# [b]Magnetic bridge plates affect tone?[/b]

Discussion in 'Tele-Technical' started by alheeley, Jan 20, 2006.

1. ### fullerplastTele-Afflicted

Mar 17, 2003
The inductance of a coil of wire is independent of everything other than the size of the coil, number of turns, length of the conductor, and diameter of the wire.

What you are changing is the permeability of the core, in this case the pickup pole pieces. In the case of the wah inductor, the core material.

What inductance meter are you using and at what frequency?

2. ### alheeleyTele-Meister

Nov 24, 2005
Re: I wouldn't say natural, but (T.Downs help?)

This is my gut feeling on the whole matter, which brings us right back to my first post in this intriguing thread

3. ### TwangbangerTele-Holic

Mar 16, 2003
Marble Falls, TX
The inductance meter that I used was an older CRL(capacitance, resistance, inductance) meter.

An inductors inductance is changed by the amount of ferrous meaterial near it. That's why there are inductors with a moveable ferrous rod in the middle. Those are tuner inductors for adjusting the inductance in a reactive circuit.

Maybe the way you learned this and the way I learned it are allong the same lines, but focused on the subject a little different.

4. ### fullerplastTele-Afflicted

Mar 17, 2003
Re: I wouldn't say natural, but (T.Downs help?)

Both have an effect. I personally think that the thickness of the plate has a more noticeable effect tonally than the magnetic properties.

5. ### fullerplastTele-Afflicted

Mar 17, 2003
Yes, I see where you are coming from. A tuneable inductor is operating a bit differently than a pickup. You are actually moving the core, which will change the inductance of that type of device. It's still the permeability of the core that you are altering, but in a variable inductor, that permeability works into the overall inductance equation.

6. ### tdownsFormer Member

Nov 8, 2003
Texas
Well I avoided this post because I don't like discussions about which tone is better. But....I was doing a search on my name AND inductance to find an old thread. I found this post in the search. I guess I didn't think this would get into the theory so much. See, I caught you folks talking about me.

Respectfully, that is not correct. But I can definitely see how could get that idea by looking up inductance formulae that are very common for air coils. They don't generally include permeability.

I hope I can explain this and make sense.

Materials have a property called permeability. That is the ability to conduct magnetic flux. Ferrous (and some special other) metals have a higher permeability than air. When a high permeability material is placed in the magnetic lines of flux around an inductor, they conduct the flux (reduces reluctance) and will therefore increase the inductance.

This is how a metal detector works (at least the old beat frequency oscillator type). A metal detector has two oscillators. The inductor of the reference oscillator is isolated. The inductor of the detecting side is big and exposed to the ground. The reference oscillator is tuned to match the detection oscillator. When both are at the same frequency, they provide a null output. If the detector inductor is place near a ferrous or high permability material, the inductance of the coil is increased. This causes a difference frequency and is heard by the operator.

The general simple equation for the inductance of a coil.

L = Inductance in henries
μ0 = permeability of free space = 4pi × 10^-7 H/m
μr = relative permeability of core material
N = number of turns
A = area of cross-section of the coil in square metres (m2)
l = length of coil in metres (m)

As you can see, the permeability is linear to inductance. Doubling the relative permeability of the core will double the inductance, assuming all lines of flux pass through the core.

In the case of a nearby permable object, the permability will conduct whatever lines of flux passes through it. So, how much the inductance is increased is dependent on the flux density and number of lines affected.

Respectively, that is not true either. When a non-ferrous metal is placed in magnetic lines of flux, eddy currents result. Eddy currents make their own flux opposite to the ones inducing them. So when a non-ferrous material is placed in the lines of flux, it will reduce the inductance.

I wrote an article on PSPICE Modeling of Guitar Circuits with Effects of the Instrument Cable. It basically shows how unloading the guitar circuit with large value potentiometers makes the resonance peak of the pickup inductance/cable & amp capacitance quite noticeable and therefore tuning it can change the sound. The same thing could happen with changing pickup inductance. Maybe we would find users of 250K pots would notice less difference with ferrous bridges than 1Meg pot users.

Only thing is, I don't know how much the inductance increases when a pickup is mounted on a ferrous bridge vs. a non-ferrous bridge.

Another thing to consider is that the permeable ferrous bridge conducting the flux lines around the pickup is distorting the lines of flux. Eddy currents are generated in the ferrous material too, so the lines of flux would not be as uniform. This could actually "focus" more or less lines of flux where the string vibrations are picked up.

Now look what you jerks have done. I gotta go make some inductance measurements now. 8)

Hey Twangbanger, did you write down any of those inductance measurements?

Great discussion.

Personally, I have guitars with both ferrous and non-ferrous bridges. I seem to make as many mistakes playing the ferrous ones as I do the non-ferrous. So I'll say there is no difference.

7. ### alheeleyTele-Meister

Nov 24, 2005
I love this place! So much learnt I never knew of, through a civilised discussion between intelligent enthusiasts. Hope you don't mind me sitting in!

8. ### TwangbangerTele-Holic

Mar 16, 2003
Marble Falls, TX
i was too late on making measurements on my tele, but rapping the inductor with a thin sheet of iron did increase the inductance from 30 to say maybe 60-100henries.

I ended up rewinding the inductor with finer wire and doing several taps so I could be sure.

I don't have the meter anymore, it was at my last job(Winding transformers)

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