Measuring a capacitor value in a tone circuit

[jmcorey]

Hi guys,

One of my guitars sounds dull on the high end. 2 humbucker super strat design, 250k tone, 500k volume. I am not the first owner, so I wonder if the previous owner changed the cap to make the guitar sound darker.

I have a new multimeter and I want to measure the capacitance of the cap in the tone circuit.

Do I need to de-solder the capacitor (take it it of the circuit) to do so?

Also, while I'm at it: I believe the relationship between capacitance and tone is that the lower the cap value, the higher the frequencies that are allowed to be heard. Is this correct?

Thanks

 

[gregory]

Re: Measuring a capacitor value in a tone circuit

Change the 250k tone to 500k or even 1M. It is true that the cap lowers the cuttoff frequency, but this frequency is far lower than what you might realize, perhaps as low as 80Hz, so going from a 47nF to 22nF or even 2.2nF isn't going to buy you any additional brightness when the control is at 10.

 

[ItsaBass]

Re: Measuring a capacitor value in a tone circuit

A typical digital multimeter (DMM) doesn't measure capacitance. You need an additional meter: a dedicated capacitance meter, or an inductance-capacitance-resistance (LCR) meter.

I'm not sure if the cap has to be desoldered or not. I've never come across a situation in which I've wanted to measure a guitar cap while it's connected. Usually I want to measure them before installing, in order to check the actual capacitance vs. the marked capacitance. What I would do is measure it both ways, and see if there is any difference.

Anyone messing around with guitar electronics should have a basic LCR meter before they have a DMM. The LCR meter gives you the three main things you need to measure in passive guitars, and nothing you don't need.

All that being said, I doubt cap value is your issue. I would be tempted to put 1M pots in a humbucker-equipped Strat, myself. But for the tone problem you describe, I would use at least 500K all the way around.

 

[jmcorey]

Re: Measuring a capacitor value in a tone circuit

A typical digital multimeter (DMM) doesn't measure capacitance. You need an additional meter: a dedicated capacitance meter, or an inductance-capacitance-resistance (LCR) meter.

Anyone messing around with guitar electronics should have a basic LCR meter before they have a DMM. The LCR meter gives you the three main things you need to measure in passive guitars, and nothing you don't need.

All that being said, I doubt cap value is your issue. I would be tempted to put 1M pots in a humbucker-equipped Strat, myself. But for the tone problem you describe, I would use at least 500K all the way around.

1. Mine does.

2. I was unaware of LCR meters, and I wish I knew about these before, so thanks.

3. I appreciate the advice regarding it really being about the pots.... Thanks.

 

[DavidRavenMoon]

Measuring a capacitor value in a tone circuit

Also, while I'm at it: I believe the relationship between capacitance and tone is that the lower the cap value, the higher the frequencies that are allowed to be heard. Is this correct?

You need a LCR meter to read caps.

The larger the value the lower the frequency cut.

But the cap in the tone circuit is only in effect when the control is at 8 or lower.



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[PicoRiveraTele]

Re: Measuring a capacitor value in a tone circuit

LCR meters are only useful for measuring inductance, and for them to do that correctly, they have to support a low test frequency. The capacitance measure fails because a pickup is not an isolated capacitance, and the resistance measure is an AC impedance measurement. The cheapest suitable LCR meter that is able to accurately measure inductance costs about $100. The underlying problem is that LCR meters are meant to measure capacitors and inductors, and a guitar pickup is not an ideal representation of either, though it's closest to an inductor.

 

[ExplorersRock]

Re: Measuring a capacitor value in a tone circuit

I wish I could offer more on the cap measurement, but I can say that I agree with previous posters about the tone pot. If it were me, I'd replace it with a 500k pot and if I was uncetain about the cap, I'd replace it too. I like .022 mfd caps with my HBs.

 

[Clint 55]

Re: Measuring a capacitor value in a tone circuit

The cap in the tone circuit is only in effect when the control is at 8 or lower.

That's incorrect. That's like saying if you bridge the jack's hot and ground with a 250k resistor, it's not in the circuit. Caps affect the sound even when the tone pot is on 10. They are most noticeable with 250k tone pots.

To the OP: You can brighten your sound without buying anything by no loading your tone pot:

https://www.youtube.com/watch?v=dN3SlH-cEAg

I prefer to use 250k no load tone pots with everything because they're as bright as you need it, but they take effect as soon as you turn it down to 9.

