Quick'n'Dirty Pickup Resistance Measurement

[duhvoodooman***]

** EDITED FOR ACCURACY & COMPLETENESS**

Here's a fast way to measure the approximate (within a couple of percent) DC resistance of your pickups without having to take anything apart on the guitar. All you need is a multimeter and a patch cable. Insert the patch cable into the guitar, turn the guitar's pickup selector switch to activate the pickup you wish to measure, turn the volume pot all the way up, and then contact one meter probe to the tip of the exposed cable plug and the other to the shaft below the insulating ring. You've now completed a circuit through that pickup and by turning your multimeter to the expected resistance range (I use the 20K range setting on my cheapo Rat Shack meter), you'll be able to read the pickup's resistance.

The photo below shows how this is done. In it, I'm measuring the middle pickup of my new Xaviere XV870, which reads 5.18 Kohm. Easy!! However, keep in mind that this is an approximation. To get a truly accurate measurement of a pickup's resistance, measure across its "hot" and ground wires while the pickup is isolated from the rest of the guitar's circuitry.

WHAT'S ACTUALLY GOING ON HERE (Warning: high techno-geek factor!): You are actually measuring the resistance of the selected pickup and the guitar's volume pot in parallel. Because the pot has a much higher resistance than the pickup (250 - 500 Kohm typical vs. 5 - 15 Kohm for most pickups), the way parallel resistances combine makes the measured value just slightly lower than the actual pickup resistance. Mathematically:

For the pickup and "dimed" volume pot in parallel:

M = measured resistance at output jack
P = pickup resistance
V = volume pot resistance

M = P*V/(P+V)

For example, for a 5.0 Kohm pickup and a 250K volume pot, the expected measured resistance at the output jack would be 4.90 Kohm, just 2% low. Similarly, for a "hot" humbucker at 15.0 Kohm and a 500K volume pot, the expected value at the jack would be 14.56K, about 3% low.

IMPORTANT: The accuracy of this method is tied to how close to "full on" your volume pot can get. For a decent quality pot in good condition, this shouldn't introduce any significant error. However cheaply constructed or very old/worn pots may introduce errors by leaving residual resistance in series with the pickup, even when the pot is "dimed". For example, if in the first example given above, the volume pot only goes to 99% of it's total span when dimed (i.e. 247.5 Kohm), the measured resistance would be 7.28 Kohm--a significant error. Other elements within the pickup's signal chain, like the pickup switch, the internal wiring, the output jack and the patch cable, contribute an insignificant amount of resistance and don't introduce any meaningful amount of error.

Incidentally, applying this same logic, if you turn your volume pot down as you make this measurement, you will see the measured resistance climb quite quickly. That's because you are adding a portion of the pot's resistance in series with the pickup, and at the same time, decreasing the portion of the pot resistance that is in parallel with the pickup. Consequently, the measured resistance climbs to a maximum value at about 50% of the pot's nominal resistance range, and then falls again, ending at zero (or very close to it) when the volume pot is all the way off. At that point, there is zero resistance to ground for the guitar signal through the pot, which is exactly why the volume shuts off completely!

(OK, major geek detour ends here.)

This technique is also a good way to check for a broken wire or connection in your pickup signal chain through the guitar. If you have an open circuit in the pickup, the measured resistance will be that of your volume pot. By the same token, a "dead short" will show as zero resistance or very close to it.

[ShortBuSX]

WOW! Great tip!
Im gonna go grab my meter now!

[duhvoodooman***]

Just remember to turn the volume pot all the way up or you'll get an inaccurate result. I forgot to mention that initially, but have edited the tip to include it.

[mrmudcat]

nice brother!

[ShortBuSX]

Just remember to turn the volume pot all the way up (i.e. zero resistance) so that you're not measuring the pot resistance in series with the pickup. I forgot to mention that initially, but have edited the tip to include it.

I also notice if you turn the tone pot all the way down the reading is higher.

I think Im a lil confused though...is this accurate? Is this the actual resistance of each pickup? I know the tone pot has got to cut into it a lil, right?
On my 2 MIM, my MIJ, and my Squier Tele there is no consistancy as to which pup is hotter and where its located...the only one that makes any sense is my LP. On all 3 Strats the resistance was almost identical with all 3 pickups but only shying by .10s at the most.

[Spudman***]

I'm glad you posted this Voodoo. Thanks.

I'd thought about mentioning it a while ago but I wasn't sure if you can really get an accurate reading. I figured cable, pots, and switch would add some resistance. Does it?

[sunvalleylaw]

Cool! Will have to try it just for fun.

[tot_Ou_tard]

Coolio!

I'm gonna give it a try as well.

Does it make sense to also do this measurement at the cable plug just before your amph or your first effect?

That is, does the resistance added after the pickup affect the sound of the pickup "in the same way" as resistance in the coil windings themselves?

[duhvoodooman***]

I've updated the original post in this thread to clear up some inaccuracies and provide a more detailed explanation of what's going on with this measurement technique. To answer some of the questions raised:

I also notice if you turn the tone pot all the way down the reading is higher.

I think Im a lil confused though...is this accurate? Is this the actual resistance of each pickup? I know the tone pot has got to cut into it a lil, right?
On my 2 MIM, my MIJ, and my Squier Tele there is no consistancy as to which pup is hotter and where its located...the only one that makes any sense is my LP. On all 3 Strats the resistance was almost identical with all 3 pickups but only shying by .10s at the most.

