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Author Topic: Homebrewing 1/4" Tuned-Slug Coils  (Read 20765 times)
KB1WSY
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Posts: 633




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« on: February 07, 2012, 10:22:02 AM »

I am thinking of trying to homebrew some vintage-type 1/4" tunable-slug coil forms, which are usually ceramic or phenolic. This is partly because supplies of the real thing are a pain to find -- they are available, but the number of suppliers seems small and supplies are episodic. But also, and mainly, for fun. One thing that *is* widely available is quarter-inch "PVC Rod" that can be bought cheaply in multi-foot lengths to chop down. Also widely available are the threaded powered-iron or ferrite slugs for inductors. So I was thinking of using a drill on a vertical stand to drill a pilot hole down the center of the PVC rod, then threading it with a tap so that the slugs could be screwed into the centers. For fastening the upright coils "thru chassis" I suspect one could use a plumbing collar with a 1/4" internal diameter (either plastic or metal). Has anyone else tried this? Do y'all see drawbacks with this plan?

On a related subject, has anyone built the "Gingery" coil winder or have experience with other coil winders at the "homebrew" level?
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G3RZP
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« Reply #1 on: February 07, 2012, 11:05:46 AM »

PVC is rather lossy. If you can get polystyrene or Teflon rod, they would be better. Cut a thread at the end, and use nylon nuts to attach the former to the chassis. It's really a job for a lathe, though.

Been there, done that. Made some phenolic rod hexagonal using a mill, drilled it through for a trimming tool, and cut the same thread as on the tuning slug. Very good fixing method.
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KB1WSY
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« Reply #2 on: February 07, 2012, 11:52:25 AM »

Hello G3RZP, that's very encouraging. I can see that it would be much easier on a lathe and I may be able to "borrow" the lathe at my daughter's high school. Perhaps I can persuade the workshop teacher that it is a novel educational project. I think one can also buy adapters that will turn a household electric drill into a small horizontal lathe -- I will check that out, my dad had one of those years ago.

Thank you for your advice about the material. I checked and it is easy to buy 1/4" Teflon rods. I think there are special drill bits for cutting into that kind of material.

Do you have advice concerning the slugs? Where did you get them from, and what was the best material/size/length? It seems there are "powered iron" and "ferrite" versions. Also, for 1/4" coils there are two versions: the "Red" and the "Green." I heard that the Reds were for HF and the Greens were for higher frequencies but I am not sure where the frequency cutoff between the two comes. (Earlier threads on eHam mentioned 20MHz as the possible cutoff.)

After threading the slugs into the plastic did you find that the slugs fit nicely (tight enough so they won't budge on their own, but also loose enough for tuning)?

Very refreshing to find someone who has "been there and done that," avoids having to reinvent the wheel!
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N3QE
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« Reply #3 on: February 07, 2012, 12:04:04 PM »

Something similar is done (most often with brass slugs but I see no reason it wouldn't work for any other kind of tuning slug) and it you can find details by googling "soda straw vfo" or "kd1jv straw".

The coil form is just a soda straw, and slug positioning is done by gluing nut(s)/mounting brackets to one end to move the slug in and out. Different variations use nylon and/or brass hardware depending on intended magnetic properties.

Other variations (especially with brass slugs) start with a prethreaded nylon spacer as the coil form.

If you insisted on the traditional threaded form, I would think it'd be 100 times easier to mold a plastic or a fiberboard-impegrated paste around a release-coated threaded form, rather than drill and tap starting from solid rod.
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KB1WSY
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« Reply #4 on: February 07, 2012, 12:25:27 PM »

Great, I'll check out the soda straw idea.

>>mold a plastic or a fiberboard-impegrated paste around a release-coated threaded form<<

To get me started in researching your idea, any idea what "craft" that technique is derived from? For what it's worth, I spent a little bit of time last week trawling the "ceramic pottery" websites in hopes that I could make a pottery form or matrix from which I could produce and bake my own coils, but didn't get very far. I also found a source for ready-made hollow porcelain rods, but I didn't see how that material could be worked (cut or threaded) without cracking it. I didn't think of doing it with plastic or fiberboard, thank you for the suggestion.
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W5FYI
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« Reply #5 on: February 07, 2012, 02:46:49 PM »

"...I suspect one could use a plumbing collar with a 1/4" internal diameter (either plastic or metal)."

