Coil Winding Information For Vintage Whites BFO Metal Detectors

[Cascade Steven]

While reviewing some more of my archived files I found the coil winding specifications for one of White's Electronics BFO metal detector coils.  Data was provided by the White's Factory.  This coil was called the "Triplet Coil".  It was manufactured in the late 1960's and early 1970's.  My information is from May 1969.  During this period there were several BFO coil sizes offered: a 3-inch coil for nuggets, a 6-inch coil for coins and a 12-inch coil for large objects.  The Triplet was designed to combine both the standard 6-inch coil and the 3-inch nugget coil with an additional 2-inch coil for added sensitivity for small objects.  Single coils were wound using a wood based enclosure.  The Triplet Coil was encased in a molded plastic housing.  For those of you who have a vintage White's BFO metal detector and would like a combination coil or those of you who wish to build a replica of a vintage White's BFO metal detector I have attached a PDF of the winding details for your viewing pleasure.  BFO Triplet Coil winding instructions.pdf

[Geotech]

I'll betcha I'm the only other person here who found this interesting. I had a White's BFO with a Triplet coil and was curious what made it a Triplet coil, but not curious enough to destroy the coil to find out. Thanks for the tidbit, curiosity satisfied.

[PimentoUK]

I found it modestly interesting, from a technical viewpoint.
If anyone is making this coil, I suggest winding the wire bifilar, that is two wires together. So for example the large winding would be made as 29 T of two wires together. Then join the 4 wire ends together so they function as a centre-tapped coil.

My only old Whites machine is a T/R from the late 1970's , so a league away from BFO machines and quirky coils like this.

Carl and George's geotech1.com forum would be a place where this info may be of interest, (though I've observed that BFO tech gets little mention.)

https://www.geotech1.com/forums/forum.php

[Cascade Steven]

PimentoUK: Thanks for the input.  You have made an interesting suggestion about an alternative method of winding.  I am curious as to whether it would improve performance?  Unfortunately I do not have a working machine on which to test it. 

[PimentoUK]

Rather than considering it an alternative construction method, I feel it is the only correct way, and gives the greatest chance of achieving a coil that is truly CENTRE-tapped. Methods such as " Wind 29T, bring out a tap, wind another 29T " are guaranteed to result in a coil that has the 'centre-tap' some modest distance away from the centre. The total end-to-end inductance should be much the same for both bifilar winding ,and half-then-half.

As for performance improvements, it's hard to say, without knowing the rest of the oscillator circuit design. Some designs may tolerate 60% / 40% split in the tap position.
Also relevant is the fact there are three coils working together. If they were all closely matched in terms of centre-tap accuracy, the coil as a whole would be more consistent.

[Geotech]

Hartley oscillator, which is pretty tolerant of the coil tap. But getting the 3 coils more closely matched may be more critical.