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Quiet The Hummer

Shielding and re-wiring a passive P-bass
by Mark Shannon
These instructions were derived from the instructions for shielding and star-grounding a Strat guitar on the Guitarnuts.com website. This is the basic method. In addition, you can install high-value capacitors on the ground connections to the bridge and shell, in order to ameliorate some problems you might come across when playing at gigs with questionable house wiring. The GuitarNuts site will have instructions for doing this.

I'm going to assume that you are either using new components, or that you are going to undo all the solders in your current electronics (in either case, there's nothing soldered on to either pot when you start. If you have pot shell-to-shell solder connections in your current wiring, I'd suggest you start over. Pot-to-pot connections can be very hard to undo correctly, especially without a 40-watt iron or better, or if one of the terminals of a pot is directly soldered to its shell.

You will need:
  1. Copper tape (available with the Carvin shielding kit)
  2. Electrical tape or other insulating tape (for insulating the ring-terminal used in the star-grounding)
  3. One linear taper pot, one audio taper pot (either from your current set of components or
  4. Extra wire (in case the wire from your old wiring won't do; you can save a little money if you use all your old parts)
  5. A ring terminal or piece of solderable metal large enough to solder four or five leads to
  6. 30-watt or better soldering iron or gun (I used a 40-watt pencil-type, and it worked just fine)
  7. A little patience
With all the components removed from the control cavity of your bass (you shouldn't have to remove your pickups, unless they are removed with the pickguard, i.e. Fender P-basses):

1. Start by shielding the control cavity. With scissors or utility knife, cut pieces of tape to fit in the control cavity. Make sure all the pieces overlap each other by about a half-inch, and that you're not rounding corners. Space will probably be tight in that control cavity, so make sure you're pressing the tape down into the corners of the cavity. Also make sure to get the back of the plate or pickguard you had to remove to get to the control cavity, and make sure the control cavity's part of the shield will make good metal-to-metal contact with the tape on the plate or pickguard. Cut out holes for the tunnels leading to the pickups and bridge so those wires can come through, and also punch out the holes for the pots and jack. You can use the pots themselves.

2. Now that you've covered the control cavity and access plate with the tape, we're going to make sure that the seperate pieces form a good conductive shield. With the soldering iron and some lead-free solder (either rosin core or coreless, NOT acid-core), tack solder the overlapping pieces of the shield together. The best way would be to have little dots of solder between the two parts. Mine ended up looking like I tinned the borders between pieces with solder. As long as you have a good conductive connection, either is fine. Then, solder a length of wire to the copper shell somewhere where it will not interfere with other components. This will be your ground lead from the shield. Solder it to your ring terminal.

3. Now to the components. It would probably be best to solder the connections outside of the control cavity before you install the components. Space can be tight inside that control cavity, and you're working with a 900 degree tool designed to melt metal. Also note that, while I have not bothered to specify the length of the wires used (since it would be impossible to get it exactly right for every case with justone set of lengths), you need to be aware of how long or short you are making your leads. Ideally, they should be long enough to allow all the components to be put in place with just a little slack left over. Too long and you've got an unneeded pile of spaghetti in your control cavity that could possibly produce short-circuits, too short and you won't be able to install the components correctly. From this point forward, when I talk about terminals of a pot, I will refer to them as follows. With the shell of the pot, not the knob, facing you, with the terminals pointing up, terminal A is the leftmost terminal, terminal B is the center terminal, and terminal C is the rightmost terminal.

From your pickups there should be two wires coming into the control cavity. In my bass they were long enough to allow me to work on the leads while comfortably outside the control cavity. You may have less luck. The hot lead is usually the one that is not black or blue. Mine was white. If you put the bass together and it won't make a sound, or it sounds bad, you've probably gotten these wires crossed. Solder the end of the hot lead to terminal C of the volume pot. To this same terminal, also solder the lead that will go to the tone pot. If your capacitor will work in this case (make sure all bare wires are covered with electrical tape or stripped insulation from a piece of wire), use it; otherwise solder a length of wire to terminal C as well as the pickups' hot lead.

Now, terminal B of the volume pot will have the lead to the tip of the output jack. This is where your desired output signal leads from. Solder a length of wire to terminal B of the volume pot. We'll connect it to the output jack later. For now, label it, or just don't forget about it.

