First page of the potentiometer archive.

Pot confusion – the electrical type

Posted by is9582 on March 7, 2015 with 2 Commentsas , , , , , , , , , , , ,

I tweeted earlier that I’d write an article today, relating to this subject. This is for anyone that might need to replace a potentiometer in a circuit, or is building an electric guitar. I thought about writing this, as I finally finished wiring my bass this morning, and at first I couldn’t recall which of the connectors on the pot went to which wire. After I thought about it for a moment, and had my coffee, it came back. But, what if someone else doesn’t have any prior electronics training or experience?

Most pots have three lugs, to which wires or other components can be attached, and then controlled by rotating the shaft of the pot.

The same pot, showing the three lugs, with no probes. The lead to the left, in the photo, I bent from it's original position, like the other two.

The same pot, showing the three lugs, with no probes. The lead to the left, in the photo, I bent from it’s original position, like the other two.

Many times a pot will be used as a volume or tone control, especially when talking about electric guitars and basses. I’ll provide a very basic explanation as to what is going on inside the pot, which allows this to “control” the volume. When the shaft of the pot is rotated, a wiper inside is moved in a section of a circle. The material the wiper rubs against inside is usually a path of carbon, which behaves much like a carbon resistor, and when made a specific amount of carbon is placed in the track. This allows them to create a specific resistance, for certain uses.

The wire from the guitar/bass pickup attaches to the center lug, and one of the end lugs is grounded, while the other end lug connects to the output jack. You can use a multimeter to determine which end lug you should connect to the output, depending on how you use the pot. I’ll walk you through this test.

First, turn the shaft of your pot in the direction you usually turn it, in order to raise the volume, and turn it until it stops. For most audio equipment, and instruments, this would be turning the shaft clockwise. This is what I did in my example.

Now, with your multimeter turned to it’s resistance mode, place one of the probes’ metal tips into (or against) the lug on the right end, and the other probes’ tip into the center lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and right lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and right lug.

When you look at your meter’s screen, you will see how much resistance is between the two tested lugs. The results from my first test are shown below.

With the multimeter set to Ohms (resistance), it reads .4 ohms, which is extremely low.

With the multimeter set to Ohms (resistance), it reads .4 ohms, which is extremely low.

Next, remove the probe from the right end, leaving the center probe in place, and place the moved probes’ tip into the left lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and left lug.

Shaft of pot rotated clockwise until it stopped, then holding multimeter leads on center and left lug.

The results from my second test are shown below.

With the multimeter still set to Ohms (resistance), it reads 416.7K ohms, and this is a 500K ohm pot.

With the multimeter still set to Ohms (resistance), it reads 416.7K ohms, and this is a 500K ohm pot.

If you think about how the wiper would move, inside the pot’s case, you can visualize where it is when you reach the end of the shaft’s travel. Whichever outside lug the wiper is closest to, when the shaft stops turning, will have a very low resistance value. This is because there is very little resistance material between the wiper and that outside lug. Hopefully this makes sense. This is exactly why the results in my above tests, turned out like they did.

While I’m talking about some of the things relating to the bass’s electronics, and basic wiring, I’ll also share a few tools I use that are helpful. Don’t worry, I won’t go into great depth at this point, but may in the future if anyone is interested.

One of the really useful tools I own is a wire stripper, that is somewhat automatic, in the sense that it can handle a range of wire thickness, without the user needing to do anything extra to have it do it’s job. On some strippers, they have small half-circles in the blade section, that only fit a very small range of wires. You have to choose the half-circle that is the right size for whatever wire you need to strip, and hopefully there is one that will work. This may show just how nice the stripper I have can make this work. I might have three or four different wire sizes in a project, and not worrying about anything other than just squeezing the handles, for it to do its job, is great.

Automatic wire stripper, used to remove insulation a specific distance from it's end, prior to soldering.

Automatic wire stripper, used to remove insulation a specific distance from it’s end, prior to soldering.

