Rev P+J build: bass drops out on B+E when blend knob middled?

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cleary
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Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 06 Nov 2018, 22:25

Hi electronics wizards,
I've mentioned in another thread I'm building a reverse mount P/J in a 5 string Fender Jazz body -

I'm experiencing an issue where the body (bass frequencies I guess) drop out of my B+E strings when I middle the blend knob - the other strings are fine.
Pickups are a random assortment - the J I pulled out of the bridge of my main MIM J bass, the P came off the forums from a 5 string Fender P I believe. I have not measured resistance across them (yet).
The P is mounted with the 3 poles covering BEA, mounted closest to the bridge, the 2 poles cover the other strings closer to the neck.

I am currently running them through a Bart mk1 9v preamp, but I have run it passive with the same issue.
Some height adjustment (raise the bridge/J, drop the neck/P) does help.
Also, if I remember correctly back when I was first wiring it up (a few months ago now, I had to put it all back in the J for a bunch of gigs), I was able to reverse the effect across the A/D/G strings by reversing the wiring on the P.

I'm certain this is a common issue, I was wondering if there's anyone who could explain what is going on for me before I drop cash on new pickups?

(New pickups are the intent anyway, this is just a proof of concept currently - but I'd rather have wrapped my head around what this issue actually is)

Thanks a lot!
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cleary
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 06 Nov 2018, 22:44

... I think obfs secret weapon may have answered my question elsewhere:
https://www.talkbass.com/threads/phase- ... t-16304079

I'll reverse one side of the P tomorrow night hopefully, and report back
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 07 Nov 2018, 22:16

ok, lots of testing later, and I've still got a problem one way or another - diagram below indicates the different P wiring permutations and what strings they knock out. Side note, I've also gone back to a basic passive VBT circuit while testing:

Image

Any further assistance very much appreciated!
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by packrat » 08 Nov 2018, 00:26

I rather think you might have more luck with a bigger gap between pickups.

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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by Bill Fitzmaurice » 08 Nov 2018, 01:20

If the blend knob blends the J and P the problem is the J and P are probably out of polarity. Judging by the pictures you should wire the P per the second picture, then reverse the polarity on the J.

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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 08 Nov 2018, 09:23

Bill Fitzmaurice wrote:
08 Nov 2018, 01:20
If the blend knob blends the J and P the problem is the J and P are probably out of polarity. Judging by the pictures you should wire the P per the second picture, then reverse the polarity on the J.
Thanks for the reply - when you talk about "reversing polarity" are we talking just swapping +ve and ground on the J, or are we talking throwing a different J in with magnets with opposite N/S poles ?
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 08 Nov 2018, 09:28

packrat wrote:
08 Nov 2018, 00:26
I rather think you might have more luck with a bigger gap between pickups.
The distance between the pickups is pretty standard - some are closer, some are further away. It *seems* closer than a regular P because of the reverse orientation:

Image
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by Bill Fitzmaurice » 08 Nov 2018, 09:31

cleary wrote:
08 Nov 2018, 09:23
when you talk about "reversing polarity" are we talking just swapping +ve and ground on the J
That. Pulling and reversing the magnets would do the same thing, but it's a lot more work.

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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 08 Nov 2018, 09:59

Bill Fitzmaurice wrote:
08 Nov 2018, 09:31
cleary wrote:
08 Nov 2018, 09:23
when you talk about "reversing polarity" are we talking just swapping +ve and ground on the J
That. Pulling and reversing the magnets would do the same thing, but it's a lot more work.
Thanks - I think I may have buggered up my testing results somewhat so I'm going to begin again and work through the permutations. I did a bit of a magnet polarity test (with one of my 2yo wooden trains :p), and it's currently looking like:

Code: Select all

 N S   N
[:]   |:|
  [:] |:|
Note I'm not actually sure if it's North or South, but it's opposed in that orientation. Not sure if this makes it an easier problem to solve, but I'm learning quite a lot :eyebrow: :D
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by jimmyjam » 08 Nov 2018, 18:38

I personally am yet to find a PJ setup that I like, and I’ve tried a lot!
On my only PJ I added a phase switch on the P pickup so as to compensate for phasing issues when switching the bridge pickup from split/series/parallel. Also can get some “interesting” sounds.
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by bassbass » 08 Nov 2018, 22:16

With a multimeter, there's no need to go through all the permutations. A compass will also help but you can do this without.

(BTW, that's my post on TB there. The guy had a set of dimarzios that had lost a magnet (common problem) and had been put back the wrong way.)

I presume the J pickup is a single coil, right?

Anyway, first you need to make sure the two P bobbins are reverse magnetic polarity from each other. You can do this with a compass, or if the pickup is out on the bench, you can check that the tops attract. (It's rare, but I have come across a few Ps over the years that for whatever reason have the same magnetic polarity on each bobbin.)

If this checks out, then you need to test their phase. You do this with a multimeter. Arbitrarily set one wire from one of the bobbins as earth. Connect the black probe to this, and the red probe to the other wire. Set the meter to millivolts and then lay a little screwdriver on the top of one of the magnets. As you lift it up you will get a small voltage. Note whether it is negative or positive,. In fact, write this down, plus the colours of the earth and hot wires for that first bobbin. Then do the same with the other bobbin. If you got a voltage reading that was say, positive on the first bobbin, then that's what you are aiming for with the second bobbin. If you get a negative reading, then reverse the probes and you should get positive. Then write down the colours for the earth and hot wires from the second bobbin. Then you need to connect the hot from one bobbin to the earth from the other for (standard) series wiring. You can then check again if you like after the series connection is made. Both bobbins should give the same voltage polarity. (Negative or positive, it doesn't matter, as long as they are the same).

