snuff]

Well, the first thing is, I've seen many, many reports from people that have changed out their Gibson pots, and the responses are so overwhelmingly favorable that I'm led to believe that Gibson pots just aren't that good.

The second thing you need to realise is that your volume controls are wired in such a way that they provide a constant load on the pickup. Turning down a 500K pot will not sound like a 300K pot. Your tone control, on the other hand, is wired differently and turning it down will be exactly the same as using a lower value pot.

The third thing to consider is that you now have linear volume pots and if you change them you have a choice of linear or "audio" (log). If you just turn down a little bit to adjust your place in the mix, or to tweak your distortion, you might prefer linear. They will make initial changes happen slowly, then everything will roll down at the end. If you turn down a lot, use volume swells, or play without a lot of distortion, then you're likely to prefer audio taper pots which will make the changes over the whole range seem smoother.

Now, the difference between 500K and 300K is with a higher value, you'll get more resonance. The pickups will seem brighter, slightly louder, and more responsive. For most people, this is a welcome change. "Dramatic"? Well, it's not as big a change as changing a speaker, but it sure costs a lot less.

You don't need to change your tone pots, but as I said in the beginning, there's some question about the quality of the stock pots, and it can't hurt. While you're at it, changing the caps to Orange Drops or PIO isn't a bad idea, either. But that's a whole 'nother thread.

[billyz]

Many of the Gibson pots are of Poor Quality to begin with. I have experienced way too many bad/loose lugs with them. I have never understood why Gibson ever changed to Linear Pots. 300k , what are they thinking. Maybe it was to tame the shrill and or harsh sounding pickups they have produced.

In my experimentations I have found 500kA for both Tone and Volume work best. use the Vintage wiring schematic it makes a small change in the tone control when rolled down. Gives the most useful sounds for my tastes.

[MistaT]

I agree with Billyz. 500k across the board. If you can find pots made by CTS, those are pretty good ones.

[Chuck H]

CTS pots can be hard to source. Bourn's makes some really good pots with a 10% taper that have a very low torque. Perfect for guitar use. And, FWIW, changing from 300k to 500k pots... You will wonder if the difference is imaginary or not unless your using high output pickups with a very high Z. Just do the math for the Z of your pickup in parallel with the pot load to figure the real world difference. All lore and mojo aside, this is the electronic facts of the matter. Oh, and piss on orange drop caps, use any film cap you have. If you hear a difference between any reasonable film cap in this app then your hearing goes beyond the test gear. Cheap ceramics ARE best avoided though.

Don't believe the lore and mojo. It is always better to understand something than to simply know it because someone told you.

Chuck

[pdf64]

See
Kinman - FAQ
I've not worked through the data and done the math, but taken at face value it seems a plausible explanation as to why a difference in vol pot values would be noticeable.
If you've got a HB guitar with a 500k vol pot handy, it would be simple to compare it's tone as stock and with a bleed resistor across the pot; 680k would get the equivilant resistance to ~290k, 470k to ~240k.

[uvacom]

CTS pots can be hard to source. Bourn's makes some really good pots with a 10% taper that have a very low torque. Perfect for guitar use. And, FWIW, changing from 300k to 500k pots... You will wonder if the difference is imaginary or not unless your using high output pickups with a very high Z. Just do the math for the Z of your pickup in parallel with the pot load to figure the real world difference. All lore and mojo aside, this is the electronic facts of the matter. Oh, and piss on orange drop caps, use any film cap you have. If you hear a difference between any reasonable film cap in this app then your hearing goes beyond the test gear. Cheap ceramics ARE best avoided though.

Don't believe the lore and mojo. It is always better to understand something than to simply know it because someone told you.

Chuck

I agree that the type of cap is unimportant, but a guitar has a complex impedance, and the electronics fact of the matter is that changing the pots from 300k to 500k can boost the amplitude of the resonant peak by about 6dB (and slightly shift it's frequency upwards). This is also dependent on the system's capacitance, which is predominantly a function of cable capacitance. The difference between pots is less noticeable with a very short, high-quality cable than with a long, cheap one.

Why? Because at audio frequencies, you can no longer view the coil's inductance as a short and the capacitance as an open, and these reactive components have a complex impedance that varies with respect to frequency. Then the potentiometers are only a part of the equation, but they interact with these other parameters heavily.

[guitician]

Yes, any resistance change is going to make a difference, mostly, the higher the resistance, the more treble you'll get.
I've made this dual humbucker guitar and used a 1Mohm vol., a "no-load" 250k tone, and switching that makes it isolated "pickup lead only" out to the jack. I used .015 film cap and when I load the tone control I hear a drop in highs, with it on 10. Also, when I "lift" the 1Mohm Vol. (while the tone is unloaded), I'll notice an increase in highs too. All very subtle changes of coarse, but if you play close to the saddle, it will make a very noticeable change in how those overtones are presented to the amp.

[Chuck H]

I agree that the type of cap is unimportant, but a guitar has a complex impedance,

Yes

and the electronics fact of the matter is

No need get snippy

that changing the pots from 300k to 500k can boost the amplitude of the resonant peak by about 6dB

Really, 6db !?! Now, I'm no expert and I'm happy to learn I'm wrong if I can learn something in the process. So I'm open to convincing, but...

An electric guitars output impedance is pretty low compared to the volume pots resistance. For example let's use the not unreasonable figure of 10k. If I parallel a 300k load with that guitar the resulting impedance is about 9.6k. If I parallel the same guitar with a 500k load the resulting impedance is about 9.8k.

