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Acoustics

MAGNETIC PICKUPS



Winder

I use a drillpress for a winding machine (I couldn’t find a cheap sewing machine). I clamp a piece of plywood to the table and clamp a guide (plastic tubing glued in a drilled hole in a block of scrap wood) to the plywood. A strip of velcro mounted on angle bar at the edge of the sheet provides the tension and another strip of the soft (loop) side prevents snagging. I use a spring clamp or a C-clamp when I need more tension. I use 40 and 42 gauge wire, the 42 needs a lot of clamping. I put the wirespool on a milkcrate about two feet below the drillpress table. At highspeeds the wire flops around a lot and and it’s important to avoid places it can snag and break. I adjust the table to the low end of the bobbin and use the action of the press to distribute the wire vertically, moving the bobbin up and down (one could set the depth stop to the top, but I haven’t needed to), or I set the table to the high end of the bobbin and guide the wire with my finger (this is actually easier). I count winds with a stopwatch (the rpms make this very easy –at 1390, 6000 winds takes 4’19"), stopping to apply lacquer every 60 seconds or so (on smaller pickups I wait till the end). The whole set up took me less than ten minutes (I already had a drill press). Clamping, instead of permanently attaching, the wireguide to the table allows adjustment for different sized bobbins. My largest bobbin so far has been 8" which wound in 3 minutes at 1390 rpm.





Because I make a variety of sizes the drill press is more useful than a normal pickup winder would be and is handy to have around when I’m not winding pickups, saving ever valuable shop space. It is harder to pot the lower part of the coil because of gravity (the lacquer doesn’t soak up) and I’ve gotten some ugly looking winds, but with enough lacquer (I turn them sideways when they’re wound and glop it on) and very tight electric tape they work quite well. If there is any microphonic distortion I pad the pickup mount with neoprene washers or run the mounting bolts through rubber grommets.

I ended up buying a second drill press (home depot cheapo) for winding as the one I generally use for steel has a lot of metal fragments (and cutting oil) around it that end up getting onto the magnets and could probably short out the coils. I now use this one for all my non-metal drilling which also keeps sawdust and other flammable material out of the welding area. A lathe (something I don’t have) would probably work even better, as it would eliminate the gravity problem and hold the bobbin from both sides. A bench grinder, with a threaded chuck would also work, but would only turn at 1750 rpm and that might break the wire.

I put off attempting this for a long time because I thought it was harder than it turned out to be. Essentially any coil of decent resistance around any piece of metal with a magnet attached to it will function.

I originally used an 80 watt (Weller) soldering iron with a huge chisel tip (after years of frustration using 15-25 watt irons as most books advise) for more than a year. I find it easier to be quick with a big, hot iron than to wait for a smaller, cooler one as most books recommend.  A friend donated a temperature controlled 40 watt (Hakko clone) iron with a pencil tip and I've been using that instead since I got it.  I strongly recommend having temperature control (about 700 degrees) if you're doing a lot of this. 

BOBBINS

I couldn't find forbon and was having a frustrating time working with plexiglass, so I decided to give steel a try for bobbin material. I'm fairly sure I'd read somewhere that it wouldn't work. A drillpress, with its powerful (1/2hp) motor may not be suitable for fragile bobbins. Superglued plexiglass tended to fly apart in mine.

However an all steel bobbin (I generally use 1/8 in) works fine and rail pickups are easier to make to the size you want them. 

I make bobbins in three basic styles.

1. Spot pickups (designed by Jason Lollar from the Bart Hopkin book -a must have for anyone interested in this).

2. H bobbins which have a rail and top and bottom plates. For example, I make a 71/2 x 3/4 x 1/8 rail and weld it to a 8 1/2 x 3/4 x 1/8 top plate then to 8 1/2 x 1 1/4 1/8 bottom plate, I have also used 1/2 x 1/2 for the rail, welding it through drilled holes to the top and bottom plates.




3. Rails
Exposed coils which consists of a blade and a bottom plate. A single 1 1/4 rail, ground out to receive a 3/4 coil, welded to a bottom plate is easier to make, but leaves the coil protected only by tape. These take up more space than the H bobbins do. Any time they will fit I will use these.




I weld a rod to the top or bottom plate (centered) and insert that in the chuck. I grind the welds smooth, lacquer and tape it, and wind it. Recently, instead of taping the bobbins before winding (I was having shorting problems when the laquer dissolved the adhesive on the electric tape), I have used plastidip instead. I have potted one pickup with it and had no problems.  For struck strings I've taped, plastidipped, or glued neoprene to the top of the bobbin to prevent string-on-pickup noise.

