Consistency and Quality Control

[Helmholtz]

What you call a „coil within the coil“ does not produce your typical eddy effect, where you only have one current loop. Instead it has to be viewed as a transformer with a shorted secondary. The coupling of inner turns is relatively tight and increases with the number of shorted turns. And so do the detrimental effects.

Zollner showed that a short over two layers in a strat PU reduces the unloaded resonant peak by 10dB and the loaded peak by around 4dB thereby essentially eliminating the peak.

This is far more than the effect of different alnico types (the conductivity of all alnico grades is comparatively low and the variation between grades is small, around +/-20%).

Some great sounds require bright sounding PUs, think Dickey Betts with the Allman Bros or SRV's „Lenny“ and „Riviera Paradise“. If customers can't get their desired sounds, they might not return the PUs but give a low rating.

Right now I'm having two strat PU's from 1970 rewound because they showed the typical symptoms of shorted turns. Their values even react strongly on squeezing and temperature changes.

Whenever a PU sample shows abnormally high AC resistance or dissipation factor this should give rise to a fault analysis and if necessary a change of production methods or materials.

[Antigua]

Zollner showed that a short over two layers in a strat PU reduces the unloaded resonant peak by 10dB and the loaded peak by around 4dB thereby essentially eliminating the peak.

The fact that I've never seen a Strat pickup show an inexplicable -10dB at the resonance, after having tested a few hundred, makes it seem all the less likely to me that this is something that actually happens.

-10dB sounds a bit extreme for two layer short. That must be close to 50 turns of wire, depending. With a shorted tapped SSL-3, I did see a very dramatic drop in resonance, beyond 10dB, but that was half of the entire coil. The bode plot of the Duncan Designed SC101, which has a piece of foil wrapped around the coil, showed only a drop of maybe 5dB at best, from what is typical of a Strat pickup without foil shielding.

[Helmholtz]

The fact that I've never seen a Strat pickup show an inexplicable -10dB at the resonance....

Fine, after all the purpose of QC is to prevent flawed products from entering the market.

[Antigua]

Fine, after all the purpose of QC is to prevent flawed products from entering the market.

Supposing there was a QC process, and that the QC process would detect such a thing. I've wound a few dozen of my own pickups, I've never seen such anomalies in my own pickups, either.

Someone mentioned that shorts could occur in a very old pickup, after the enamel had become brittle. Notwithstanding the potential of this actually happening, no QC process will account for something that happens decades later.

[Helmholtz]

Anyway, it doesn't hurt to check. And the builder/OP will only find out if he does.

[Joe Gwinn]

This is actually equivalent to my proposal above. The Q value of an inductor is proportional to L/Rs, Rs being the equivalent AC series resistance. Often meters display the D or DF (dissipation factor) value. As D=1/Q, it gives the same info as your method.
Q, D and AC resistance measurements should preferably be taken at 1kHz and not at 100/120Hz, as the losses caused by inter-winding shorts increase with frequency.

Almost. The difference is the DC resistance, which is fairly high in low-Q inductors like pickup coils, and so obscures the effect we seek. The DC resistance changes very little when there are shorted turns, so tracking the excess of AC over DC is a more sensitive indicator of electromagnetic-coupled loss due to shorted turns.

[Helmholtz]

The difference is the DC resistance...

Difference between what?

...tracking the excess of AC over DC is a more sensitive indicator..

More sensitive than what?

Edit:
Seems I misinterpreted this sentence:

There have been discussions of this before, and one defense is to take inductance and AC resistance readins of event pickup coil made, and look for units with uncommonly high excess of AC resistance over DC resistance.

So you use Rac/Rdc.
But Rdc is less sensitive to partial shorts than L. Thus the ratio Rac/L , given by D= (Rac/L)/(2*pi*f) is the better indicator.
Rac here means the equivalent serial resistor, it is close to Rdc for low loss PUs and increases with losses/shorts. To measure, the meter has to be in serial mode. Parallel mode gives wrong readings. When using the D or Q measurement you don't need to care for the parallel/serial mode.

The most sensitive sign of shorts, of course, is an exceptionally low resonance peak.

[Joe Gwinn]

Difference between what?
So you use Rac/Rdc.

No, I subtract Rdc from Rac to yield the excess resistance. A five or ten turn short out of 5000 turns isn't going to be detected from Rdc, given that our instruments have at best 1% error.

The most sensitive sign of shorts, of course, is an exceptionally low resonance peak.

That will do it for sure. But it's easier to measure Rdc and Rac (at 1 KHz), and plot their difference from unit to unit. The oddballs will stick out.

[Helmholtz]

No, I subtract Rdc from Rac to yield the excess resistance.

Thanks for clarification.