Experimental rail pickup construction: wiring question

[Jason Rodgers]

Hello, MIMF-ers! So, you've probably seen my various posts on pickup construction: pickup winder, bobbin platen, flatwork materials, and all of the questions and discussion that go with those pieces of the DIY pickup puzzle. Now, I'm getting close to finishing my shopmade bobbins and start some winding. Here is what I have, so far...

The black flatwork pieces are the laminated countertop laminate scraps described in the flatwork thread (black, wood grain samples provided by John Sonksen). This works well and I will do it again. As the intended guitar is a 7-string, I thought I'd bypass the issue of pole pieces and string spacing and just build rail-style pickups for total coverage. These rather interesting cores are simply 3/4" wide x 3/16" soft steel bar stock from the Depot.

Moving forward, I'm epoxying the cores into the flatwork. The steel cores have been shot with Krylon rattle-can clear, but I'll additionally tape them with Kapton tape (thanks to David King for that suggestion, and other tips, as well) before winding to avoid shorts. To gang the coils, I will use another piece of laminate for the baseplate.

Each bridge coil will get about 5000-6000 turns of 42 gauge wire, and about 4000-5000 each on the neck. They'll be powered by a row of 1/2" x 1/8" x 1/8" neodymium magnet blocks on the bottoms of the cores. I'm going to wire these as 4-wire 'buckers so they can be coil split: 26 gauge stranded black, white, red, and green (according to one manufacturer or another's color-coding). The cores will also need a ground. I will eschew the shielded 4-wire cable for twisted leads, which offers some effective shielding, and well-shielded and grounded cavities.

So, my question is pretty simple: will this work?

Does a pickup need to have a metal baseplate with a ground wire? Is there any special way I should ground the steel cores?

Thanks again to David King for a lot of technical advice along the way, and John Sonksen for material and moral support!

[David King]

You can solder to steel with enough thermal mass in your iron's tip and some good old acid flux. You want to tin and solder the ground leads on before you glue up the bobbins.
Use superglue to construct your bobbins. It's just so much quicker and easier to work with than epoxy and it's going to hold up better to heat.
I guess I would wind the coils first and test them with and without a steel base plate to see what sounds better. Most baseplates are nickel-silver or brass which are non magnetic. Sometimes a steel baseplate is used as a part of the magnetic circuit to push the field out over a wider section of the string's length. That's typically only used in single coil pickups when you want a second pole pointing at the strings from outside the coil for the sake of efficiency.

[Jason Rodgers]

Well, I don't think the magnetic field needs to be any bigger, so I'll start with the laminate base.

Snap, I already began glueup with epoxy. The bottom bobbin flats are on. I'll double up with CA and attach the top flats with CA, as well. And I'll get the grounds on before anything else.

[Jason Rodgers]

Ooookay, so soldering is not a strength.

I have some zinc chloride flux - the kind you get with a copper pipe sweating kit - and some basic solder that comes with an inexpensive Weller 25 watt soldering iron. On a test piece, I smeared some flux on the steel, held the iron tip to it for a good while, then brought the solder in beside the tip. It melted, but rolled right off.

If I try on the pickup cores, would it be better to attempt to solder to the webs between the holes, since that is a smaller cross section of material?

[David King]

A smaller cross section might help. Did you tin the tip first so that it was completely covered in a blob of molten solder? That's the only way you are going to get heat transfer from the tip into the steel. The steel has to be very freshly sanded and the flux may be the wrong stuff. I just did a little test on some hot-rolled steel. I sanded it to uniform bright with 320 paper, heated it up quickly from behind with a butane plumbing torch to pale straw color (about 450º F) and touched it to the end of my roll of rosin core lead solder. The solder flowed instantly and looks ok to me so it is possible.
I suspect you're not getting the steel hot enough to melt the solder directly. You would need a 100W iron with a good sized tip.

[Jason Rodgers]

Hmm, ok, so I really didn't think this through. With a clear coat on the cores and the bottom flats glued on, I'm not going to be able to use a torch to do this job. I'll need to remember that next time...

