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Even though this bass [my DeArmond Starfire] features a 3-way selector switch, Fender has used the same "loudness pot" wiring as required on the Jazz Bass, which lacks a 3-way switch. This is an error.
Where there is no "Gibson-style" switch provided to combine and solo the pickups, the volume pots MUST be wired "backwards" as fader pots. All Fender Jazz basses are wired this way. The reason for the requirement is simple: with no pickup switch in the circuit, setting one pickup's volume to "off" will mute the other pickup. no matter where that volume knob is set. So, fader pot wiring is a concession to allow either pickup to be soloed, while the other is silent.
The problem is... it doesn't sound the same as "volume pot wiring", as seen in most Gibson guitars. When the pots are wired "normally", and a pickup selector switch is set in the center "both pickups" position, setting either volume knob to zero mutes the entire instrument.
Some people consider this limitation to be annoying, and prefer the J-Bass style wiring, where either volume knob can be set to zero without muting the instrument by mistake. However, this is not a simple matter of deciding how you'd like the two (or three) volume knobs to interact with each other.
"Fader pots", as wired in J-Bass and your Starfire, reduce loudness by progressively "loading" the pickup until it is eventually shorted out to produce silence. Because the individual pickups are shorted out, not the mixed output of the cable, the other pickup is free to be adjusted to any level you choose. However, there is a large downside to this operating compromise: pickups sound progressively worse as they are loaded and shorted out. As the parallel resistance on each pickup is lowered, approaching a dead short, the treble harmonics are first affected, then the mis, and then finally the bass harmonics and fundamental tones. As a result, a Fender-style "fader pot" cannot be operated AT ALL, without shifting the response and EQ shape of the pickup.
The only solution to this tonal problem with "fader pots" is to wire them "normally", as volume pots.... but this requires a 3-way selector switch, to avoid the "automatic muting issue" described above. This still maintains a potential operating error, because neither volume pot may be set to zero when the center switch position is chosen... unless you were trying to mute the guitar. But, once the player is accustomed to the benefits of a 3-way selector switch, volumes are generally left at high levels until the guitar is to be muted, and set aside on a stand.
"Volume pots" are wired in most instruments having a pickup selector switch (Stratocaster, Telecaster, Les Paul, SG, ES-335, PRS, etc) and in single pickup instruments (P-Bass, etc). The tonal benefit is significant: now, as loudness is reduced, the parallel resistive loading on the pickup remains CONSTANT... at the rating of the pot, and there is no "shorting out" of the pickup as silence is approached. Instead, the output cable to the amp is loaded down with parallel resistance, until silence is eventually reached when the volume pot on "0" completely shorts out the cable. Note that the pickups are now silent because the amp is silent.... not because they have been "loaded" or shorted out. As a result, the pickup enjoys a uniform loading at all volume settings, and tone remains very consistent from "10" down to about "2".... there is no progressive loss of treble, them mids, then bass, as volume declines. It's not a perfect system, but it's MUCH BETTER than "fader pot" wiring.
OK - if this makes sense to you, then let's plan to re-wire your dual "fader pots" as "volume pots. This will sound better, pickup blending & mixing will be improved, and a flatter response will be heard over a wider loudness range. Again, the ONLY DISADVANTAGE is that you cannot set one of the volume knobs to "0" when you select the center toggle position.
Second problem in the Fender wiring diagram:
The two tone pots are improperly-wired to the same wrong center terminals as the pickups are wired. So, when the first problem... fader vs volume pot wiring.... is remedied, the tone controls (to be discussed below) will also be wired to the correct terminals... on the left side of the pots. The CENTER terminals will be connected to the 3-way toggle switch.
Now, let's look at your options for correcting & improving your control harness:
If you will be retaining the original Fender knob assignments (recommended), the poor blending problem will be greatly improved by making the change to "volume pot wiring", described above, plus changing to 500K LINEAR TAPER volume pots. The reason why you are finding a very small angle of adjustment shifts from the "both pickups" sound directly to a "solo pickup" sound, is because your original fader pots have way too strong of an audio taper. I'll save you further boredom in describing why LINEAR pots will work better.... just consider that these are very inexpensive and will not be a wasted experiment!
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And if anyone wants to comment, whether to say 'yes, that's how it should be' or 'hmm...something wrong there', I'd be delighted to hear from you.
