Metal Spacer in humbucker

[ZeroCool7]

What is the purpose of having a metal spacer on the adjustable screw polepieces only?

[big_teee]

What is the purpose of having a metal spacer on the adjustable screw polepieces only?

To have better contact with the magnet, and to make a better magnetic field.
The pole Slugs are larger and make firm contact with the magnet.
You can use two Adjustable screw bobbins if you like.
I make a model like that.
I can't tell that it has many benefits, just more adjustable.
This Thread should have been posted in the beginners Corner.
Good Luck,
Terry

[belwar]

It really serves two purposes - One direct and one indirect. The first as stated above is to allow contact with a standard 1/2" magnet. Two slug coils would not require it because they would line up with a standard mag.

Secondarily, it adds to the sound that we know as a humbucker. It's a substantially ammount of steel which is circuit, which in turn increases the inductance

[Peter Naglitsch]

A bit OT, but anyway: I had a HB that were just a tad too bright and I couldn't tame that with alternative magnets and I was in a bit of hurry so a rewind wasn't a real option. I had an aluminium bar stock that had the right dimensions, so I chopped of a bit and substituted the wooden spacer under the slug coil. That tamed the pickup just the way it needed and also migh give a bit of understanding to how the steel keeper bar affect the sound.

[LtKojak]

A bit OT, but anyway: I had a HB that were just a tad too bright and I couldn't tame that with alternative magnets and I was in a bit of hurry so a rewind wasn't a real option. I had an aluminium bar stock that had the right dimensions, so I chopped of a bit and substituted the wooden spacer under the slug coil. That tamed the pickup just the way it needed and also migh give a bit of understanding to how the steel keeper bar affect the sound.

So you delivered a humbucker with TWO aluminum spacers and you say THAT tamed the highs? Did I get it right?

[Peter Naglitsch]

No, I substituted the wooden spacer with one made from aluminium. I figured the position right under the slug coil might move the sound in the same direction as a cover do and my gut feeling was right. It didn't radically reduced the highs, just a subtle reduction that made the HB more pleasing in that particular guitar. I'm not going to speculate if it was an increase in inductance or the efect of eddy current in the aluminium or whatever, but my ears told me it worked.

[Mike Sulzer]

No, I substituted the wooden spacer with one made from aluminium. I figured the position right under the slug coil might move the sound in the same direction as a cover do and my gut feeling was right. It didn't radically reduced the highs, just a subtle reduction that made the HB more pleasing in that particular guitar. I'm not going to speculate if it was an increase in inductance or the efect of eddy current in the aluminium or whatever, but my ears told me it worked.

No need to speculate. It was the eddy currents. Aluminum does not increase the inductance since it is not a ferromagnetic material.

[Peter Naglitsch]

Curiosity got the best of me...
I took a few standard HBs, a couple of non ferrous metal cover and a LCR meter. I measured the inductance of one with the cover off, 4.41H, measured the inductance of the same HB with one of the the covers on, 4.33H, repeated on all pickups and all covers and the result is more or less identical, a change in inductance (even tough it decrease) of a few percent. To round off the experiment I placed a quite thin piece of aluminium on top of one of the HBs (something I had at hand) and the inductance decreased once again although not very much. I don't know why, but i can read a LCR meter and the inductance does change when non ferrous metal is in proximity of the pickup. Interesting...

[big_teee]

Curiosity got the best of me...
I took a few standard HBs, a couple of non ferrous metal cover and a LCR meter. I measured the inductance of one with the cover off, 4.41H, measured the inductance of the same HB with one of the the covers on, 4.33H, repeated on all pickups and all covers and the result is more or less identical, a change in inductance (even tough it decrease) of a few percent. To round off the experiment I placed a quite thin piece of aluminium on top of one of the HBs (something I had at hand) and the inductance decreased once again although not very much. I don't know why, but i can read a LCR meter and the inductance does change when non ferrous metal is in proximity of the pickup. Interesting...

