Building Scroll Basses- Pickups 1

The pickup system of the Series IV Scroll Bass is my own design, and is quite unusual. I make almost all of the components mself.

Here's the pickup system installed in the body. The cluster at the front is the M-pickup, which is a group of four oval coils, one for each string. At the back, under the bridge, is the P-pickup assembly. The coil assembly for the P-pickup is mounted down in the pocket under the spring frame assembly.

Here are the two pickup coil assemblies. The P-pickup coil assembly, on the left, is made up of two large coils mounted to a base plate, with an aluminum bracket on top. The M-pickup assembly, which is face down in the picture, is made up of the four coils fastened to a pair of machined aluminum bars.

The core of the pickups are the bobbins, which are the forms that the coil wire is wound around. I make up my own bobbins by casting them from polyurethane casting resin, in silicone molds. I've designed my bobbins with integral terminal lugs, which make the solder joints to the fine coil wire simpler and more secure.

The first step in making up custom plastic parts like these is to make up a master, from which the molds will be made. I usually make my masters from aluminum, machining the main shapes in the milling machine and lathe, and hand shaping the corners. The masters have to be cut accurately and polished to a nice finish. Then the master is assembled into a box of waxed wooden blocks, and silicone mold compound is poured over it. When cured, the silicone mold is gently pulled off of the master. I usually make up multiple copies of the molds, so that I can cast batches of parts at one time. The masters are stored away, so I can later make make up new molds as needed when they get worn or damaged.

Once the molds are made, casting a batch of parts is simple and fast. The polyurethene casting resin is mixed and poured into the molds, and it cures in about 15 minutes. I add some black powdered ainiline dye to the resin to make the parts a dark smoke grey color.

Here's the master for the M-pickup bobbin, and the wood blocks that will surround it while pouring the mold. It's actually one half of the bobbin, including one flange. It's machined from a solid block of aluminum, leaving a lower base surface. The locations where the holes will be drilled are marked with centering spots, so they will be in the cast part.

Pouring a mold: The M-pickup bobbin master is surrounded by a form of waxed wood blocks, temporarily held together with clamps. The silicone rubber molding compound is mixed and poured in, filling the box.

Casting a batch of M-pickup halves. I have eight molds lined up so that I can cast one whole instrument set at a time. The polyurethane resin is quickly mixed and poured right into the molds. Eight raw cast parts are in the foreground while another set is curing in the molds. I don't use any mold-release spray when casting polyurethane parts. It isn't necessary and it roughens the surface finish of the parts. However, mold-release spray is important when casting epoxy in silicone molds (later).

This is the master for the P-pickup bobbin, also cast in two halves. Since this bobbin is round, I turned the master from round aluminum stock in the lathe.

Here's what the molds look like, after curing and pulling them off of the masters.

Here's a set of raw P-pickup bobbin halves and the base plate, which I cast by the same process.

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