Terminal A of the volume pot will be the ground lead from the volume pot. Turn the wiper closer to this terminal, and signal will be thrown away rather than simply increasing the resistance of the signal. You don't have to have this lead; as the resistance increases, more signal will go through the tone pot, but without this lead, the tone will start to darken and turn muddy as the volume is turned down. So, solder a piece of wire to terminal A, connecting the other end to your ring terminal.

4. Now the tone pot. This works because a capacitor's resistance to a current flowing through it is inversely proportional to the frequency of the signal. For bass frequencies, the capacitor presents a high resistance to the tone circuit, so they'll likely choose the other path, leading through the volume pot and to the output. High frequencies, however, will find the capacitor relatively easy to pass, so the pot will be their major obstacle. A tone pot is wired as a variable resistor, with only two terminals having leads. With resistance at its minimum, the tone is very dark since most of the high frequencies are going to ground through the tone circuit. With resistance at its maximum, the resistances for both low and high frequencies is quite high, usually higher than that of the volume circuit, so most of the signal goes to the output instead of ground (whenever you have a path to ground, no matter how high the resistance, some of the signal will take it. A bright tone doesn't necessarily send all of the signal generated by the pickups to the output).

You need two leads for the tone pot. One is coming from terminal C of the volume pot. Solder the other end to terminal A of the tone pot. You'll also need another lead from terminal B to the ring terminal. If you didn't use the capacitor as the lead from volume to tone pots, solder one end of the capacitor to terminal B, and the other end to the ring terminal or to a lead to the ring terminal, making sure that you insulate any bare wire that could possibly bend to touch other bare metal components. Now, solder the lead coming in from the bridge to this terminal. This is the bridge ground lead, and grounds the strings themselves. Also, solder the other wire coming from the pickups to this terminal. This is the pickup's ground lead, and it completes the circuit to the pickups. Now, solder one last piece of wire to the ring terminal. This will be the lead to the ground side of the output jack. You should now have six leads soldered to the ring terminal, from the pickups, bridge, volume pot, tone pot, shielding, and one lead that will go to the output jack. Cover the ring terminal with two pieces of electrical tape in a ring sandwich, or, if the ring terminal with its connected leads looks less like a star and more like a tree, you can simply wrap around the terminal with a couple of layers of tape. Make sure you don't break any of the wires while doing this, and make sure you don't have any bare metal showing outside the tape.

5. Lastly is the output jack itself. There are several terminals coming from this thing. At least two should be the hot leads, and at least two more should be ground terminals. Insert a plug into the jack and start tracing contacts. One or two spring-loaded prongs should contact the tip of the plug. These are your hot contacts, and should lead to two seperate terminals. Solder a small length of wire between the two corresponding terminals (NOT the contacts themselves). There should also be at least one spring-loaded contact that makes contact with the shaft of the plug. This is one ground contact. The other is the jack's faceplate, the part of the jack that's visible from the outside. Both of these should also have corresponding terminals. Solder these together with a length of wire as well. Now, solder the lead coming from terminal B of the volume pot to one of the hot terminals, and the lead from the grounding ring terminal to one of the jack's ground terminals.

6. Re-install all of the components. Now the pots and the jack should make contact with the shield, grounding them. Reattach the access plate and plug in. Your bass should work, and more importantly you should hear very little hum until you turn the amp gain way up. You'll have more mids and treble, since you can now get more gain at the amp without hum becoming objectionable.

If your bass doesn't work, you have probably crossed some wires. Go back over the wiring and make sure there are no short circuits, and that the signal through both the volume circuit and the tone circuit are correct. You may have reversed some of the terminals, and in this case the controls will work opposite normal, for instance turning the volume pot clockwise will decrease volume instead of increasing it. For an audio pot, this may make a large difference, but for a linear pot, it's just backwards. In either case, switching the leads from the two outer terminals will solve the problem.

If hum is actually louder, make sure you have the connections to the jack laid out correctly. You may have bridged the wrong terminals when joining hot leads and ground leads, or you may have connected hot to ground and vice versa. The last thing to consider is that there is actually a built-in ground loop in this wiring. I put it in because not all jacks will make contact with the shield well enough to ground the shield that way, so I had you solder that lead from shell to ring terminal. There are two fixes: putting a piece of insulating tape between the jack's base and the shield, so there is no more metal-to-metal contact between the two, or simply cutting the shield grounding lead at the solder joint between lead and shield, and taping off the bare metal so the end won't make intermittent contact with the shield. If this solves the problem, just leave it this way.

This is Mark Shannon's first article for ActiveBass.com