When it’s wound, lacquered, soldered to 30 gauge leads, and taped, I grind the rod off (testing continuity first to make sure the coil hasn’t shorted) and put the magnets on the bottom plate. When I’m sure of the alignment I superglue them in place (I dry and spread the superglue with a hairdryer which has improved my low success rate with it).

All in all this isn’t very hard. The fine wire requires patience (and for me a pair of old lady reading glasses and very bright light), but it’s tougher than you’d think and can be wound at high speeds (at 2050 rpm it wouldn’t work on an 8in bobbin) without breaking as long as it doesn’t snag and the speed is even (I pity those who wind on turntables –6000 winds at 45rpm takes over two hours). I wind a 5 inch pickup in about ten minutes including the stop-and-pot time. To save time during the winding-potting I use a hairdryer to speed the lacquer drying process (this also drives the laquer into the interstices in the coil). This has made the centrifugal force on the wet lacquer less messy and kept the winds tighter. (Nail polish is perfect for the spot pickups, I don’t stop the press and brush the polish on every 15-20 seconds.) I’ve been using 1/4 x 1/4 x 3in ceramic magnets, four per pickup on large ones, though they sound okay with two.








I mount the pickups and magnets on a pieces of 1/8" flatstock that has a hole for the jack and bolt them together. This assembly bolts to the instrument. I use grommets on these boltholes to isolate the pickup electrically and vibrationally.

When it’s impractical to bolt down the pickups I let the magnets (glued to the bobbin) themselves keep them in place. On instruments with strong vibrations (which rattle the magnets and transmit the sound of this) I put a layer of gasket cork or 1/8" neoprene between the magnets and the mounting.

Since I’m not trying to duplicate the sound of a PAF or a 60’s tele and I don’t have to fit the pickups into pre-routed spaces (in traditional terms my pickups are enormous), the whole process is far less fussy than it is for replacement pickup makers (who have to deal with guitar players who are never satisfied with their sound). As long as the coil doesn’t short out, the pick-up works. I seek a passive (single coil) sound anyway as the distortions I’m interested in are implicit in the instruments themselves.

Bigger pickups seem to work better than conventional sized ones. One of the hardest parts of winding is getting the wraps evenly into 1/2 in or smaller spaces (I’ve had a far higher success rate with 3/4" rails) . Tall bobbins actually work better as more of the coil is closer to the rail (this is true of long pickups as well). Any size and shape seems to work. I usually do 6000 winds on smaller pickups and 5000 on longer ones. I don't generally test for resistance, I adjust their hotness by proximity to the sound source –with softer bass strings or long flexible tines the pickup needs to be hotter (more winds) as it needs more distance. If the pickups don't sound good, I make hotter (more winds, higher resistance) ones and save the others for other applications. The important thing is that they be uniform if they are going to be used together.

When I’m hooking up multiples (my largest array is three 8 1/2"s hooked up to make a 25 1/2" field for a xylophone), I test different wire and magnet alignments until I get a sound I want.

It may be from the excellent grounding of steel instruments that I haven’t had many problems with hum. The instruments themselves provide shielding. When I get noise I use sticky back copper foil (available from StewMac). Don’t give up on pickups because they hum during a bench test. A good ground can often fix this. Test with jumperwires (you can never have too many of these) until they sound right.

I got wire from MWS (a 5 pound spool makes a lot of pickups!) and magnets (3 sizes: 3/16 x 1 alnico rods; 1/4 x 1/4 x 1 1/2 ceramic bars; 1/4 x 1/4 x 3 ceramic bars) from Master Magnetics.

As a welder, I don’t do a lot of very small delicate work and it’s never been that much fun for me (I have more of a blacksmith’s disposition –I think anybody could do this as well as / better than I do). But the time it takes me to make a usable pickup is far shorter and less frustrating than getting piezos to sound good in certain situations (particularly on large steel soundboards or bowed instruments) and magnetics don’t make anywhere near as much noise when someone kicks a stand.

If you’re thinking about trying this, don’t get frightened by the pros (they are working under far more restrictive circumstances), you can make something that works in less than half an hour. The simplicity (quickness) of this process allows a lot of experimentation as failures don’t waste that much time. If a pickup doesn’t sound good in the intended application it will most likely work in another one (i.e., a low resistance coil that sounds dull on a plucked or bowed instrument will work on a struck instrument, with a high resistance coil the opposite may be true).

If you have any questions or suggestions, please contact me through anarchestra@yahoo.com.



LINKS

http://www.ampge.com/
associated manufacturers of pickups and guitar electronics





 

 

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