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Help with spicy Jazz Bass wiring?

F

ffil

Aug 1, 2025
17
11
#1
Hi, I have this jazz bass which i changed the wiring time ago adding a push pull switch for series/parallel options, (I followed the classic Seymour Duncan wiring that appear as the first result in google images).
Days ago i was wondering if i could find a way to balance the sound between the pickups when i'm in series, so i added a pot to trim the neck pickup, keeping the bridge one always on full.

Now comes the problem, in series it work fine, but in parallel the neck pickup just doesn't work and i can't find out why, doesn't seem like a phase problem neither a soldering one, probably there's a logic problem in the circuit but I'm not an electrical engineer so I'm asking for help.

here's the wiring:

disegno.jpg
 
#3
tone and bridge work fine, the trim pot doesn't affect anything as it should and the neck pot with the relative pickup is completely dead like the volume is turned down all the time, also there's no noise when i turn the pot
 
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#6
I am not an electronic engineer either, but so far here is my understanding:
• Compared to Seymour-Duncan's design, you inverted the neck and bridge pickups (which has of course absolutely no negative impact, it works as fine).
• From the original design, you cut the ground wire of the bridge pickup and added the trim pot.
• Before you installed the trim pot, everything was working fine.

Am I correct?

In such a case, the surprising part is that you touched only the bridge pickup but the one which does not work is the neck pickup.
 
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#7
To me, the circuit diagram seems fine but it looks like the neck volume pot is bypassed in series. Why add a new trim pot instead of using that pot?

Confirm the neck pickup is actually working in series—tap the pickup with something metal. Assuming it is working, the only wires not being used in series but used in parallel are the wire from switch post 3 to the neck pot and the pot wiring. Recheck those.
 
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#8
With the bass in series mode, that will not work to reduce the volume of just one pickup. As drawn, I believe that will reduce overall volume of both pickups because you are in series and shunting signal to ground of both pickups. I could be wrong.

Just a guess, but I think you would want the trim pot going across plus and minus of the pickup you want to trim, and not to ground. When The trim pot is at zero ohms, you bypass the trimmed pickup completely, when it is at max, you go through the pickup mostly entirely.

Again, just a guess.
 
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#9
ffil said:
Hi, I have this jazz bass which i changed the wiring time ago adding a push pull switch for series/parallel options, (I followed the classic Seymour Duncan wiring that appear as the first result in google images).
Days ago i was wondering if i could find a way to balance the sound between the pickups when i'm in series, so i added a pot to trim the neck pickup, keeping the bridge one always on full.

Now comes the problem, in series it work fine, but in parallel the neck pickup just doesn't work and i can't find out why, doesn't seem like a phase problem neither a soldering one, probably there's a logic problem in the circuit but I'm not an electrical engineer so I'm asking for help.

here's the wiring:

View attachment 7289668
Click to expand...
A no-load tone/volume or whatever pot would/should allow you to dial back the additional volume that occurs in a discrepancy between two pickups in parallel, versus two pickups in series... without loosing the high end due to rolling back the volume when in series. I am not 100% sure, but this seems to me like the easiest, and most reliable method to balance the outputs between series/parallel.
 
#10
Theobald said:
I am not an electronic engineer either, but so far here is my understanding:
• Compared to Seymour-Duncan's design, you inverted the neck and bridge pickups (which has of course absolutely no negative impact, it works as fine).
• From the original design, you cut the ground wire of the bridge pickup and added the trim pot.
• Before you installed the trim pot, everything was working fine.

Am I correct?

In such a case, the surprising part is that you touched only the bridge pickup but the one which does not work is the neck pickup.
Click to expand...
all correct, at this point i'm start to think that is not a logic problem but technical but i am pretty sure that i've checked everything
 
#11
SharpCat said:
To me, the circuit diagram seems fine but it looks like the neck volume pot is bypassed in series. Why add a new trim pot instead of using that pot?

Confirm the neck pickup is actually working in series—tap the pickup with something metal. Assuming it is working, the only wires not being used in series but used in parallel are the wire from switch post 3 to the neck pot and the pot wiring. Recheck those.
Click to expand...
normally in series mode the neck pot became the master volume and the bridge is bypassed, i tried using the bridge pot but i didn't managed to success, so i tried different things and the first that worked was this, adding a new pot.
Anyway in series it work fine so i don't think i will try to change it.

yes i'll check that wires better
 
#12
newfuture said:
With the bass in series mode, that will not work to reduce the volume of just one pickup. As drawn, I believe that will reduce overall volume of both pickups because you are in series and shunting signal to ground of both pickups. I could be wrong.

Just a guess, but I think you would want the trim pot going across plus and minus of the pickup you want to trim, and not to ground. When The trim pot is at zero ohms, you bypass the trimmed pickup completely, when it is at max, you go through the pickup mostly entirely.

Again, just a guess.
Click to expand...
yes the trim pot trim the entire signal, but it does it between the neck and bridge pickup (after the neck, before the bridge), so when the bridge signal get in it's on full while the pre bridge signal is trimmed, that's why the pickups are inverted, to have the right signal chain (idk if i could place the trim in another position but doesn't really matter).
i think it's like this, if not i don't know why but it works, the only complaint is the linearity of the pot but its just the type (b250k a250k...ecc), at least i think
 
Last edited:
#13
Thunderthumbs73 said:
A no-load tone/volume or whatever pot would/should allow you to dial back the additional volume that occurs in a discrepancy between two pickups in parallel, versus two pickups in series... without loosing the high end due to rolling back the volume when in series. I am not 100% sure, but this seems to me like the easiest, and most reliable method to balance the outputs between series/parallel.
Click to expand...
i didn't express myself well, i meant balance between the pickups in series mode, because normally you have only master volume, anyway it work fine, my problem is when i'm in parallel
btw i like your solution to the volume jump, i should consider that
 
#14
ffil said:
i didn't express myself well, i meant balance between the pickups in series mode, because normally you have only master volume, anyway it work fine, my problem is when i'm in parallel
btw i like your solution to the volume jump, i should consider that
Click to expand...
Cool thanks. It's quite possible and just as likely you expressed yourself just fine, and I read it how I (reflexively) read it.
 
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#16
guys, i checked everything and i found some bad weld joints, i redid these and now everything work perfectly, i also changed the trim pot with an a250k in order to have more linearity

thanks to everyone for the help

talking about the sound, it's not a big change but now i can remove the boomines of the series sound keeping the mid/output boost, essential for me.
anyway i found this: Jazz Bass series with blend and i am tempted to rewire it but i want to keep my creation ahahaha
 
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#17
I'm back after doing another wiring, the concept is the same as before, but now i used only 3 pots.
everything work fine except for the fact that in series, the neck pot should only trim the neck pickup, which it does but it also cancel a ton of volume and reduce the general treble.
with the old 4 pot wiring there weren't any problems like that, it was trimming perfectly.
here's the wiring, push=parallel & pull=series, all the pots are now a500k and not b250k as before.

parabol a.png


99.99% sure that isn't a soldering problem, so i think it's logical but cannot find out why, maybe is the b250 to a500 switch?
 

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