Question on Ibanez True-Duo Pickup Wiring

[DrAlx]

An Ibanez True-Duo Pickup is a humbucking pickup with 3 coils (A,B,C) each of roughly 3.5k resistance.
Coils A and B are stacked and share the same Alnico pole pieces.  Coil C is adjacent to the other coils, and uses a ceramic magnet.

In single-coil mode, coils A and B are wired in series and coil C is shorted out.  So you effectively have a single-coil Alnico pickup of about 7k resistance.  Single coil mode is not hum-cancelling (i.e. it's not like a stacked humbucker).

In humbucker mode, coils A and C are wired in series, and coil B (the lower of the stacked coils) is shorted out.
So you have a humbucker of about 7k resistance.

The idea behind the True-Duo pickup is that the single-coil mode sounds more like a true single-coil pickup than a regular coil-split humbucker.
It works well in that regard.  The drawback is that the humbucker is relatively weak compared to other humbuckers.

Here's a schematic of the two modes using stock wiring.  The stock wiring uses a DPDT push-pull pot to switch between the 2 modes.  The DPDT wiring is a bit messy, so a small PCB is used on the push-pull pot to simplify things.



I think there is a much simpler way of wiring things.  The idea is to wire all 3 coils in series A--C--B and then short out either coil B or C.
This means only half a push-pull pot is required (i.e. a SPDT will do).  So make the junction between coils B and C the pole, and the other ends of B and C would be the 2 throws.  Schematically the two modes then look like this...



Now the humbucking mode is the same as with the stock wiring, because both ends of coil B are connected directly to ground.
But the single-coil mode with coil C shorted out is not the same as the stock wiring, because the ends of coil C are not directly connected to ground.
I know coil C won't pick up hum, but is this approach bad in other ways.
In particular, would the shorted-out coil C pick up other noise (I'm thinking RF) because its ends are not directly grounded ?

[Rob Strand]

Nice summary.

In particular, would the shorted-out coil C pick up other noise (I'm thinking RF) because its ends are not directly grounded ?

That's what I'd be thinking too.

I'd question the idea of shorting the unused coils in the first place.  Why not let them be open. That way you could permanently wire coils B and C to ground and just wire A to B or C with an SPDT switch. The only care required is to shield the switch wires (ground shield).    To me it's much better than letting coupled noise in at the pickup since you have no control over that.

It would be an interesting experiment to see if the tone is affected between open and shorting.  Open unused coils seems more of a natural starting point and so shorting would need some justification.  Even if the tone is affected, what case creates a "better" pickup?
(Maybe there's a patent? giving some reasoning.)

[amptramp]

Inducing a voltage in a shorted coil might produce a dampening effect whereas leaving the unused coils open would not.  A shorted coil would be like a shorted winding on a transformer with current flowing through the resistance of the coil.

I am not sure why coils A and B have to be separate with the C coil going to the middle.  The C coil can be on one end (electrically) of the coil stack.

[DrAlx]

Thanks for the replies guys. A bit of back-story regarding how this question came up ...

I'm going to swap out the pickups on my HSS Ibanez guitar and replace them with DiMarzio's.  So the existing True-Duo pickup that I have will be replaced by a regular humbucker.  The push-pull pot for the True-Duo is not appropriate for use with the new humbucker pickup unless I remove the small PCB on the push-pull pot, so I figured I'd just get new pots as well.  I've also taken this as an opportunity to tidy the wiring in the control cavity and I made a PCB for that purpose.  I have the PCB in the control cavity along with the new pots.





All parts in the cavity are now wired to the PCB instead of to each other, and I have terminal blocks so I can swap pickups without needing a soldering iron.  I have push-pull switches on both the volume pot and tone pot.  So when I get my new pickups I'll have this scheme:

Volume Push ==> Humbucker coils in series
Volume Pull ==> Humbucker coil split, or humbucker coils in parallel (depending on PCB DIP switch settings).

Tone Push ==> 5-way switch gives regular pickup combinations (N, N|M, M, M|B, B)
Tone Pull ==> 5-way switch gives modified pickup combinations (N+M, N+(B|M), M+B, M+B, M+B),
so I can have the middle pickup in series with the other two.
                                          
I designed the PCB for use with my new pickups, but I later realised (by accident, not design) that I could also use this PCB with my existing True-Duo pickup, and get both of its modes so long as I use the odd wiring scheme I described in my question.  I simply need to use the right combination of True-Duo pickup wires in the terminal block holes, and set both DIP switches on the PCB to Off.  No soldering or other rewiring is required, and that's what I have set up in the guitar at the moment.

I can't use this PCB for the simpler (correct) True-Duo wiring that grounds coils B and C on one end as suggested in the above replies.  That's why I asked my question.  I suspect what I have done might be a no-no with regard to noise.  I have not noticed a massive increase in noise with my hacked True-Duo wiring, but then it's not been easy to do a comparison either.  I could just be in an environment with low noise.  It's not a big issue really as the True Duo is going to be swapped out.  I was mainly asking out of interest.

BTW, today I did some more reading around and found this post on Seymour Duncan forum.

https://forum.seymourduncan.com/forum/the-pickup-lounge/336095-what-to-do-with-unused-coil-when-coil-splitting-a-humbucker

There are graphs there where someone has both simulated and measured the difference between having the unused coil grounded on one end versus grounded on both ends.  It's a negligible difference, measurable apparently, but not audible.  Grounding both ends gives slightly larger output (I would have expected the opposite).

[Rob Strand]

The push-pull pot for the True-Duo is not appropriate for use with the new humbucker pickup unless I remove the small PCB on the push-pull pot, so I figured I'd just get new pots as well. 

Sometimes I think it's easier to just remove all the old stuff and put it away for safe keeping then replace the innards with new stuff.   The exception being with parts like the 5-way switch.  IF the pull pots are good quality it makes the decision less clear.

I suspect what I have done might be a no-no with regard to noise

If the pickups have grounded shields then maybe it's OK.  You can only do the experiment.  Try touching the pole pieces.

BTW, today I did some more reading around and found this post on Seymour Duncan forum.

Interesting result.  Even less than what I was expecting.

[DrAlx]

If the pickups have grounded shields then maybe it's OK.  You can only do the experiment.  Try touching the pole pieces.

I just tried touching the pole pieces when I'm not grounded and it's the same effect in both modes, so I think I got away with it.