Would a mechanical connection work? Here is a thought I had:
- Drill a small, blind hole in the steel.
- Using solid copper wire, wedge a folded end of the wire into the hole.
- With a nail set or some sort of "peen," tap the copper securely in the hole.
- Tape it off immediately (with that crazy strong Kapton tape!) to avoid any movement and loosening of the ground wire.

We use mechanical connections in household electrical systems. I don't know if this situation is analogous, though. Once the coil is wound and the pickup potted, there would be little chance of that ground shorting. Of course, that little chance, buried in the center of the coil, could make or break the whole deal. At this point, unless I backed up several steps, I'd be ok with that chance on an experimental attempt.

[David King]

Just place a strip of copper tape with conductive adhesive on the steel and call it good. Oh yeah solder the wire to the tape first. Again free samples are your friend. I'd first try drilling a 3/32" hole in the bottom edge and tap that with a 4-40 tap so you can screw a brass solder lug to the bottoms of your rails. That should hold up pretty well to the ravages of time.

When you wind these you might want to fill the slots in the bottoms of the rails with something rigid (perhaps your epoxy got in there already?). Imagine 20 grams of tension per turn x 6000 turns and you get the picture pretty quick.

[Jason Rodgers]

Free sample of adhesive backed copper shielding tape: ordered.

Yes, the epoxy and CA got down in those grooves. I see what you mean: the tension of the wire could conceivably squeeze those gaps closed if left unfilled.

I tried mushrooming the wire into a hole on scrap, and it worked. I drilled a tiny 1/32" hole, doubled the end of the wire over (26 or 24 gauge, I think), and gave it a tap with a nail set. The first time, I had to hold the steel in the vice and pull the wire out with pliers. The second time, the wire kinked and broke when I tugged. I think as long as the wire is taped well so it can't wiggle around and weaken the plug point, it'll work fine. This will also be minimally invasive. But, of course, the tape is less so. We'll see how long it takes for the tape to arrive, and how patient I can be: by golly, I wanna wind!

[Halgeir Wold]

For this kind of soldering , a 25W iron is way too small...... I'd say 75-100 would do, even a simple cheap one...
Regular electronic solder also works......

[Jason Rodgers]

For future iterations of this design (assuming this works at all!), I will need to have a better plan for the grounding. If soldering is the best means, I may need to go see what Harbor Freight has for irons.

[Mark Swanson]

Watch your local tool sales and yard sales, Craigslist. I have seen a lot of them that way. I use one of the pistol-grip type for bigger jobs, they can get hot enough.

[Jason Rodgers]

I had a fair iron gun, but it died. It would have done the job. Can't remember where I read it, but this source said those types of trigger action irons shouldn't be used with pickups: something about the magnetic field messing with your magnets. I don't need to worry about that with these pickups, but I'm sort of glad that iron died, because I would have used it on this project and probably would have nuked a pot or something!

[David King]

That's BS about demagnetizing via the soldering iron. The transformer's field is contained and it's too far away anyway. Most folks charge their magnets just prior to assembly using some cheap neodymium magnets so not an issue.

[Jason Rodgers]

Ha! Look at that: I fell for some of the guitar electronics lore!

[Halgeir Wold]

Well, - I'm afraid that the field in a solder gun is not that well contained, as the tip is part of the secondary coil, with a fairly heft current,
That's how it works, OTOH - I don't think that it will do any harm for the few seconds that is needed

[Jason Rodgers]

Ooo, dramatic lighting. The bobbins are assembled. The Kapton tape should be in the mail today or tomorrow. I'll install the ground wires as described above, insulate the cores with the tape, and should be ready for winding this week.

What with yard sale season starting up, I'm in the market for a small crock pot - party bean dip size - for wax potting.

[David King]

Jason, I got one for you.

[Jason Rodgers]

Sweet! Did you scrape out the old bean dip?

[David King]

Velveeta.

[Jason Rodgers]

These really ARE going to be experimental pickups: potted in cheese!