MIM jazz neck pickup: why a tone difference w\ volume knob 100% vs 80%

Is the volume for your bridge pickup all the way up as well or are you only playing with the neck pickup?

 

You are talking about the volume pots on the bass, not the amps, right?

With a VVT wiring, the volume pots act as tone pots for a few reasons. The first is that with the pickups both turned up full, you get a parallel humbucker with about half the inductance and half the impedance of either single coil on its own. This raises both the frequency and height of the resonant peak. As soon as you start to turn one down, you quickly lose this effect.

With one pickup all the way up, and the other all the way down, the other reason becomes easy to hear. With VVT wiring, the pickup hot wires are connected to the vol pot middle lugs. So as the volume pot is turned down, the resistance between the pickup hot wire and earth diminishes rapidly. This is an impedance load on the pickup, and as the pickup has to drive a lower and lower impedance, the treble gets more and more attenuated. This is exactly how a tone pot rolls off the treble for most of its sweep, (until the cap comes into play).

There's other stuff going on, with the cable capacitance, and the load of the other vol pot and pickup, but those are the two main ones. The classic VVT jazz bass circuit looks pretty simple, but there's surprising complexity in the interactions going on...

 

There are lots of interactions going on with J wiring.

@RobbieK nailed it above. Treble bleed involves a change to the wiring where you install a small capacitor across the volume pot to compensate for the loss of treble by filtering some highs back in when the volume is rolled back a little. The paradox is that, with VVT wiring, when the pot with the bleed cap is fully off to solo the other pickup, the cap actually works in reverse and cuts treble from the output...

 

For sure. Actually there was a thread just the other day about treble bleed caps in a jazz bass.

I'm pretty sure the jazz bass was the first mass produced 2-pickup guitar without a selector switch, so after they ditched the stack-knob circuit with its summing resistors, Fender had to work out a cheap and simple way of making it seem (at least on the surface) that the two vols were working independently. Even with its problems, I still think swapping the middle and outer lugs was an inspired work-around.

Most jazz bass players (TBH, I'm not one), just seem to embrace the eccentricities of the arrangement and make the most of it. But if you want your two volume pots to retain a steady tone, there are a few ways around it, but you'd need to go with an active circuit, like a twin buffer, or active mixer I'm afraid.

 

With the volume full up, the inductance of the pickup is resonating with the capacitance of your cable. When you turn down, the resistance of the pot alters the characteristics of that filtering, and damps that resonance. Some of us enjoy the sound character at full volume more, some of us like the character backed off a bit.