Troubleshooting pickup volume mismatch, Gibson Les Paul BFG

[slowfoot]

Hello fine folks,



I've got a Les Paul BFG from the original 2007-2009 run that comes stock with a Bridge Burstbucker 3, a neck p90, and the wiring is volume/volume/tone on the lower bout, with the pickup selector in the neck tone hole and a kill switch on the normal LP pickup selector position.



I bought this guitar second hand, last year, and I've always had trouble with the neck p90 dramatically overpowering the volume of the bridge pickup in the middle position, even with the p90 lowered all the way down and the bridge pickup all the way up. I have a couple more LP style guitars and this hasn't ever been the case in any of those.



I blamed the BB3 bridge pickup and replaced it with a hot Fralin Big Single trying to go for a bit of a tele vibe and the volume mismatch continued. I decided to pull that pickup out and put in a slightly hotter humbucker, from Arcane Pickups, and the problem persists. I had not (foolishly) checked the resistance of the original BB3 when installed into the guitar, but here are the values shown by the other pickups involved.





Neck P-90 installed: 7.6k

Fralin Big Single installed: 14.0K

Fralin Big Single removed: 15.3K

Arcane 3x3 instealled: 8.6k

Arcane 3x3 not instealled: 9.1k





Is the drop off in resistance installed versus removed normal, or is this indicative of a further problem I'm chasing? If there's anything I can provide to give more information, please let me know. I am (sadly) new to the tinkering-on-guitars world, but haven't ever had any trouble like this in the past. It might also be that the Gibson P-90 is just that much meatier than my other pickups and I'm not going to get them to balance out at all but at this point I don't know how to prove or disprove this hypothesis.





Thank you for your time and for reading my post.





Slowfoot.

[slowfoot]

Further information that might (or might not) be helpful:



Arcane Humbucker is a 3x3 gold foil cover, and an Alnico 5 magnet. It does have the four wire (and an unshielded ground) that I am not using for coil split purposes at the moment, the split wires (red and white in Arcane wiring schematics) are soldiered and shrink wrapped out of the way.

The Big Single was a 43 gauge wire version, with a 2% overwind. Given that it was a simple gibson style braided shield wire, I would like to think I did not mess anything up there!


Also:  I had originally posted this in the pickups forum, but then I saw it was for describing pickups not for questions so I reposted this here.


Thanks again 

[Yogi B]

Nov 5, 2024 0:00:45 GMT -5 slowfoot said:

Neck P-90 installed:  7.6k
Fralin Big Single installed: 14.0K
Fralin Big Single removed:  15.3K
Arcane 3x3 installed:   8.6k
Arcane 3x3 not installed:  9.1k

Is the drop off in resistance installed versus removed normal, or is this indicative of a further problem I'm chasing?  If there's anything I can provide to give more information, please let me know.

It is normal that the measured resistance will be a little lower when installed as the resistance of the pickup is now in parallel with the volume pot. However, not usually by as much as what you've observed, so I think there is something a little more going on.


R_\text{total} = {\vcenter{\dfrac{1}{\dfrac{1}{R_\text{pickup}} + \dfrac{1}{R_\text{other}}}}}
\implies
R_\text{other} = {\vcenter{\dfrac{1}{\dfrac{1}{R_\text{total}} - \dfrac{1}{R_\text{pickup}}}}}

Using the above I get R_other as 165k or 157k for the two different humbuckers. Don't read too much into these numbers being different. As the installed/uninstalled numbers are still relatively close, even a little rounding has a large impact on the end result — the worst possible cases would result in a range from 153k to 178k.

In the past Gibson have used 300k pots, but values of around 160k would suggest two of them in parallel. Assuming normal wiring practices, whilst in the bridge position only the bridge volume should have an influence on the reading. The neck volume should be disconnected from the output; and, though the master tone is in the circuit, normal wiring has the capacitor (which blocks DC) in series with any path to ground (thereby eliminating the tone pot from the resistance measurement).

However, in looking for images of BFG wiring, I have come across a couple in which it appears that lug 3 of the tone pot is grounded — adding the resistance of the tone pot in parallel to that of the volume pot. I don't think either guitar has 100% stock wiring, so I can't eliminate that as a factor, but it's odd that two people would do the same weird thing. The first guitar has had treble bleeds added and the the tone cap replaced with a bumblebee type (mostly obscuring what I hoped to show, which is unfortunate); the second is closer to the original wiring, but it looks like the pickups have been swapped.

SPOILER: Click to show

Sonically this won't make much if any difference, and doesn't account for the volume mismatch, but if your guitar is wired with 300k pots and has the unusual tone pot grounding it would explain the resistance measurements you're getting.

---

I bought this guitar second hand, last year, and I've always had trouble with the neck p90 dramatically overpowering the volume of the bridge pickup in the middle position, even with the p90 lowered all the way down and the bridge pickup all the way up.  I have a couple more LP style guitars and this hasn't ever been the case in any of those.