Try not to use metal collars--they will have a "shorted turn" effect, stealing some power from the circuit and changing the coil inductance. Go for the threaded hole in the former and fasten it to the PC board with nylon screws. Radio Shack used to have them, and most hardware stores carry them. GL
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G3RZP
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« Reply #6 on: February 10, 2012, 05:51:24 AM »

I always use standard jobber's drills for Teflon.  Polystyrene has some advantages, but is more difficult to work because of the need to keep the tool cool. If I need to thread lock, there are various ways. One is a squirt of silicon grease: another good one is very fine monofilament fishing line down the slug. When drilling or working with Teflon, DO NOT SMOKE! Teflon dust in a hot cigarette produces some nasty chemicals.

Dust iron - see manufacturers recommedations. From memory, red is good up to 20 or 30 MHz. Ferrite - depends on the grades, and the ones I know best are not, I think available any more. Some ferrites, especially those of high permeability and low loss at VHF (they used to use them for pager antennas) can be ruined by exposure to a magnetic field. Brass cores are good for reducing inductance.

Personally, I wouldn't attempt plastic moulding around  the core. It might work if you have a suitable  tool and moulding press, but that's getting a bit beyond most home workshops. If you can find bigger diameter slugs, you can do nice larger diameter forms, too.

Have a look at Amidon - they used to do threaded cores. As '5FYI says, avoid metal collars.

I've looked at the Gingery coil winder book. It appears that it needs a lot of setting up, because when you do a wave winding (I think the US term is 'universal winding')after every turn, the arm moving the wire has to lag by one wire diameter so that it doesn't pile up every turn on top of every other turn. I suspect a proper coil winder would be an interesting model engineering project, just as a gear hobbing machine is!
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N3QE
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« Reply #7 on: February 10, 2012, 07:28:47 AM »

>>mold a plastic or a fiberboard-impegrated paste around a release-coated threaded form<<
>  To get me started in researching your idea, any idea what "craft" that technique is derived from?

The video linked below shows a "roll your own" carboard tube, starting with paper and glue and a form. Craft-wise this is something that model rocket and telescope makers might do when they need a custom size.

I don't know exactly how to get machine-screw class threads on the inside but looking at vintage NOS cardboard tube slug tuned inductors... the thread is coarse and is not a "full" thread, in fact it's just barely threaded, and I would think they must've been made with a similar technique over an appropriate threaded form. Perhaps with more pressure or goopier paper pulp to conform to the thread. Often the inside of the threaded tube appears waxy, I don't know if this is an aftereffect of my hypothetical release compound, or if they do that after manufacturing the threads.

http://www.youtube.com/watch?v=KHLxoqsE_UY
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KB1WSY
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Posts: 633




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« Reply #8 on: February 10, 2012, 10:06:40 AM »

I have found, on a British forum, an article from the 1950s describing how to build a fairly sophisticated small-coil winder (including "wave" winding) entirely from Meccano (U.S. Erector Set) -- "A Simple Tuning Coil Winder" by P.A. Robinson. The author was laid up in bed with the 'flu when he got the idea, and comments that "it is advisable to quietly procure the bits and pieces before the influenza sets in, for to ask the wife to go round to the local toy shop for Meccano parts will only promote sidelong glances and a hurried return to the doctor. This is no idle remark, as the author well knows." Anyway, I've asked my brother to root around in our mother's basement, which houses a large stash of vintage Meccano. When I have the parts I will try to build the thing and report back!
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KB1GMX
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Posts: 709




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« Reply #9 on: February 11, 2012, 10:44:16 AM »

Winding coils is far easier than some make it out to be.

I've found phenolic tube in supply houses to be useful
to do "replica" forms or to fix damaged forms in older radios.

the windings are often by hand and rare is the special Pi wound or
weave wound that cant be reproduced by hand.  Q-dope is handy
and you can buy or make it (dissolve Polystyrene in MEK till you
get a thick syrup, note: flammable).

To duplicate the older projects out of handbooks. Usually they specify
a manufactured coil, that you can't buy now.  All is not lost as you can
look at the circuit and knowing the C across or in series, the frequency
you can calculate the L.  There are calculators on the 'net to do that
and other to calculate of you have form of said diameter and need an
L of so many uH how many turns need to be wound.  if the coil is
slug (usually powered iron but sometime brass or aluminum) tuned
and its generally the case then extreme accuracy is not required as
it gets tuned later. Slugs for that can be found in old or other forms. 
Note the old slugs were often powered iron flavor which increase
inductance when nearest to the center of the coil.  If you need to
reduce the inductance use a aluminum or brass screw.