The tone pot shouldn't make any difference, since the tone capacitor blocks DC current. Turning the tone pots on my XV870 made no change in the measurement. However, if you have your tone pots wired up with some type of "treble bleed" resistor bypass across the cap, that may well cause accuracy problems. I can't vouch for this method once you get into wiring modifications on the pots. As far as your Strat pickups all measuring close to the same, that doesn't surprise me for a "matched" set. The p'ups in my new Xaviere are all within a couple of tenths of a Kohm of each other.

I'd thought about mentioning it a while ago but I wasn't sure if you can really get an accurate reading. I figured cable, pots, and switch would add some resistance. Does it?

While the volume pot can introduce inaccuracy as described in my modified original posting, the other stuff adds negligible resistance that doesn't produce any significant error. Again, all bets may be off with certain wiring and pot modifications.

Does it make sense to also do this measurement at the cable plug just before your amph or your first effect?

That is, does the resistance added after the pickup affect the sound of the pickup "in the same way" as resistance in the coil windings themselves?

The guitar cable--even a long one--should add negligible resistance vs. the pickups and pots. However, cable capacitance can induce significant treble loss under some conditions. There's a good article HERE that explains the effect.

As far as that "post-pickup" resistance impacting the tone--well, you have that well known treble-loss issue when you dial the volume down. But that's not affecting the sound of the pickup "in the same way as the resistance in the coil windings themselves", as you put it. In general, more resistance--a higher output impedance--is a good thing for your tone, as it preserves the treble. The dreaded "tone-sucking" occurs when low impedance allows the higher frequencies to be lost to ground. And it's important to remember that the resistance is but one of several characteristics of a pickup that determine it's overall tone and response behavior.

[tot_Ou_tard]

I loved the high techno-geek factor.

Let's have more of that please!

[Bloozcat]

Excellent post, DVM...:

[ShortBuSX]

The tone pot shouldn't make any difference, since the tone capacitor blocks DC current. Turning the tone pots on my XV870 made no change in the measurement. However, if you have your tone pots wired up with some type of "treble bleed" resistor bypass across the cap, that may well cause accuracy problems. I can't vouch for this method once you get into wiring modifications on the pots. As far as your Strat pickups all measuring close to the same, that doesn't surprise me for a "matched" set. The p'ups in my new Xaviere are all within a couple of tenths of a Kohm of each other.

Thanks for clearing that up, kinda.
I dont have any wiring mods except like the one you just did, where you moved the - bridge pup to the tone pot leaving the middle without.
I swore it read higher values when the tone was turned all the way down...but then I also kinda question where you say the tone pot doesnt have any affect, and to that I have to reffer to the "no load tone pots"...Im not trying to be a smarty, cause I dont really know...Im just trying to figure a couple of things out...and Im really finding this info useful, so thanks.:

[duhvoodooman***]

Thanks for clearing that up, kinda.
I dont have any wiring mods except like the one you just did, where you moved the - bridge pup to the tone pot leaving the middle without.
I swore it read higher values when the tone was turned all the way down...but then I also kinda question where you say the tone pot doesnt have any affect, and to that I have to reffer to the "no load tone pots"...Im not trying to be a smarty, cause I dont really know...Im just trying to figure a couple of things out...and Im really finding this info useful, so thanks.:

Well, let's see if we can get rid of that "kinda"....

The reason that the tone pots have no effect on this measurement is because they place a capacitor between the pot and ground, and caps block DC (which is what the multimeter uses to measure resistance) while letting AC (the pickup signal, in the case of guitars) through. At least they damn well better, because this is the whole basis of capacitive coupling!

The whole "no-load" tone pot thing is designed to avoid AC signal loss to ground through the tone control, even on the full treble setting. Since we're dealing with DC here, it doesn't apply, since it's blocked by the cap.

So if you see the measured pickup resistance changing with variation of the tone control, sumthin' else is going on!

[ShortBuSX]

Thanks for the explanation, Im gonna try it again...Im sure youre right:
Arent you usually???

BTW, you know this is why I like buyin pedals from you?
I mean I can only fake it for so long...before it starts gettin all "ohms law" and all

[duhvoodooman***]

Just to check this out, I took the back off my Fullerton ST4 HSS tonight and measured all four pickup configurations (neck, middle, bridge-humbucker mode and bridge-coil split mode) directly . This also allows direct calculation of the parallel-pickup resistances for the neck-middle combo at position 4 of the switch, and the two combos (middle/bridge-humbucker and middle/bridge-coil split) at position 2. This exercise gave me seven "actual" values to compare against the values obtained by measuring at the ouput jack with the meter/patch cable method described above.

In all cases, the output jack measurement was 1 - 3 % low vs. the measured or calculated actual value--a very close approximation. And when I plugged the seven actual values and volume pot resistance into the equation given in the first post in this thread, the predicted output jack measurement was within 0.01 - 0.02 Kohm in all cases. Right on the money!

[Bloozcat]

Thanks for the explanation, Im gonna try it again...Im sure youre right:
Arent you usually???

BTW, you know this is why I like buyin pedals from you?
I mean I can only fake it for so long...before it starts gettin all "ohms law" and all

Now that's funny, I don't care who you are....