This stripper also has a “stop” against which, you hold the end of the wire, so the amount of insulation removed is the same each time. This stop can be moved to a range of distances, that the manufacturer thinks are most useful, when working on electronics. I may have needed a slightly longer uninsulated section once or twice, but it was easy enough to make this work, too. Below you can see the blue wire has the insulation from both ends already removed.

Blue wire in background has the insulation from both ends already removed with the stripper.

Blue wire in background has the insulation from both ends already removed with the stripper.

Another useful tool is a third hand. Actually, this is what I call it, but I honestly can’t recall what it was called, when I bought it. Basically its a hefty stand that has two small clamps, with which you can hold a wire or components, when soldering or performing an operation that is difficult with just two hands. The photo below shows one of the clamps grasping a metal lead on the output jack for my bass. The red arrow points to the “hot” lead, or in other terms, the lead that will have the signal carried on it.

Output jack, held by the stereo lead, so I can solder a wire to the

Output jack, held by the stereo lead, so I can solder a wire to the “hot” lead.

 

Perhaps this will whet some appetites, and hopefully also answer a question or two that may have led some to have others work on their gear, when understanding the basic layout might allow you to do the work yourself.

Thank you for checking out the article and as always, let me know if you have any questions or comments. I’ll be glad to answer or help, if I can.

 

Lee Laird

Bass electronics – Lace connections

Posted by is9582 on March 1, 2015 with 6 Commentsas , , , , , , ,

I finally found a little time to do some more work on my bass, and wanted to share a few things. Where I last left the build, was I’d made up a gizmo to hold the Lace Sensor pickup in place (under a little tension), so I could decide my favorite pickup location. This was cool, and the Sensor comes with wires coming from the pickup, terminating in a rubber shrink-wrap-coated female jack. So, once you plug your 1/4″ guitar cable into the Sensor’s jack, and the other end into your amp (or other similar source) the pickup is live and will detect and transmit sound. This was incredibly helpful, with the pickup placement, as I didn’t need to do anything that is normally required in the electronics end of guitars/basses. Specifically, I didn’t need to drill a path for the cabling, or create a control cavity, install a volume control, or even drill and install an output jack. I had actually already completed a few of these mentioned tasks, as I’d drilled my wire pathway and had created the control cavity. This is one of the few pickups, that I’m aware of, that comes out of the box with this type of setup. To mount the pickup, I used some #6 brass screws that were 1″ in length. I carefully marked two diagonal holes, drilled to the correct depth, and inserted the first two screws. Now that the pickup was secure, I marked for the two remaining holes, and repeated the process. I find it’s much easier to handle the four holes, and their accuracy, in this manner. It’s too easy to have the part shift ever so slightly, during the marking process, when marking all four holes at the same time.

Bass showing Lace Sensor solidly attached with brass screws, and pickup's cable/jack.

Bass showing Lace Sensor solidly attached with brass screws, and pickup’s cable/jack.

Now that my pickup was secured, it was time to move forward with the electronics end of things. I first need to remove the sheathing from the shrink-wrapped output jack, and de-solder the wires, to feed the wires down into the control cavity. I used a small razor knife to carefully the shrink-wrap material, and this jack had some of the toughest wrap that I’ve seen. So, working carefully, I was able to nibble away at the sheath (with the knife) until I was able to completely remove the wrap.

Closeup of Sensor attached, and the jack's sheath cut open.

Closeup of Sensor attached, and the jack’s sheath cut open.

After the wrap was off, I better understood Lace’s use of this material (thought continued in a few lines). When I first saw the uncovered jack, it was somewhat reminiscent to the male portion of the guitar cords. It had a screw-on cover, that completely protected the wires, and the solder joints inside. I examined the jack a little closer and it turned out that the flared tip (where the male 1/4″ guitar cord is inserted) was also screwed onto the main body. With flared tip removed, it left a threaded shaft, with connections for left and right (which you can use for stereo, or for mono) as well as a ground.

