At it’s very simplest – a guitar pickup circuit conducts small electrical currents generated by the pickups to an output jack, where they are, in turn, passed on to an amplifier. As with all electrical circuits – any potential flows from the “hot” or positive, and always seeks the quickest way to ground, or negative. So in building the Jaguar wiring circuit, the first step is to build in the ground side, and link all the elements together – so there is one, continuously linked ground, which can then be accessed and shared between all the “grounded” components.
I’ve already lined each of the recess chambers with copper foil. This is done to screen all of the electrical components from outside interference, and is a pretty effective first line of defence on it’s own. While the pickups detect small changes in the electro-magnetic field, caused by the movement of the strings – the pickups and wiring are also susceptible to other, interfering, electro-magnetic fields in the environment. Most noticeable is the 60Hz mains hum, which is induced by all the domestic wiring hidden in the walls of our homes, workshops and studios. The copper foil produces a Farraday cage effect – screening cables and components placed within the cage, from outside interference. It’s a good way of helping to cut down the annoying hum that is, notoriously, produced by single-coil pickups in unscreened guitars. Since your amplifier will only exaggerate the effect by amplifying the signal – keeping the signal clean from the off, is important.
In fitting the copper foil screening – as well as providing a screened interior, I have also, at the same time, built a modern day version of the brass plates used in the original, 1962, Fender design. These plates were used primarily as shielding electrical conductors – long before self adhesive copper foil or conductive paint were in common use. The foil I have used does exactly the same job. In theory – all I need to do is to make sure the foil is contiguous, or is linked via soldered wires or other means.
However, as I’ve already said in an earlier post – I’m going belt and braces here. There’s always quite a bit of discussion when it comes down to just how any particular diversion from standard specification can have, even the slightest, unauthentic effect on the overall sound or character of a guitar. It’s debateable at best, but staying close to the original design interests me in a few aspects. There’s something about the Jaguar’s brass plates that speaks to me of the technology of the time – even down to the way Leo Fender used ultra lo-tech glazing brads to hold the plates in place. I’ve already put conductive copper foil in place to cut out any unwanted hum. But I’m going to double-up, and build another layer into the ground side circuit. This time using the original, brass plate method.
Now – getting hold of an original Fender set of grounding plates in the UK seems nigh on impossible. Nothing for it – I’ll have to make them up myself. My metal working skills and resources are pretty primitive – but I manage to get hold of some 0.5mm brass plate and, by tracing the outlines of the cavities onto paper, and then by tracing that outline onto the brass backing plastic – I have something resembling a set of plates, which I can hopefully cut out with a pair of heavy duty tin snips. When it comes to it – maybe 0.5mm is too thick. It’s hard to cut straight – especially the inside corners on the large “C” shaped plate. The results won’t win any plaudits for accuracy – but they’ll get the job done and, with a bit of hammering and filing away the sharp edges, we have a, just-about-acceptable, set of plates to work with.
The original plates were held in place by small, triangular, glazing brads. In my framing work, I’ve removed countless of these little beggars from old Victorian and Edwardian frames. I still, occasionally come across them in frames from the ’60’s and ’70’s. However, unfortunately, I’ve never thought to keep any back for guitar building. The modern brads are smaller and straighter, and made out of a cheap alloy – but I’m pretty sure they’ll do the job. The plates in the two switch recesses, and in the main control rout are pinned into place by tapping two brads, sideways, into the body of the guitar – using a small ball hammer and a punch. They are then secured, in place against each plate, with a little solder. (I also cover the solder joints with a small patch of copper foil – just in case anything ever comes adrift. I don’t ever want to find a small, metal brad rattling round in my circuitry).
All the metal hardware of the guitar is capable of picking up, or transmitting, interference to the circuit. Ever had that thing where you touch something metal on a humming guitar – bridge? tremolo? strings? – and when you touch it, the hum goes away? That’s because your hand has now grounded the signal. Installing effective grounding makes sure this is never a problem. Anything metal – tremolo, bridge, control plates etc., all need to link to a common ground somewhere. Part of the complication of the Jaguar layout is getting these various links installed in logical places, with minimum cable runs and flexible access – should you need to ever do any maintenance. There’s enough spaghetti in there already, with all the switching wires.
