Les Paul Bass Low Z specs

[dcoyle]

I bought two seemingly matched low impedance pickups from the guy mentioned at the end of the winding capacitance theory thread and the DC resistance is not as he said.

DC Resistance

Using a BK DVM, with a zero reading of .1 ohms, I got the following readings, (so for a guess at absolute value, subtract 0.1, and expect at least +/- .1 error.)
PUPs are out of circuit.

Bridge pickup (judged by the width of the wear grooves from strings)

Top coil

brown to red, 24.9,
br/blue, 51.8,
br/green, 113.8

Bot coil

br/r25.9
br/bl, 53.4
br/gr115.2


Neck pickup

top coil

br/r, 25.6
br/bl, 53.2
br/gr, 115.0

bot coil

br/r, 24.6
br/bl, 51.45
br/gr, 112.5

All in ohms, and all probably .1 ohm high, +/- .1 ohm.

The smallest taps have a variance of 2.7% in the samples, the middle taps have a variance of 1.9 % and the full coils have a variance of 1.5%.

The averages, based on measuring four coils, and correcting for the .1 ohm offset in the measuring device, is

25.1 ohm, inner tap,

52.3 ohm, middle tap,

114.0 ohm, full coil.



Magnet

Looking at what I can see from the bottom of the PUPs, they have a black epoxy, but where it has run thin, it appears the magnet is a bar which is 3 inches long and a 1/4 inch wide, and has been reported elsewhere to be a 1/2 inch tall, then a sheet of metal, then another magnet of the same size for the top.

The magnetic polarity of the magnets is the same for both PUPs, and opposite, top to bottom. The north pointing end of a compass is drawn to the top of the pickups.

The strength is roughly that of a Fender P bass.


Inductance

I only measured the inductance of the neck top coil, and believe the first two significant digits .

br/r, .010 H
br/bl .040 H
br/gr .161 H

Turns

The outside of wraps is 1/2" from the magnet surface, and there is no bobbin. There is a transverse wrap of tape around the coil to keep it in.

My measurements and calculations give a 7.6 inch average path per wrap. At 65 ohms per 1000 ft, for 28 ga, the total coil is about 1775 feet, and roughly 2771 turns. The middle tap may be at 1362 winds and the inner tap may be 684 turns.

As on other low Z coils, I couldn't get a cap value.

Cheers,

Dan

[bbsailor]

I bought two seemingly matched low impedance pickups from the guy mentioned at the end of the winding capacitance theory thread and the DC resistance is not as he said.

DC Resistance

Using a BK DVM, with a zero reading of .1 ohms, I got the following readings, (so for a guess at absolute value, subtract 0.1, and expect at least +/- .1 error.)
PUPs are out of circuit.

Bridge pickup (judged by the width of the wear grooves from strings)

Top coil

brown to red, 24.9,
br/blue, 51.8,
br/green, 113.8

Bot coil

br/r25.9
br/bl, 53.4
br/gr115.2


Neck pickup

top coil

br/r, 25.6
br/bl, 53.2
br/gr, 115.0

bot coil

br/r, 24.6
br/bl, 51.45
br/gr, 112.5

All in ohms, and all probably .1 ohm high, +/- .1 ohm.

The smallest taps have a variance of 2.7% in the samples, the middle taps have a variance of 1.9 % and the full coils have a variance of 1.5%.

The averages, based on measuring four coils, and correcting for the .1 ohm offset in the measuring device, is

25.1 ohm, inner tap,

52.3 ohm, middle tap,

114.0 ohm, full coil.



Magnet

Looking at what I can see from the bottom of the PUPs, they have a black epoxy, but where it has run thin, it appears the magnet is a bar which is 3 inches long and a 1/4 inch wide, and has been reported elsewhere to be a 1/2 inch tall, then a sheet of metal, then another magnet of the same size for the top.

The magnetic polarity of the magnets is the same for both PUPs, and opposite, top to bottom. The north pointing end of a compass is drawn to the top of the pickups.

The strength is roughly that of a Fender P bass.


Inductance

I only measured the inductance of the neck top coil, and believe the first two significant digits .

br/r, .0010 H
br/bl .040 H
br/gr .161 H

Turns

The outside of wraps is 1/2" from the magnet surface, and there is no bobbin. There is a transverse wrap of tape around the coil to keep it in.

My measurements and calculations give a 7.6 inch average path per wrap. At 65 ohms per 1000 ft, for 28 ga, the total coil is about 1775 feet, and roughly 2771 turns. The middle tap may be at 1362 winds and the inner tap may be 684 turns.