Closeup of Sensor's jack, showing the wires still attached, and the long threaded shaft. Red arrow points to the unscrewed  flared tip,

Closeup of Sensor’s jack, showing the wires still attached, and the long threaded shaft. Red arrow points to the unscrewed flared tip,

With this design, I knew I could handle the jack’s installation a bit out of the norm, by feeding it through a 1/2″ hole in the bass. I wouldn’t need to use the normal larger sized female jack, and the mounting plate that attaches that jack to the bass. This is a cool setup! I unsoldered the ground connection, as well as the main lead, and then wrapped the tips of the wires together tightly, with some tape. This will make feeding the wires through the body, a bit easier, since there is less chance their leads will get caught in the passage. Feeding the wire(s) was a simple process, although I could have gone with a drill bit a size or two larger, when I drilled the passage. The output wires on most pickup are fairly small, which is what I originally anticipated, but the Sensor’s thicker wires were so large they almost wouldn’t fit through. If possible, it is better to have all of your parts on-hand, prior to starting the instrument’s build, to limit any potential little hiccups (which this almost became).

I bought a couple of extended shaft Gibson 500K potentiometers, to use as the volume control for the bass. I believe this style pot, with it’s long threaded shaft, was originally made to use in Les Paul guitars, so it would work on their carved-top versions. When I created my control cavity on the bass, I left around 1/4″ of wood between the top and the cavity, which I just estimated while I set the stop on the drill press . I didn’t want to get too thin and have the tip of my bit break through, or even telegraph it’s presence. When installing the pot, I first drilled a small 1/8″ hole from the cavity side, so I’d know with certainty the pot would fit nicely. This helped know exactly where the full sized hole needed to be, but I’d have risked some wood blow-out, if I’d drilled the full-sized hole from the inside of the cavity. To prevent this, I flipped the bass over, so I was drilling in from the top side, and drilled my 3/8″ hole centered on the earlier hole. The 3/8″ hole was just large enough to allow what was almost a press-fit, for the pot’s shank. With the longer threaded shaft on this style pot, I can easily control how much of the shaft is projected above the top of the bass. To do this, I threaded a nut down onto the shaft, far enough so the adjustable portion of the pot is the correct height, and then with a washer and another nut on top, tighten it down so it is firm. This has a nut flanking the 1/4″ thick top, from top and bottom. If you have any trouble keeping this concept from loosening up, just bring a third nut into the mix. Place two nuts on the underneath side, tightened (jammed) against one another, and then handle the top nut in the same manner as i described above. This should stay put no matter what happens.

I was looking at some of the different plastic knobs I have on hand, to use on the volume pot, but none really looked appealing. Then it hit me. Why limit myself with plastic knobs? I chucked a blank of some nice colored Pecan, into my wood lathe, and I turned it to a pleasing shape. When I was satisfied, I sanded and added some oil finish, while it was still turning on the lathe.

Turned knob, still attached to blank in lathe. Red arrows point to the knob and to the parting point.

Turned knob, still attached to blank in lathe. Red arrows point to the knob and to the parting point.

Before cutting the knob loose, I put the whole blank into my vise, and drilled the hole to fit over the pot’s tip. I marked my drill bit with some of the blue painter’s tape, so I wouldn’t accidentally drill too deep and waste my efforts. After drilling, I cut the knob loose with a fine-toothed saw, and pared away some extra wood, which also added a little texture on the very top of the knob. Now it is just waiting for installation.

Knob finished and sitting next to a pot (not the one used), to show scale.

Knob finished and sitting next to a pot (not the one used), to show scale.

I hope to have a little time in the next few days, when I can drill and install the output jack, and complete the wiring. Then it’ll just be a few more little details to finish up all aspects of the bass. Until then….

Thanks as always for checking out my article. Please let me know if you have any questions or comments.

 

Lee Laird