Kurt Cobain “Jag-Stang”. Shielding the body, and grounding the bridge

The polished Jagstang body can now be prepared for assembly. Although the guitar will eventually have a humbucking bridge pickup there’s still a traditional, single coil Mustang pickup fitted in the neck position. Since single coils can notoriously produce unwanted noise due to electromagnetic interference – I may as well shield all of the internal cavities on the Jagstang with self-adhesive, conductive copper foil, and then connect all of this to the ground side of the eventual circuit. I can also take the opportunity to install a grounding wire from the tremolo. This should, in turn, ground the drop-in bridge unit, via direct metal-to-metal contact.

JagStang body – Reaming out the thimble holes

Since I aim to bring all of the grounds together at a central, “star” grounding point – I could look to connect the bridge directly with a single wire although, practically speaking, it’s probably easier to do it in two halves – bringing the two togther with the copper foil shielding at a suitable location, somewhere in-between. Since the controls will be using smaller, “mini pots” – there’s plenty of available space in the main control cavity. I usually aim to locate the central point somewhere around there, and no reason not to do so again here. I locate a suitable point on the side of the main control cavity, and create a small hole with a sharp awl.

There’s already a small conduit drilled between one of the tremolo spring cavities and the large pickup rout. I can run a wire from one of the bridge “thimbles”, across to the tremolo spring cavity at its’ closest point, loop it around and then run it through the interconnecting conduit, to the pickup rout. Once the tremolo plate is screwed down – the wire will be hidden, and crucial metal-to-metal contact with all of the major metal components of the bridge will have been achieved.

The body has been pre-drilled by Warmoth, to receive 9.0mm thimbles. These appear to match Fender “import” dimensions. (Fender US thimbles seem to be slightly larger, at 3/8″, or 9.5mm). Instead of the TOM type bridge fitted on the prototype and later Japanese Fender Jagstangs – I want to use a drop-in, “Mustang-type” bridge. This will allow me to match the 7.5″ neck radius of my chosen neck. A HOSCO replica Mustang bridge unit, obtained from Guitars Electric, comes supplied with the necessary 9.0mm thimbles, and appears to be of decent quality for the money. Crucially – the bridge is available with my chosen 7.5″ pre-set radius.

The thimble holes have become slightly narrower, due to accumulating lacquer around the openings during the finishing process. I first work the edges of the openings slightly with a countersink tool – just enough to expose the wood again, at the edges of the openings. This will help stop any splits in the lacquer from progressing out onto the surface of the guitar as the thimbles are pushed home. An accurate 9.0mm reamer is then worked into each opening in turn. Once the holes have been cleaned out – the thimbles drop in snugly, and yet are still just loose enough so that they can be removed again, relatively easily.

JagStang body – Running a ground wire

A short length of black, cloth covered wire is prepared. The ends are stripped, and enough covered “slack” is left in the mid-section to allow for the wire to be coiled away from the tremolo spring within the spring cavity. One stripped end is inserted deep within the adjacent thimble hole, and then run across the body – just the short distance to the spring cavity. Here, the wire is coiled down the side of the rout, and fixed with a short piece of adhesive copper foil. This will stop it getting caught up in the spring mechanism. The other end of the wire is threaded through the conduit – towards the tremolo side of the main pickup rout. There – a good length of the wire is sandwiched between sheets of adhesive copper foil and secured, out of the way, to the side of the chamber.

JagStang body – Tremolo and bridge grounding – Continuity check

The thimbles are re-inserted. This time with the grounding wire held alongside one of them. This makes fitting that thimble a little harder – but a good shove with a suitable screwdriver down in the base of the thimble, does the job nicely. The screw holes for the tremolo plate itself, have already been located on the body – so fitting it again is straightforward enough. The thimble flanges sit very snugly within the openings on the tremolo plate and that alone creates a good direct contact. Additionally – as the plate is tightened down – the exposed wire between the thimble and the spring cavity is also brought into direct contact with the underside of the tremolo plate. Because of the pressure exerted on the wire at various locations – I’ve chosen a wire with a pre-tinned core. This is much less fragile than many, individual, finer strands and withstands compression better over time.

The bridge unit is dropped into place, and the post height adjusted so that the metal support posts sit right down on the bottoms of the metal thimbles. The other end of the ground wire is then secured in place between suitable strips of copper foil. Continuity is checked with a multimeter.

JagStang body – Running the ground wire on to a central “star” grounding point

A second ground wire is then prepared. This is, again, stripped at both ends. One end fits down into the small hole, previously located for the central “star” grounding point in the main control cavity. A small screwed lug holds this end of the wire securely in place as the screw is tapped home. This creates a good contact, although the effective area of contact can be extended along the side of the cavity by sandwiching the stripped ground wire between pieces of copper foil. The rest of the wire then runs along the side of the main control cavity, through the small pre-driled conduit hole, and on into the large pickup chamber. Here – it’s stripped back, and runs alongside the tail of the previous wire from the tremolo and bridge. It’s held securely in place with more strips of copper foil, and these provide a point of connection, directly linking the tremolo unit to the main “star” ground. The whole length of exposed wire is then covered and secured with copper foil, and then checked for continuity with the multimeter.

JagStang body – Fitting the strap buttons

While I have a screwdriver handy – I can install the strap buttons back into their pre-drilled holes. These are modern, chrome, Fender USA units, (Fender parts number 006-3267-049) – rather than the vintage style, nickel, “original” type. They fit just the same way however, and are each padded with white felt washers.

JagStang body – Shielding the body cavites with conductive copper foil

Then – back to shielding tasks. The body cavities are completely lined with overlapping strips of heavy-duty, self-adhesive copper foil. The adhesive is conductive, and the resultant lining helps create the necessary “Faraday” cage effect, which shields the guitar circuitry from outside, electromagnetic interference. There are only two chambers on the Jagstang – but the deeper side recesses on the large pickup rout make fitting some of the foil pieces, a little bit more tricky. A thin burnishing tool and the edge of a credit card both come in handy to help get the foil pressed firmly into place in some of the tighter spots. Once the lining has been completed, the whole surface is firmly burnished down with an agate burnishing tool. The copper surface is checked for electrical continuity and, since the ground wire is also in direct contact with this foil lining, all of the “ground” components can be seen to be connected back to the single, lugged “star” point in the main control cavity.

JagStang body – Shielding the body, and grounding the bridge

Finally – a few extra tabs of foil are run up and over, onto the face of the guitar – where they can directly contact, and yet still remain concealed by, the covering plates. Since the covering plates are held down securely by screws – it makes sense to locate these additional contact points directly around a few suitably located screw holes, where the screws can help maintain good contact.

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