As on other low Z coils, I couldn't get a cap value.

Cheers,

Dan

Dan,

See the below link for a 1950s era RCA bulletin about microphone loading. This comes from the same era when these pickups were made.

http://f1.grp.yahoofs.com/v1/YDyBS8E...oading_rca.pdf

Note that at the low impedance coil tap of about 25 ohms for one coil equals 50 ohms for two coils in humbucking series. Using the resistive pot loading rule of thumb of making the load 40 times higher than the resistance, that puts the volume pot used in the low Z circuit of 2.5K within this range (40 X 50 ohms = 2000 ohms). As you select progressively higher resistance taps the higher frequencies and harmonics are increasingly loaded (decreased) while increasing the output level for lower frequencies.

Try to replicate the Gibson Low Z circuit and you should have a pretty wide tonal selection range, especially with two pickups working in and out of phase.

I would not worry too much about the coils not being what were initially stated. The multiple two higher taps were intentionally designed to mismatch with the lowest tap being the clean sound that optimumly matches the microphone matching transformer at the amp end.

Thanks for posting this detailed resistance measurement.

Joseph Rogowski

Edit: PS When the coil turns are doubled (2 times) the inductance increases by 4 (2 squared).

The turns taps are approximately double of the lowest tap: 25, 53, 114 ohms. The inductance of the br/r of .0010H tap should probably measure .01H or 10mH. The second tap is 40mH and follows the above formula for inductance increase when turns are doubled. The Epiphone Jack Casady bass has about a 25mH coil inductance and this would track with two 10 mH coils being linked in series with some mutual coupling between the coils. Try this: wire the two low tap coils in series (in phase) and remeasure the combined inductance and post the result. I suspect it should be near 25mH.

[dcoyle]

typo corrected

Joseph,

Thanks for catching my typo on the inductance value, I have removed the extra zero on the above post, so it now reads correctly.

Dan

[dcoyle]

mas info, re coupling

The cap coupling between the the two brown wires in one PUP is 188 pico fd and between the two green wires, 194 pico fd, so there is some capacitive link, although with the low Z's, I guess it's above the audio band. I believe cap coupling between bucking coils would lead to cancellation, and with this low R, probably way above audio freqs.

Re. inductance, hooking the PUP as the schematic does, I measured the br to br wires, with the red (least winds) taps joined, at .0185 H, and with the green (most) windings commoned, at .249 H . This value threw me because it was less than it should be for the two inductors in simple series, so I remeasured the upper coil and measured the lower coil, individually.

The inductance values are

top coil

br/r, .010 Henry
br/gr, .153

series .0185



bot coil

br/r, .011
br/gr, .166

series .249

It dawned on me that I could only get a value lower than the sum if the winds were out of phase, which, Duh, would make sense if the mag poles are the same orientation for both coils.

So I measured the coils with one coil reversed and got

.390 H br/gr/br/gr,

and

.022 H br/r/br/r

So , I construe that to be an indicator that the wind handedness is different between the coils, and that the inductive coupling is fairly low, especially given their proximity. That goes along with the reported sheet of steel between the two coils.

Re. the seller, I had no concern that the values were not as stated, rather I wanted to let anyone who came on this thread, later, to know the correct value. I felt buying the pups was an indulgence, but the sharing of knowledge was part of my rationalizing why it wasn't so bad. As if.

Dan

[dcoyle]

broken link

Joseph,

I couldn't get that link to work.

Thanks,

Dan

[bbsailor]

Joseph,

I couldn't get that link to work.

Thanks,

Dan

Dan,

I tried to attach the pdf file but it is too large and it would not upload.

Try this new link: http://www.coutant.org/micspecs/micspecs.pdf


Joseph Rogowski

[bbsailor]

The cap coupling between the the two brown wires in one PUP is 188 pico fd and between the two green wires, 194 pico fd, so there is some capacitive link, although with the low Z's, I guess it's above the audio band. I believe cap coupling between bucking coils would lead to cancellation, and with this low R, probably way above audio freqs.

Re. inductance, hooking the PUP as the schematic does, I measured the br to br wires, with the red (least winds) taps joined, at .0185 H, and with the green (most) windings commoned, at .249 H . This value threw me because it was less than it should be for the two inductors in simple series, so I remeasured the upper coil and measured the lower coil, individually.