The bigger problem is where shielded coils are used as you
have to duplicate that shield.  Be creative.

In older especially tube circuits fixed coils and variable or
variable plus fixed caps are interchangeable especially at lower
HF and VLF frequencies.  As you get closer to VHF care in circuit
layout is advised.

Can you use ferrite and powered iron toroids?  Yes. the key here is
that you will have to adjust the capacitance rather than the inductance.
Toroids have a small range of inductance adjustment by stretching or
compressing the windings but only if the form is not full. Varying the
capacitance is allowed and reasonable.

One last item axial lead coils (aka chokes) are still available and in
the right inductance values with adjustable caps make fine tuned circuits.
Watch for the Q, higher is better as some chokes are for RFI suppression
and have very low Q.


Allison
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KB1WSY
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Posts: 633




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« Reply #10 on: February 11, 2012, 12:49:33 PM »


Wow, what a creative set of responses. I have lots of ideas to be going on with:

(1) My original idea, validated by the fact that someone else has been there before me:

>>Been there, done that. Made some phenolic rod hexagonal using a mill, drilled it through for a trimming tool, and cut the same thread as on the tuning slug. Very good fixing method.<<

(2) The soda straw or the nylon spacer:

>>The coil form is just a soda straw, and slug positioning is done by gluing nut(s)/mounting brackets to one end to move the slug in and out. Different variations use nylon and/or brass hardware depending on intended magnetic properties. Other variations (especially with brass slugs) start with a prethreaded nylon spacer as the coil form.<<

(3a) Molding a coil from from plastic or fiberboard paste...

>>If you insisted on the traditional threaded form, I would think it'd be 100 times easier to mold a plastic or a fiberboard-impegrated paste around a release-coated threaded form, rather than drill and tap starting from solid rod.<<

(3b) ... or from cardboard:

>>The video linked below shows a "roll your own" carboard tube, starting with paper and glue and a form. Craft-wise this is something that model rocket and telescope makers might do when they need a custom size. http://www.youtube.com/watch?v=KHLxoqsE_UY<<

(4) and Allison's great post:

>>I've found phenolic tube in supply houses to be useful to do "replica" forms or to fix damaged forms in older radios. (details)<<

(5) in which the obvious but important point is made that with a bit of math, you can figure out an alternative to the original coils specified in project instructions;

(6) and that besides the dimensions of the coil and type of form, there are alternatives such as using capacitor tuning, toroids, or building your own tuned circuits using choke coils (a personal note here is that I found on the 'Net a great post by someone who has been building IF transformers using this method, with two chokes in close proximity).

Lots of food for thought. Thank you all!!

One more thing, completely off-topic. Some of you have managed to post in a format that keeps the column width relatively narrow (much easier to read). How did you do that???

73

Martin (I passed the Technician and General tests today, so within days you will find that my CHESTNUT moniker changes to a real call sign!!!)
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KB1GMX
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Posts: 709




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« Reply #11 on: February 11, 2012, 01:08:46 PM »

Major congrats, happy building.

Narrow posts, hit return at the end of a reasonable line.  Typical fonts these
days make that at around 80 characters or 12-14 words.

Wide posts, dont hit return at the end of a reasonable line.  Typical fonts these days make that at around 80 characters or 12-14 words. Same thing but I didn't hit return at "these days".

Or you can edit in your favorite editor or word processor and set the line
for decent breaks and copy/paste to here..


Allison
« Last Edit: February 11, 2012, 01:14:56 PM by KB1GMX » Logged
KB1WSY
Member

Posts: 633




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« Reply #12 on: March 13, 2012, 05:29:42 AM »

So, I have a bunch of NOS Miller 4506 coils from which I will be removing the existing windings and then re-winding with new wire -- I don't need 4506's but the underlying coil form is a 4500 which is exactly what I want. So, a few questions people may be able to help me with. Please note that I have done a lot of research but would still rather hear from people who've actually got experience doing this stuff.

--There's a yellowish varnish gluing down the original wire, where it meets the metal clips. Not sure what this stuff is, what do y'all suggest for getting it off without damaging the underlying coil form? Turpentine?

--Once I have wound the coils, what do I use to replace that varnish on the ends of the coils? Is that what Q-dope is for?