I blamed the BB3 bridge pickup and replaced it with a hot Fralin Big Single trying to go for a bit of a tele vibe and the volume mismatch continued.  I decided to pull that pickup out and put in a slightly hotter humbucker, from Arcane Pickups, and the problem persists.   I had not (foolishly) checked the resistance of the original BB3 when installed into the guitar, but here are the values shown by the other pickups involved.

When mixing pickups in parallel it's not really about 'hotness', it's more about 'efficiency': the voltage output per ohm of the coil's impedance.

If we focus on lower frequencies (mid-range & bass), such that the DC resistance of a pickup remains a good approximation for its AC impedance, then the voltage output each individual pickup contributes to the total can be given by a simple resistive voltage divider:

V_\text{contribution} \approx V_\text{pickup} \times \frac{R_\text{loading}}{R_\text{pickup} + R_\text{loading}}
When the pickups are combined in parallel R_loading consists not only of the 'external' loading such as volume pots or an amplifier's input impedance, but also the impedance of the other parallel connected pickups (this, partly, is why parallel connected coils aren't usually louder than the individual coils). Additionally, since the external loading is constant and affects both pickups: it doesn't affect how the signals are mixed. Thus, the only loading we need to consider is the impedance of the other pickup — and so, for the ratio of neck pickup's output to the bridge pickup's output, we get:


\frac{V_\text{N-contribution}}{V_\text{B-contribution}}
\approx \frac{
   V_\text{N} \times \dfrac{R_\text{B}}{\cancel{R_\text{N} + R_\text{B}}}
}{
   V_\text{B} \times \dfrac{R_\text{N}}{\cancel{R_\text{B} + R_\text{N}}}
}
\approx \frac{
   V_\text{N} R_\text{B}
}{
   V_\text{B} R_\text{N}
}

Now, if we imagine overwinding the bridge pickup by some amount, then — because the number of turns a pickup has is approximately proportional to both its output and its DC resistance (everything else remaining the same) — we can write:


\begin{aligned}
V_\text{OW} &\approx k V_\text{B}
\\
R_\text{OW} &\approx k R_\text{B}
\end{aligned}

And so:


\frac{V_\text{N-contribution}}{V_\text{OW-contribution}}
\approx \frac{
   V_\text{N} R_\text{OW}
}{
   V_\text{OW} R_\text{N}
}
\approx \frac{
   V_\text{N} \bcancel{k} R_\text{B}
}{
   \bcancel{k} V_\text{B} R_\text{N}
}
\approx \frac{
   V_\text{N} R_\text{B}
}{
   V_\text{B} R_\text{N}
}
\approx \frac{V_\text{N-contribution}}{V_\text{B-contribution}}

Therefore, despite an overwound pickup having boosted output when used individually, the same isn't true of its contribution in a parallel mix.

(After the fact, it occurs to me that thinking of the pickups as current sources with magnitude V_pickup / R_pickup is more concise, since if V & R increase by the same factor then I remains unchanged and current sources in parallel simply add together.)

---

Nov 5, 2024 0:01:34 GMT -5 slowfoot said:

The Big Single was a 43 gauge wire version, with a 2% overwind.

Using higher gauge (thinner diameter) wire enables the winding of hotter pickups as it allows the same number of turns of wire to occupy a smaller volume of the bobbin. But, by itself, that doesn't increase the output, just the resistance (thereby making efficiency worse). Increased output only comes from the greater number of turns that can now fit on a fully wound bobbin.

An exception to this would be something like the Bill Lawrence microcoils, where the ability to wind the same number of turns in a smaller space is used to put the entire coil closer to the top of the pickup where the magnetic influence of the string is greater. Unfortunately, the opposite — increasing the bobbin height — is another method people have used to get more turns of wire on a pickup. Again increasing overall output, but at the cost of efficiency.

---

As for potential solutions the simplest thing to try would be adding a small (1k to 5k) resistor in series with the neck P-90. This will reduce the contribution of neck pickup in the middle position, especially in the bass & mid-range frequencies, but should have little to no impact (in most scenarios) when the neck pickup is selected alone.

[JohnH]

I'd agree that the small resistor added in series with the neck pu is likely to be the easiest way to make this change.

This guitar has two volumes, and the effect of adding the fixed resistor will be similar to just turning the neck volume down a tad. It would probably be an extremely small pot turn, probably too small to set on the fly, but this might be a way to test at least.

If you measure the resistance of the guitar across the jack when set like this, the increase created by the volume pot not quite at max will be close enough as a guide to what fixed resistor might be wanted. Also, test with just the neck with volume set this way to be sure that the neck-only tone isn't affected adversely.

[gckelloch]

My GFS XV-585 came with an AlNiCo V powered 7.8K neck P90, and a bridge VEH HB (AlNiCo V PAF-type wound to 11.2k with 43AWG wire). I used a 10k trim pot on the P90, which worked great to fine-tune the combined sound. I have since swapped out the P90, but I think I set the trim pot to ~5k with not much effect on the tone. Amazon sells various trim pot kits with every value you'd ever want.