The inductance values are

top coil

br/r, .010 Henry
br/gr, .153

series .0185



bot coil

br/r, .011
br/gr, .166

series .249

It dawned on me that I could only get a value lower than the sum if the winds were out of phase, which, Duh, would make sense if the mag poles are the same orientation for both coils.

So I measured the coils with one coil reversed and got

.390 H br/gr/br/gr,

and

.022 H br/r/br/r

So , I construe that to be an indicator that the wind handedness is different between the coils, and that the inductive coupling is fairly low, especially given their proximity. That goes along with the reported sheet of steel between the two coils.

Re. the seller, I had no concern that the values were not as stated, rather I wanted to let anyone who came on this thread, later, to know the correct value. I felt buying the pups was an indulgence, but the sharing of knowledge was part of my rationalizing why it wasn't so bad. As if.

Dan

Dan,

Here is an experiment to try to obtain a little more data about the pickup.

Put the top pickup low tap coil on a scope and tap it with a screwdriver to see the output level. Try it again with the same low tap coil on the bottom and observer the output. I suspect that the lower coil has less output and is only contributing to the humbucking effect and contributes little, if any, signal. The two coil low impedance taps (10 mH and 11 mH) in series have very low mutual coupling as their combined series inductance is 22 mH by your measurement.

The next interesting test is to connect the pickup to a microphone matching transformer (low Z side) and measure the unloaded output from the high Z side. I suspect the output will be about 300 to 400mv P to P. Just hold the pickup over a plucked string.

Joseph Rogowski

[dcoyle]

question tone pot values

Joseph mentions the value of the volume pot in the LP low Z bass as 2k5 ohms, and David posted a schematic showing the vol pot at 2K5, but no values for the pots for treble and bass. Can anyone fill me in as to their values?

Thanks

Dan?

[dcoyle]

You mean you want me to clean my bench far enough back to get to the scope?

Jeesh.

I will, but it may take a bit.

I have done that sort of test before and found it's good for seeing polarity, but not so good for amplitude judgments. Maybe I can drop the screwdriver tip on it from a predictable distance, and get some repeatablity.

I have had good luck, when I was designing the preamp, using the scope to establish peak to peak max for a good wack on the bass. To be meaningful, first I'll have to test what the turns ratio of the transformer is and that means excavating the frequency generator, too.

I'll compare the coils and report.

Dan

[David Schwab]

Dan, if you get a chance, can you take some photos of the back of the pickup? That guy had them posted once, but they were kind of small.

I'm curious about the outer coil not fitting in the bobbin.

[bbsailor]

Joseph mentions the value of the volume pot in the LP low Z bass as 2k5 ohms, and David posted a schematic showing the vol pot at 2K5, but no values for the pots for treble and bass. Can anyone fill me in as to their values?

Thanks

Dan?

Dan,

The treble pot is 1K, linear.
The bass pot is 2.5K, linear.

Joseph Rogowski

[dcoyle]

There is no bobbin, David.

There is only a coil stuck in the case and epoxied. There is one wrapping of 5/16" electrical tape, perpendicular to the coil wraps, to hold the mid bulge in.

There is also a piece of tape going in the direction of the wraps to hold the various taps in place on the ends of the pup.

I wonder if the ebay vendor was putting two things together for a "kit"?

Dan

[David Schwab]

There is no bobbin, David.

There is only a coil stuck in the case and epoxied. There is one wrapping of 5/16" electrical tape, perpendicular to the coil wraps, to hold the mid bulge in.

There is also a piece of tape going in the direction of the wraps to hold the various taps in place on the ends of the pup.

Oh, so no bobbins at all?

I wonder if the ebay vendor was putting two things together for a "kit"?

Yes, that's what they said. They said the bobbins were for the recording guitar, but they threw them in for free with the cover.

[dcoyle]

David

The overall width of the coil (1 3/32") is a little narrower than the inside of the case and is slightly (1/32") rotated, viewed in azimuth.

If the magnet is 1/4", which was my impression from 35 years ago and the indication by the flow of epoxy in these pups, then that would give a 27/64" wide coil thickness, per side.

With this measurement and a more detailed approximation of the coil geometry, I now think the amount of turns may be roughly, 685, 1255 and 2692 turns for the three taps.

Joseph

The screwdriver test roughly indicates that for a metal plunck from above, the lower coil has about one tenth the output of the upper coil.

Please,

Who can tell me the values of the treble and the bass pot?

Thanks

Dan

[dcoyle]

whoops

Thanks Joseph for the pot values. I had missed the post when I reasked the question.

Dan