| Parallel / series switch mod for Jazz Bass
You need a DPDT (double pole double throw) switch to achieve that.
They have 6 legs arranged like this :
.\.
ACE
BDF
First, cautionously unsolder the wire connected from the neck PU to the ground.
It's usually black.
Then do the same with the one connecting the bridge PU to its volume pot.
It's yellow, white or red depending on basses.
Install your switch and link like this
A : to ground
B : to bridge PU volume pot
C : to neck PU ground (the first wire you unsoldered)
D : to bridge PU signal (the second wired you unsoldered)
E : to F
F : to E
In one position you will get your regular Jazz Bass sound.
In the other one, your PUs will be wired series. Bridge volume will be deactivated, and neck volume will act as a general volume.
The sound is very surprising coming from a Jazz Bass. Beefy, with a huge low medium rumble.
You can also get a 3 position on/off/on switch, in this case central position will act as a mute.
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For a bass with volume/balance/tone arrangement
You need a TPDT (triple pole double throw) switch.
They have 9 legs arranged in a square.
.\.
ADG
BEH
CFI
You then have 3 wires to unsolder :
- from neck PU to balance
- from neck PU to ground
- from bridge PU to balance.
It goes like this :
A : to balance (where the neck PU was previously connected)
B : to ground
C : to balance (where the bridge PU was previously connected)
D : to neck PU signal (previously wired to balance)
E : to neck PU ground (foreconnected to the... ground !)
F : to bridge PU signal
G : to the volume pot (soldered on the same leg as the wire coming from the balance. DON'T unsolder the balance, keep them together)
H : to I
I : to H
Your usual setup and sound will come out in one position.
In the other one, balance will be deactivated, volume will still work the same, but you'll have the series wiring sound. | 
