Blueshawk Wiring

The volume pot is 300kΩ and linear. The value is a little high but understandable if you want to preserve brightness. What's strange is the choice to use a linear taper pot instead of an audio taper. In my experience this makes the volume pot annoying to use, as the first 2/3 of its rotation makes almost no difference to the level. There is a treble bleed cap on the volume pot however, which helps retain the high frequency content as you turn the volume down.

The tone pot is 500k, log taper.

This is where it gets a little crazy. Most Gibson guitars with 2 pickups and a neck/both/bridge selector use a bat-style selector switch. Here we havea a blade-style switch. But this is not a typical 2p3t selector. Instead we have a 4-pole, 3-throw switch, made from 4 independent 1p3t sections.

In this part of the schematic the labels (b) and (w) stand for black and white wires from the pickups, although in my case the white wire is more of a pale blue.

This is a resonant circuit with a choke (7.2H inductor) and capacitor to ground in parallel with the pickup. A resistor in series with this resonant circuit dampens it otherwise it would be a drastic volume cut. With the resistor it acts like a midrange cut. There are 5 different capacitor options on the 6-way switch; the remaining switch position does not select any resonant circuit.

In addition, there is a DPDT switch to bypass the resonant circuit entirely. This switch is part of the push-pull tone pot.

Many people make the mistake of thinking the pickups in the Blueshawk are P-90 pickups. While they use the same soapbar-shaped cover and look almost the same from the outside, inside they are different. P90s have two bar magnets below the coil, facing inward at steel screws which act as pole pieces. The "Blues 90" pickups in the Blueshawk have magnetic pole pieces and lower impedance. They sound thinner and have lower output compared to most P90 pickups. The only outward clue of the difference is that the Blues 90 pickups do not have screw heads for pole pieces, whereas most (all?) P90s do.

There is a parallel resistor+capacitor network in series with the output when the pickup selector is in the middle position (both pickups). This makes taking a resistance reading tricky, because if you just probe the output jack you'll get this parallel resistance confounding your measurement. But here are the readings from my guitar at the pickup selector switch for each of the pickups and the dummy coil:

It is not surprising to see they're within about 100Ω of each other, as they appear to all use the exact same bobbin and were likely all made to the same spec before some of them went on to become pickups and others to remain dummy coils. The difference between the pickup and the dummy coil is the presence of a brass (or perhaps brass plated steel) backing plate and of course the pole pieces.

Here are the pickup combinations:

Some people have complained online about the middle position being low output. The fact that this affects some players and not others can be explained by this high output impedance. If the load on the guitar is not high impedance (1MΩ for example) this extra resistance will form a significant voltage divider and result in attenuation. My assumption is the parallel RC network is meant to attenuate the series combination of the bridge+neck pickups, so it is doing its job. But this is a real limitation. To mitigate the volume drop in the middle position, the easiest solution would be to put a high-impedance buffer between the guitar's output and whatever comes after, thereby isolating it from unpredictable loads. Alternatively, you could change this hard-wired RC filter network, or perhaps make it user-controllable. Perhaps repurpose the push-pull pot from its current duty to shorting this RC filter instead, making the middle position usable in more situations. Speaking of the push-pull pot, this is a great segue.

The push-pull pot bypasses the Varitone circuit so you get the pure sound of the guitar pickups without any extra reactive load. However, position 1 (all the way counter-clockwise) on the Varitone switch is often said to disable the Varitone from the circuit. This is not precisely true. It also seems redundant for Gibson to include a push-pull pot bypass if one of the Varitone postions already serves this purpose, so I suspect they included the Varitone bypass to appease players who wanted the option to truly remove it from the circuit.

Anyway, in the first Varitone switch position the load on the pickup is a 100kΩ resistor plus the parallel combination of all the other 10MΩ resistors for the other 5 positions, or equivalently 2.1MΩ loading the pickups, plus the rest of the Varitone circuit. 2.1MΩ is a lot compared to 5kΩ, so even though this is technically inaccurate it's still reasonable to say position 1 "effectively" bypasses the Varitone.

This reminds me of one of my favorite quotes (regarding scientific models):

All models are wrong, but some are useful.

Not Blueshawk-specific but this shows up often when discussing guitar electronics.

Inside a passive guitar, the concept of "star" grounding is meaningless. This idea comes from high power and/or high voltage circuitry where multiple return paths for current may form loops and result in extra noise. This does not apply inside a passive electric guitar because (a) the currents are in the micro amps and (b) the "ground" comes from the guitar's cable. Because the guitar gets its ground reference from a cable, the guitar itself is effectively a small circuit at the end of a long ground wire, and any loops inside the guitar are limited by the side of the guitar's control cavity.

I think the reason this myth persists is due to some kind of selection bias. A guitar with a noise problem may be (mis)diagnosed as having a ground loop, and (incorrectly) prescribed with star grounding as the cure. Then, a repair person rewires the guitar and finds that when they finish the noise problem is gone!

But I believe the real story is that the guitar originally had a wiring fault (which could include some kind of grounding problem), and during the course of rewiring the guitar, the repair person also fixes the original problem. The addition of star grounding doesn't hurt but it also provides no benefit.

First confirm all the wiring is correct. If it is, try re-orienting the dummy coil by flipping it over in its cavity. Keep the orientation with the least amount of hum. Alternatively, disconnect the dummy coil wiring and try the opposite polarity. Then repeat the orientation exercise. Keep the polarity/orientation with the least hum.

As long as you take care to avoid shorting out any signal wires, and ensure 100% of the shield is grounded, I think this is worth doing. A roll of copper tape with conductive adhesive is a few dollars and contains enough to shield several guitars.

This is on my to-do list. 300k audio taper pots with the right length of shaft are not common but they can be found. 500k and 250k pots are common and would work but might change the sound. I will try to source a 300k audio taper pot just to keep it close to the original sound, but my second choice would be 250k. It would be a little darker and a little less output but I think it would be closer to the original in terms of performance, especially considering a 250k pot will be less sensitive to different input impedances.