I agree that reactivity does increase, and this will raise the resonant peak, but 6db !?! How can a difference in overall impedance of less than 2% raise the resonant peak by 6db?

(and slightly shift it's frequency upwards)

Yes

This is also dependent on the system's capacitance, which is predominantly a function of cable capacitance. The difference between pots is less noticeable with a very short, high-quality cable than with a long, cheap one.

Agreed. I use to take my meter to the guitar store and check capacitance on all the cables before buying. They love that BTW

Why? Because at audio frequencies, you can no longer view the coil's inductance as a short and the capacitance as an open, and these reactive components have a complex impedance that varies with respect to frequency. Then the potentiometers are only a part of the equation, but they interact with these other parameters heavily.

Now this I can get behind. But even though I don't know the peaks and valleys of a guitars impedance I have a very hard time believing it swings so wide as to react 6db louder on the resonant peak with a 500k pot vs. a 300k pot. Then again, a typical 8 ohm loudspeaker may have an impedance of well over 100 ohms at the speakers resonant frequency. As I said, I don't know much about the impedance characteristics of pickups. If you know these figures for any common pickups (P-90, PAF, strat type single coil, etc.) that may help me get my head around it.

Chuck

[Glass Snuff]

While hardly conclusive, I offer this insight using Spice:



As I mentioned above, it changes the resonance, and the key is not so much the extra output as the added high order harmonics inferred by the steeper sides of the peak.

[billyz]

By the way, CTS pots are very easy to find.

Mojo Potentiometers Mojotone CTS 500K Short/Split Shaft Potentiometer

Specialty Guitars: CTS Potentiometers 500K

CTS Potentiometers | AllParts.com

Google them, they are everywhere.

Bourns (chinese) are ok, better than Alpha in my opinion (or different ?). Better than CTS is Allessandro (PEC). Yes, they are better , mostly the taper is as close to the old Centralab as I have found. Are they worth it? to some , others drink budwieser and are very happy about it.

If a treble bleed cap makes no difference then why not a Ceramic ?

Caps make a small difference , maybe not as much a percentage as other things. There is the law of diminishing returns, but for some , money is not the issue. I would use a sprague or mallory (sub your fav film type)for the money.

I like my pots to be on the high side, impedance wise, but they don't need to be matched. I use the higher value for the Volumes and lower for tone. When I sort them. These days, with everyone sorting the pots for value, it is not uncommon to get a bunch of low value CTS pots , like 450K- instead of 550K or even 500K, unless you pay for them.

At least in my experience. YMMV

[Mike Sulzer]

An electric guitars output impedance is pretty low compared to the volume pots resistance. For example let's use the not unreasonable figure of 10k. If I parallel a 300k load with that guitar the resulting impedance is about 9.6k. If I parallel the same guitar with a 500k load the resulting impedance is about 9.8k.

It is a parallel resonant circuit composed of the inductance of the pickup and its capacitance and that of the cable (which is usually the dominant one). At resonance the impedance becomes very high, infinite in the case of a lossless resonant circuit, as much as several hundred Kohms in a real pickup.

Thus, any resistance placed across the pickup (volume and tone controls and the input resistance of the amp) load the resonant circuit and reduce the output in that frequency range. So higher resistances sound brighter, as long as the resonant frequency is located somewhere in the typical range (2.5 to 5 KHz with the cable).

All three of the resistances mentioned above go in parallel to determine the complete resistive load. This is true of the tone control pot as well because the tone cap is like a short circuit at the resonant peak. (When you turn the tone control down from ten, the loss of high frequencies is due to loading the resonant circuit. The capacitor only has an effect down closer to zero.)

[uvacom]

While hardly conclusive, I offer this insight using Spice:



As I mentioned above, it changes the resonance, and the key is not so much the extra output as the added high order harmonics inferred by the steeper sides of the peak.

Hmm, interesting. I'm getting different data with my spice model. Of course it's just a model, but it is handy for plotting curves of LCR networks that go beyond the simple textbook series or parallel networks (such as what we have in an electric guitar) without having to do the tedious calculations.

This is my circuit:



And this is the plot (parametric AC sweep for potentiometer values 100k, 250k, 300k, 500k, and 1Meg, both pot values swept).



Do you see any glaring discrepancies? I use terry downs's model as a starting point, but I've just started making the model more elaborate by modeling the pickup as a network of distributed elements (instead of just lumped elements), and eventually I'd also like to model effects such as eddy currents, etc.

[Mike Sulzer]

...and eventually I'd also like to model effects such as eddy currents, etc.

Not so easy. You will need a leakage inductance in series with a resistor that varies with the square root of frequency across the pickup. But the effects are large enough to matter.

A new model the impedance of a humbucker, compared to measurements

[Chuck H]

Brilliant stuff. As an amp guy I didn't know a guitar pickups impedance could swing so wide. But, since it does, it makes better sense to me now that a parallel difference of 200k could make a real world change.

FWIW I have always been able to "hear" it. I test different resistance pots and resistor/cap combinations for "bright" circuits with different guitars and pickups to determine what sounds best to me. So I know there is a difference. I didn't know it could be so profound and I was playing devil's advocate to find out why

The info in this post should be known to every amp builder. Sadly there seem to be few posters here on Ampage that cross over.

Thanks

Chuck