 

[AlexR]

Re: Measuring a capacitor value in a tone circuit

All you have to do is go no-load to see the effect that a cap has even at 10......especially with a 250k tone pot you will find that the effect is for sure noticable.

It is both irresponsible and misleading to generically say any tone pot on 8 (or whatever randomly generated number you think of) is a cutoff for notice of effect. Lets face it 250k on 10 is like a 500k tone pot on 7 or thereabouts.

 

[PicoRiveraTele]

Re: Measuring a capacitor value in a tone circuit

I've found that the lower the cap value, the sooner the tone knob makes a bigger audible difference as you turn it down, which stands to reason. A 0.1uF cap value, which came stock in some Fenders in the past, is very muddy at zero, but makes for a great sweep at all points in between.

 

[Clint 55]

Re: Measuring a capacitor value in a tone circuit

If you want a long bright portion of the sweep you can use a linear taper pot.

 

[ItsaBass]

Re: Measuring a capacitor value in a tone circuit

If you want a long bright portion of the sweep you can use a linear taper pot.

Exactly what I switch to every time I swap out a tone pot. I prefer the extremely fine adjustment you can get over most of the range.

Those who do a lot of tone knob fiddling as they play probably prefer audio taper, though, as it gets you where you probably want to be faster. I am more of a pre-set tone knob user, so I prefer being able to make a very precise tone knob placement, even if it's slower.

FWIW, I usually use .01 uF or .0047 uF caps for my tone controls too (and often 1M pots).

Needless to say, I don't like how most stock tone controls are so "front loaded" in taper, and so dark in cap value.

 

[Clint 55]

Re: Measuring a capacitor value in a tone circuit

After reading your post, I had to go put in a 4.7nf cap in my sss strat. I like how it still sounds like a guitar even with the tone on 0.

I'm gonna have to score some paper in oils now. 4.7nf and maybe 15nf for the guitar I play jazz on. 22nf is even too dark for that.

 

[DavidRavenMoon]

Re: Measuring a capacitor value in a tone circuit

That's incorrect. That's like saying if you bridge the jack's hot and ground with a 250k resistor, it's not in the circuit. Caps affect the sound even when the tone pot is on 10. They are most noticeable with 250k tone pots.

You are NOT hearing the cap as much as you are hearing the loading of the pot.

At the impedance of the circuit, the cap is acting more like a bare wire when the pot is on 10.

You can test this for yourself. I have a three way switch in one of my basses to select between a .047 and .022 cap, and removing the caps from the circuit.

If the 500k tone pot is on 10, and you switch between the three settings you hear no difference in tone.

But the whole circuit is interactive depending on the value of the tone pot, the impedance of the pickup, and so on.





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[Clint 55]

Re: Measuring a capacitor value in a tone circuit

You are NOT hearing the cap as much as you are hearing the loading of the pot.

The signal's only path to ground is through the cap. 500k tone on 10 and no load sound similar, but they are not electrically identical. You can hear the difference on a guitar. Caps affecting the sound with the tone on 10 is most noticeable with a 250k pot tho.

 

[gregory]

Re: Measuring a capacitor value in a tone circuit

Not that I'm saying 500k vs no sound the same, but not electrically identical and being actually audible (I mean truly audible rather than only cognitively steered) are two very different things.

 

[Clint 55]

Re: Measuring a capacitor value in a tone circuit

Yap

 

[PicoRiveraTele]

Re: Measuring a capacitor value in a tone circuit

The signal's only path to ground is through the cap. 500k tone on 10 and no load sound similar, but they are not electrically identical. You can hear the difference on a guitar. Caps affecting the sound with the tone on 10 is most noticeable with a 250k pot tho.

They're electrically identical, because a cap with a large enough value, 0.022uF or .047uF, presents no impedance around the resonance. This can be shown with a SPICE model, where a 47n cap is bypassed and it makes no difference in the resonant curve. In order for the cap to present an impedance at the resonance, the value of the cap would have to be below 1nF, at which point the impedance across the cap is so great is great that it's as if there is no tone pot at all.

 

[Lewguitar]

Re: Measuring a capacitor value in a tone circuit

Disconnect the tone control and see if your guitar brightens up.

If it does, replace the 250k tone pot with a 500k tone pot.

I like .02 caps.

I also like a 180pf bright cap across the volume pot on my guitars with humbuckers.