First to sort out is the tremolo. The body already has a pre-drilled hole running between the main control rout and the tremolo cavity. I solder one end of a black, grounding wire to the junction between the brass plate and copper foil in the main cavity – and then run the wire through the conduit, to where it emerges in the tremolo rout. The original method was, apparently, to use a staple to hold the wire in place – with the ends of the wire frayed out under the tremolo plate. I don’t want to risk damaging the paint finish with a staple – even hidden under the plate – so I fray the ends of the wire out, but hold it in place using a few strips of copper foil running over the edge of the cavity and around one of the screw holes for the tremolo. I build up a few more layers around the hole – so that there’s a small pimple of copper to ensure good contact with the plate when it’s screwed down.
A similar process links the plates at the bottom of each of the switching compartments, to the main “C” plate which runs throughout the pickup and wiring rout. The guitar body already has lateral holes drilled to install wiring – but I’m using old style, cloth bound Luxe wire, and this is quite thick in comparison with modern, plastic coated wire. I therefore decide to drill some additional through-holes to route the grounding wires, and keep them totally separate from the circuit wiring. Each of the brass plates in the out-lying switching compartments is joined via soldered cloth-covered wires, to the large, “C” shaped brass plate running through the pickup and main wiring channels. There is no need to use brads to secure the position of this main plate in the pickup rout, because it will be partially held in place by these solder joints and ground wires. It will be further secured in place when the pickups are finally fitted – so it’s necessary to mark and drill 5mm holes at each of the pickup screw locations, before the plate finally goes into place. The wire at the rythmn circuit rout is passed through to the main cavity, and is stripped with a long tail. Once soldered tight to the “C” shaped plate by the drilled opening, the extra tail is run up and over the edge of the cavity – where it can act as an additional ground for the copper foil on the back of the scratch plate.
Just before I actually fix the position of the “C” shaped pickup plate with the solder, I decide to remove the little lug at the main control end. I think that’s a great point to put a central, shared grounding point for the bridge – at the same location as the link between the main compartments. I should also be able to wire the pickup ground wires to this central point as well, and eliminate the need to run yet more wires into the main control recess.
I strip a piece of wire, and solder it to a screw lug. I also attach another piece of cloth screened wire to the same lug, and pass that wire through the hole into the main control cavity. I can now use the screw lug to hold the brass plate down, as well as to act as a hard-wired, conductive anchor for the connecting ground wire. I should be able to attach a second lug, this time with the pickup ground wires attached, and hold everything down with one small screw.
The stripped piece of wire is routed up the side of the cavity, and across the body of the guitar to one of the bridge thimbles, where it is pushed inside and held in place by the metal thimble itself. At this point, the wire will be hidden underneath the scratchplate, (and copper foil on the back of the plate will also be grounded via this wire). Since all the components of the bridge are metal on metal – the ground effect should be conducted through to the bridge pins, the saddles – even, presumably, the strings. The remaining cloth covered wire through at the main control cavity is secured there with solder, to the last brass plate. This now completes the links between all the various compartments, the bridge, the tremolo and all four brass plates.
All that is left, is to add a few fly-leads which will, eventually, be attached to the chromed control plates which sit over each of the main compartments. As it is – the copper foil pressed against these plates around the rim of each opening will probably be enough do the job – but in the original ’62 design, seperate fly-leads were fitted to directly link the brass plates to each of the chromed control plates. At the pickup switch rout, and at the rythmn circuit rout – cables can be attached, via lugs, to one of the screws which hold the switches onto each plate. At the main control cavity, I can fan the cable wires out, and hold a grounding wire in place underneath the control plate – in a similar way as I did with the tremolo. I make sure I cut the fly-leads long enough, so as to allow for the control plates to be moved around for later soldering and manipulation with components attached. I attach the leads where required, and then solder the wires, in place, onto the brass plates.
All that is left to do is to tidy up some of the solder joints, (I like to cover them with copper foil if they look untidy or in any way insecure. Although I’m learning – I’m not the best solderer by any means, and if anything should fail, a little adhesive copper foil will stop things coming adrift, and will also keep things conductive). I’ve learned to put a piece of non-conductive electrical tape over the conductive foil, (and/or brass plate, in this case), wherever the cable jack is inserted – just to make sure the jack switch can’t short if it happens to get pushed in too far.
Once all the links are done, a multimeter is used to check conductivity between all points. Tests show that everything I put in place thus far is connected and contiguous – with zero resistance detectable. Functionally – it should be as good as I can get. Normally, it’s recommended that each separate strip of copper foil is soldered to another – all to ensure conductivity, even if the conductive adhesive should fail over time. Hopefully, by installing the brass plates as per the 1962 original design – the ground will still be in place even if the copper fails. In such a case, the shielding may well degrade – but the pickup circuit should still be fully functional.