Great Experiment!
While your at it can you do that with a double screw bobbin pickup vs. a double slug bobbin Pickup.
With the same strength magnet, or perhaps with the same magnet.
Later,
T

[Mike Sulzer]

Curiosity got the best of me...
I took a few standard HBs, a couple of non ferrous metal cover and a LCR meter. I measured the inductance of one with the cover off, 4.41H, measured the inductance of the same HB with one of the the covers on, 4.33H, repeated on all pickups and all covers and the result is more or less identical, a change in inductance (even tough it decrease) of a few percent. To round off the experiment I placed a quite thin piece of aluminium on top of one of the HBs (something I had at hand) and the inductance decreased once again although not very much. I don't know why, but i can read a LCR meter and the inductance does change when non ferrous metal is in proximity of the pickup. Interesting...

Yes, that is what your inductance meter appears measure, but it does not mean that the inductance of the coil has gone down. You can set the meter to measure an inductance with either a series or parallel resistance present, but a humbucker has both, the parallel being due to the eddy currents. You have changed the parallel resistance (as well as affecting the value of another inductor that appears in series with the parallel resistor) by adding metal near the pickup. The meter cannot handle this situation accurately.

There is a discussion of measuring inductance with an LCR meter (and with a more complicated set up) down the page a bit.

[Steve Conner]

Mike's explanation is the most technically correct. But from a back of the envelope perspective, non-ferrous metal placed in a coil reduces the inductance, ferrous metal increases it. I've seen variable inductors with brass tuning screws that reduce the inductance when screwed in.

But reduced inductance should give more highs, not tame them. Hence Mike's series and shunt resistors to model eddy current losses.

[Mike Sulzer]

Mike's explanation is the most technically correct. But from a back of the envelope perspective, non-ferrous metal placed in a coil reduces the inductance, ferrous metal increases it. I've seen variable inductors with brass tuning screws that reduce the inductance when screwed in.

But reduced inductance should give more highs, not tame them. Hence Mike's series and shunt resistors to model eddy current losses.

In any case, the meter reading is always suspect when two kinds of losses are involved. To see this, start with an inductor and resistor in series. The meter measures this correctly when in the series mode. Now add a resistor in parallel. This lowers the magnitude of the impedance and decreases the phase angle (towards real). The meter reads a lower inductance, since the projection onto the imaginary axis has decreased, but the inductance has not actually changed.

[Steve Conner]

If this inductor with two resistors were used in a LC oscillator, would the shunt resistance affect the operating frequency? That is, would it fool an oscillator the same as it fools a LCR meter?

Going on from that, would it change the resonant peak frequency?

I think a non-ferrous slug should decrease the actual inductance slightly, over and above any fooling of a LCR meter due to loss resistances. The reason being that magnetic fields can't penetrate a conductor, so there is less room for field lines, hence lower inductance. But maybe this is only significant at RF. Any audio hobbyist knows that transformer hum goes straight through an aluminium enclosure.

[Mike Sulzer]

If this inductor with two resistors were used in a LC oscillator, would the shunt resistance affect the operating frequency? That is, would it fool an oscillator the same as it fools a LCR meter?

Going on from that, would it change the resonant peak frequency?

I think a non-ferrous slug should decrease the actual inductance slightly, over and above any fooling of a LCR meter due to loss resistances. The reason being that magnetic fields can't penetrate a conductor, so there is less room for field lines, hence lower inductance. But maybe this is only significant at RF. Any audio hobbyist knows that transformer hum goes straight through an aluminium enclosure.

I think the easiest way to look at it is consider the admittances of the C and L, that is the inverse of the impedance. Put them in parallel; admittances in parallel add. They have opposite sign, and at some frequency they have the same amplitude, and so the admittance of the combination is zero. This is the resonance. Put a resistor in parallel as well; that is, add its admittance. This does not affect the resonant frequency although the admittance no longer goes has zero magnitude. It is still minimum at the same frequency.

(If you cannot decide which way the resonance would shift, it cannot shift.)

[copperheadroads]

I just use the stew mac magnets . I leave them in the plastic case & stick one on each side of the jaws of my bench vise ,with north pole facing the south pole . to degauss ,just flip one magnet