From mgarvi–(at)–anix.com Wed Nov 8 15:41:24 CST 1995
Article: 5103 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Pot Question
Date: 8 Nov 1995 06:14:05 -0500
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>A friend of mine just bought Barden pickups for his Strat. The pickups
>are the double-rail type, a humbucker which fits in the same space as a
>conventional Strat pickup. Barden has advised using 250k pots; local
>music stores advise using 500k pots Which will give optimum signal
>strength and tone?
I’d tend to go with what Barden says. He knows the optimal values
for his own pickups.
> What does the difference in pot values give you ?
>Excuse me if this seems like a dumb question, but I promised I’d post
>this for him, as we can’t seem to get a good answer from anyone we know.
>Thanks in advance for your response.
Not a dumb question at all, Tom. Here’s what I know:
All pickups are inductors by nature (as are all big coils of wire).
This means that they have a higher impedance (like resistance) at
higher frequencies. If you ‘load them down’ with a lower resistance,
then sometimes you’ll lose some signal…mostly treble. So considering
this, you might think you should use as high a value pot as possible.
Not so.
Ironically, if you put too much resistance in SERIES with the guitar’s
output, you can also diminish treble. This happens as a higher
resistance volume control is being turned down. For example, if you’ve
backed the guitar’s volume control down by 1/10th, you’ll have 25k
in series with the guitar output if you are using a 250k pot. With
a 500k pot, you’ll have 50k in series.
The treble loss caused by series resistance is actually a complex
interaction with cable capacitance, etc. and some players use it
to their advantage. Turning the guitar down automatically changes
to a mellower rhythm tone.
Conversely, players that prefer brighter rhythm sounds may benefit
>from putting a small value capacitor from the center terminal of
the guitar’s volume over to the right terminal (as viewed from
the bottom of the pot). This will allow more highs thru as the
pot is backed down. You could try a .001 cap for a start. I’m
just guessing at the value. I don’t do this on my own guitars,
so I don’t really have a personal preference.
In any case, you can see that it’s usually best to strike a balance
between high ‘paralleled’ resistance (the volume pot turned up full)
and low ‘series’ resistance (as the pot is being turned down).
For single coils, usually 250k is enough. For humbuckers, which
have higher inductance, 500k is more appropriate.
You can try some experiments pretty easily. Put a 500k pot in. Turn
the guitar volume up all the way. Get a 470k or 510k resistor from
Radio Shack or wherever –low wattage is fine. Unscrew the sleeve
>from the 1/4″ plug going into your guitar. As you are playing, have
someone hold the resistor across the two connections and see if you
can tell the difference.
Paralleling your 500k volume pot with the 470k resistor simulates
changing to a 250k pot.
—————————————————————–
I saved the ugly math for the end. Impedance is like a resistance
which varies with frequency. Inductors are lower impedance at
low frequencies, etc. The effective resistance (called ‘reactance’)
at a given freq. is:
X = 2 * pi * L
where X is reactance in ohms and L is inductance in Henrys.
A typical humbucker may be around 7.5 Henrys or so.
This means that at 5khz the pickup’s reactance will be around 235k ohms.
5000hz * 3.14159 * 7.5hy = 235500 ohms.
Remember that the objective in choosing the correct pot is not in
*matching* the reactance, but in staying about twice as high (ancient
Hippy wisdom). So in this case, a 500k pot is appropriate.
5khz may seem like a low limit for treble frequencies, but most
guitar speakers start to roll off around there anyway. So guitar
amps in general don’t respond much above say…7khz. If they did,
you probably wouldn’t like it.
Strat pickups have a lower inductance, and therefore will not
require as high value pot.
Also be aware that pots can commonly be found in two ‘tapers’:
log and linear. The log pots may be better for guitar, since
the volume increases smoothly as the pot is rotated. But I know
players who like the faster increase provided by a linear taper pot.
I hope that helps a bit.
Regards,
Mark Garvin
Composer/Design Engineer
New York City
From fto–(at)–etcom.com Wed Nov 8 15:41:55 CST 1995
Article: 5117 of alt.guitar.amps
Newsgroups: alt.guitar.amps
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From: fto–(at)–etcom.com (Tom May)
Subject: Re: Pot Question
In-Reply-To: mgarvi–(at)–anix.com’s message of 8 Nov 1995 06:14:05 -0500
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Date: Wed, 8 Nov 1995 19:02:15 GMT
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In article <47q3dt$7p--(at)--anix2.panix.com> mgarvi–(at)–anix.com (Mark Garvin) writes:
As usual, a very informative post. Thanks Mark.
>I saved the ugly math for the end. Impedance is like a resistance
>which varies with frequency. Inductors are lower impedance at
>low frequencies, etc. The effective resistance (called ‘reactance’)
>at a given freq. is:
>X = 2 * pi * L
You’ve been up too late . . . that should be X = 2 * pi * f * L,
where f is the frequency in Hz. ^^^
>where X is reactance in ohms and L is inductance in Henrys.
>A typical humbucker may be around 7.5 Henrys or so.
7.5 henrys? That seems really huge. Not that I’ve actually measured
the inductance of any pickups (just resistance), but I’m just
wondering where you got that number. I guess pickups do have quite a
few turns.
>This means that at 5khz the pickup’s reactance will be around 235k ohms.
>5000hz * 3.14159 * 7.5hy = 235500 ohms.
Another typo, you left out the “2” on the left side but got the right
answer anyway. I wish I could do that. I’m not dissing you, I just
wanted to point that out for anybody who was following along and got
confuzed.
>Remember that the objective in choosing the correct pot is not in
>*matching* the reactance, but in staying about twice as high (ancient
>Hippy wisdom).
🙂
—
Tom.