Then do the same for the J coil. And also make sure it gives the same voltage polarity when you lift the screwdriver up off the magnets.

Doing this ensures that the signal from all three coils is in phase. This way, the p will humcancel (as long its bobbins have reverse mag polarity from one another), and the J will be in phase across all five strings when you blend the two pickups.

You can see me do this with a pickup in this video, if you skip forward to 12:20.



If you still have problems with the signal dipping or anything else after you've done this, and you've also got the pickup heights adjusted, then it may be a problem with the blend pot, or something else...
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 09 Nov 2018, 07:59

bassbass wrote:
08 Nov 2018, 22:16
...
(BTW, that's my post on TB there. The guy had a set of dimarzios that had lost a magnet (common problem) and had been put back the wrong way.)
Yeah I knew it was you, it was the most cohesive answer in that thread - I won't lie, I was hoping you'd see this thread too :red face
I presume the J pickup is a single coil, right?
I'm actually not sure - I glossed over this bit in my haze this morning... I'll def check it tonight so I know, but I continued on with the instructions below regardless :D
Anyway, first you need to make sure the two P bobbins are reverse magnetic polarity from each other. You can do this with a compass, or if the pickup is out on the bench, you can check that the tops attract. (It's rare, but I have come across a few Ps over the years that for whatever reason have the same magnetic polarity on each bobbin.)
Definitely reverse magnetic polarity on the two P bobbins
If this checks out, then you need to test their phase. You do this with a multimeter. Arbitrarily set one wire from one of the bobbins as earth. Connect the black probe to this, and the red probe to the other wire. Set the meter to millivolts and then lay a little screwdriver on the top of one of the magnets. As you lift it up you will get a small voltage. Note whether it is negative or positive,. In fact, write this down, plus the colours of the earth and hot wires for that first bobbin. Then do the same with the other bobbin. If you got a voltage reading that was say, positive on the first bobbin, then that's what you are aiming for with the second bobbin. If you get a negative reading, then reverse the probes and you should get positive. Then write down the colours for the earth and hot wires from the second bobbin. Then you need to connect the hot from one bobbin to the earth from the other for (standard) series wiring. You can then check again if you like after the series connection is made. Both bobbins should give the same voltage polarity. (Negative or positive, it doesn't matter, as long as they are the same).
:notworthy: This is the information I was looking for... thank you so much!
Then do the same for the J coil. And also make sure it gives the same voltage polarity when you lift the screwdriver up off the magnets.

Doing this ensures that the signal from all three coils is in phase. This way, the p will humcancel (as long its bobbins have reverse mag polarity from one another), and the J will be in phase across all five strings when you blend the two pickups.

You can see me do this with a pickup in this video, if you skip forward to 12:20.

If you still have problems with the signal dipping or anything else after you've done this, and you've also got the pickup heights adjusted, then it may be a problem with the blend pot, or something else...
So, the outcome was almost what you said to a tee -

* Except, when I wired the two P bobbins together Earth to Hot, the voltage was reading opposites from each bobbin (?) I'm not sure if I just marked them wrong or what, but I just swapped the earth and hot on one bobbin and they measured the same voltage (polarity?)

* When it came time to test the J I found my little screwdriver has too small so I used a medium sized one with minimal magnetism to get across the poles - it still gave me a readable voltage bump on removal

I only had time to quickly test with headphones in a shitty old zoom pedal, but I'm confident it's resolved my mismatched strings issue.

My notes, just for posterity/info
Image

...and a community safety notice: How not to do a workbench :(
Image


Thanks again for the assistance and info, my 5 days of hair tearing out resolved in 30 mins with the right info. Appreciate it immensely
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by bassbass » 11 Nov 2018, 00:23

Nice one man. Glad you got it sussed...

Ps: my workbench had always been a mess. But I moved in March and set up a new little workshop with tool boxes, shelves etc and so far it's still neat and ordered. I'm pretty much surprising myself. It's a lot tidier than my apartment. :D
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by cleary » 12 Nov 2018, 07:38

bassbass wrote:
11 Nov 2018, 00:23
Nice one man. Glad you got it sussed...
Argh, I spoke too soon. Had visitors over the weekend and didn't get a chance to test til last night... it was a no go.

I pulled it all apart again today, re-checked the P with a bit more precision (I think there was a risk I confused myself flipping things over) - back together, no go.

Then I remembered a certain assumption we made that I hadn't checked, and...

Image

It's not a single coil... and that also explained a weird anomaly I got where when I used the little screwdriver to measure voltage from one side of the pickup it gave a positive reading, and got a negative reading when I approached it from the opposite side - so

At this point I think I understand the path I need to take, which I can also verify via the jazz neck pickup which is a pair to the bridge pickup I've borrowed for this build... so as tempting as it is to ask for all the answers, I will have a crack on my own from here and report back.

I forgot about a gig I had coming up this weekend too, so it's all back in the workhorse bass for now anyway, it'll likely be a few weeks before I can have another good crack at it.

Thanks again for the assistance so far
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Re: Rev P+J build: bass drops out on B+E when blend knob middled?

Post by bassbass » 13 Nov 2018, 19:37

Bummer. But the process is just the same for the other pickup. Check its mags are opposite and that the signal from each coil is in phase. And also in phase with both coils of the P. Best of luck mate, and let us know how you go.
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