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Topic: Replacing the pickups on a Supro |
Mike Bonnice
From:
Arizona, USA
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Posted 20 Sep 2008 7:15 am |
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The pickups on my Supro string-through lap steel died, so I decided to wind some new pickups. Result - it works great!
There were three major hurdles:
- making a string winder
- making the bare pickup spools
- installing the pickups
It turns out that the winding is the easy part. It's just like painting a house - painting is the easy part, the hard part is preparation before and detailing after.
To make a winder, I found a motorized back-massager at a rummage sale, then I took it apart and attached a threaded insert to the end of the motor shaft using JB Weld. To make it count the number of windings, I used the calculator tip from this page:
http://www.fieldlines.com/story/2005/10/26/72644/745
I attached a small switch to the massager housing and a bump (a screw) to the rotating part, then whenever the bump passed, the switch closed and added another 1 to the calculator. It turned at about 120 RPM.
The spools were difficult. The old paper spools fell apart, so I had to make new ones. After much trial and error, I created the center of the spool from a small piece of wood and the sides (the flatwork) from paper bowls (like picnic bowls). The advantage of the bowls was that they guided the pickup wire to the center of the spool, so I never had the case that the wire falls off the edge of the spool during winding.
To assemble, I drilled three holes (for the pole pieces) in the bottom of a bowl, inserted some screws, glued the wooden center to the bowl (it had been drilled already), then glued on the other bowl. Using nuts to hold the assembly tight, I waited until the glue dried, then mounted the assembly to the end of the motor shaft.
The motor shaft was horizontal. I had tried a vertical mounting scheme (on an old record turntable) but I was fighting gravity the whole time because the wire tended toward the bottom.
Winding was easy. I mounted a spool of 43 gauge wire on a horizontal bar to let it feed, taped the end of the wire to the bottom bowl, then turedn on the motor. I hand-fed the wire, holding it between my fingers and guiding it left to right.
After winding 3700 times, I got a resistance of about 2100 ohms.
To finish, I detached the assembly from the shaft and used a razor knife to cut the bottom of the bowls into the rectangular shape of the original flatwork. I added a couple of fatter copper wires to lead out the delicate pickup wire, just like the original pickups.
Finally, I dipped the pickups in wax according to the recipe in
http://www.stewmac.com/freeinfo/I-1465.html
I think a made the pickups slightly too tall because they are a very tight fit under the old sheet metal housing. I think it's because when I sized the wooden center piece I didn't account for the thickness of the electrical insulating tape that I need to prevent the lead out wires from contacting the sheet metal.
The pictures show the idea with the bowls. The one showing the final assembly shows the center spacer made of three pieces of tubing. This didn't work well because the tubing was flexible, so when I tightened the nuts, they squeezed and deformed and I lost control over the shape and height. That's why I made the center out of wood. I tried balsa, but that was too difficult to get very thin walls, so I switched to harder wood. My local Ace hardware store had basswood strips measuring 1/4 inch by 1/4 inch. Sorry I don't have pictures of those.
All the dimensions of my parts matched the dimensions of the original parts: the size and thickness of the flat work, the overall width, length and height of the center section.
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Mike Bonnice
From:
Arizona, USA
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Posted 20 Sep 2008 4:57 pm Here's what the final result looks like
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Here's the final result.
I also improved it by reducing the amount of tape so that everything fits under the sheet metal.
I also replaced the original cord with a cord having a female end. This enables using a modern cable without messing up the original body with a jack.
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 20 Sep 2008 5:42 pm
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| Congrats! Cool project. Try a set of Rick A. NIbro magnets in there. You will be impressed. |
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Bill Creller
From:
Saginaw, Michigan, USA (deceased)
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Posted 20 Sep 2008 7:16 pm
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Great job on that, especially because it's the first time you did it. 
Not sure I have the patience to do that! |
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Peter Jacobs
From:
Northern Virginia
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Mike Bonnice
From:
Arizona, USA
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Posted 21 Sep 2008 7:49 am How many windings
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There were a few factors in deciding how many windings:
- A little research on the web showed that most pickups are in the 4000 ohm range, with quite a bit of variation. That would mean that each of my two pickups would be about 2000 ohms. This was a big guess, but it was so typical of all pickups that I was willing to trust the guess.
- I measured the thickness of the original wire and found it to be close to 43 gauge, so that was the wire I bought.
- I started to wind a pickup using a different structure for the spool, but the wire got tangled and broke at about 1000 windings. I measured the resistance and found the number of ohms per winding. I extrapolated that to 2000 ohms (the resistance for each of the two pickups). That gave me a forecast of about 4000 windings.
- I wound another pickup to 4000 windings, but I wasn't holding the wire tight enough and the resulting coil was wider than would fit in the available space.
- So, finally, once I got the structure right and the winding technique right, I stopped at 3700 to be ensure that the result would fit. That gave the resistance of 2100 ohms. It didn't exactly match my extrapolation because the coil gets fatter with each winding, so the ohms per winding is greater for the outer windings that it is for the inner windings.
To summarize: 3700 windings worked because it fit in the space, and it produced the right resistance because (I assume) that's the original wire, and fitting the original wire in the original space produced the original resistance.
I had received the advice that I should stop winding at some point, then measure the resistance by stripping the wire without cutting it. If the resistance was too low, I was to re-insulate the stripped spot and continue winding. This was too risky, because it's scary to delicately strip the wire without breaking it, and breaking it was the last thing I wanted to do. That would mean any number of undesirable rework options, and for me the idea of perfection was a single, well-executed run of the winding operation.
I'm not sure this method will work in all situations. If you pack a spool full of windings, you might get too many, and I don't know what effect that would have on the sound. To practice, I got an old Fender pickup from a guitar shop. It appeared that there was still plenty of space on the spool for there to have been more windings if Fender had wanted more.
The ideal method would measure the length of wire wound. Since the physics are simply that there is a resistance per length of wire, the truth is that one should hit a target for total length. Of course, this is just the physics of DC resistance. When it comes to AC impedance, there are probably many factors besides length of wire that affect the result, such as number of windings and winding distribution, and probably some effect of interacting with the magnetic field.
As a footnote, the Stew-Mac advice on winding pickups is, "Pickup rewinding requires patience and study." This is especially true if you need to make your own flatwork and spools. I enjoyed the challenge, but it was a little too much like precision Swiss watch making for me to do it again. |
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Phill Martin
From:
Whitewater Kansas, USA
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Posted 23 Sep 2008 12:32 am NIbro
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"Try a set of Rick A. NIbro magnets in there."
I second that I have Supro pickup that was realy lacking in sound, got a hold of Rick and got a set of Magnets and they mad a world of difference.
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