--I may have to improvise some additional metal clips because the original NOS forms that I have a single-coil and some of the items I am winding will be double-coil (coupled coils). I think I should be able to make these with thick gauge solid wire, stripped of the insulation.

--For one of the 4500-based coils, the original project instructions from 50 years ago call for 100 turns #34 wire, "scramble wound" (along with a second, coupling coil). I assume that "scramble wound" means winding in several layers, because there is no way that 100 turns would fit on these little coil forms without layering. I further assume that this is a good use for my newly acquired vintage Morris coil-winder, since it can produce a "universal" or "basket weave" pattern.

--I am totally new to this kind of work so please excuse this basic question. I have researched this issue but am still a little confused. When there are two coils on one form: (1) they should both be wound in the same direction, I already know this; (2) however I am not sure which end of the form the "ground" end of the wiring should be, and whether the grounds of both coils should be next to each other in the middle of the coil or not; and (3) does it make any difference where I put the smaller coupling coil, above (furthest from the chassis) or below? The old project instructions simply assume that I know all this stuff already.

If anyone can suggest a really good source book for all these questions on the old craft of coil-winding, I could then stop pestering y'all with questions! There is a book several people refer to: "Coil Winding: A Description of Coil Winding Procedures, Winding Machines and Associated Equipment" by William Querfurth and published by the Stevens coil winding company in the 1950s with several updated editions through the 1960s. Available either on CD-ROM for a reasonable price, or even the original book for $40 and up. Has anyone seen this book, and what do you think of it?

TNX ES 73 DE Martin, KB1WSY
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WB6BYU
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Posts: 13014




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« Reply #13 on: March 13, 2012, 08:52:36 AM »

Quote from: KB1WSY

--There's a yellowish varnish gluing down the original wire, where it meets the metal clips. Not sure what this stuff is, what do y'all suggest for getting it off without damaging the underlying coil form? Turpentine?


Or acetone / fingernail polish remover.


Quote

--Once I have wound the coils, what do I use to replace that varnish on the ends of the coils? Is that what Q-dope is for?



Yes.



Quote

--For one of the 4500-based coils, the original project instructions from 50 years ago call for 100 turns #34 wire, "scramble wound" (along with a second, coupling coil). I assume that "scramble wound" means winding in several layers, because there is no way that 100 turns would fit on these little coil forms without layering. I further assume that this is a good use for my newly acquired vintage Morris coil-winder, since it can produce a "universal" or "basket weave" pattern.


If you make a "basket weave" pattern it will no longer be "scramble wound", the latter
meaning to wind the wire on without much regard to crossing each other, etc.  Sort of
the way you would get if you didn't watch what you were doing when you wound
a string around a stick, so the center was fatter than the ends.  Mostly it means not
to worry about making nice neat layers.


Quote

--I am totally new to this kind of work so please excuse this basic question. I have researched this issue but am still a little confused. When there are two coils on one form: (1) they should both be wound in the same direction, I already know this; (2) however I am not sure which end of the form the "ground" end of the wiring should be, and whether the grounds of both coils should be next to each other in the middle of the coil or not; and (3) does it make any difference where I put the smaller coupling coil, above (furthest from the chassis) or below? The old project instructions simply assume that I know all this stuff already.


The only time it really makes a difference whether the two coils are wound the same direction
is for an oscillator circuit where the feedback needs to have the proper phase.  The only
electrical difference is the relative phase differences between the ends of the physical coil,
and for interstage coupling that doesn't matter.

Generally you'd want the "ground" end of the winding closer to chassis ground to minimize
capacitive coupling.  Similarly a low impedance link will be less effected by the chassis than
a high impedance tuned coil.  But in most cases the circuit will work regardless of how the
coils are arranged on the former (which is more likely the case than they assumed you knew
what to do.)

Note that with some coils the link is wound separately on the former and spaced 1/4" or so
away from the primary, and other times it is wound directly over the primary winding,
depending on the level of coupling required for the circuit.  Tighter coupling gives better
power transfer but lower loaded Q.
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KE6WNH
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Posts: 126




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« Reply #14 on: March 16, 2012, 07:48:12 PM »

Strange, I salvaged a lot of slug-tuned coils of every size imaginable from discarded electronic circuits over the years. I'd gladly mail you a couple dozen of them that you could pull apart and rewind.

A few years ago, I bought a sack of 100 of just such coils from All Electronics near downtown LA.
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