General Potentiometer questions

[matteee9999]

Hey all,

Just joined up and this is my first posting. Looks like a great forum with lots of knowledgeable and helpful folks.

I'm rewiring a guitar, not for the first time, with upgraded pots, caps, etc., and I've come up with some simple questions that have occurred to me for the first time on this project. This all may be pretty elementary, but I'd like to have a deeper understanding of this stuff instead of just following a diagram.

What do the outer lugs of a pot do? Or, since I know more or less that they complete the circuit with the middle lug (the wiper) to send signal along, what is the difference between the left lug and the right? The left, looking from the back, has always be grounded to the back in all the guitars I've had and all the schematics I've looked at. If you look inside a pot, and I have, both sides would seem to do more or less the same thing, although the left lug, the one that is usually grounded out and not used, has more bare brass showing than the other side. Any explanation of this would be much appreciated.

Next question, and this may be retarded, but what is accomplished by grounding the unused lug to the back of the pot?

Lastly, does it matter which lug receives the signal and which sends it on? Basically, is it important which is in and which is out? On the guitars I've been working on, mostly Gretsch, the pickup leads attach to the center lug, with the right side carrying the signal to the pickup selector switch. From the there, it is attached to the right side lug of the master volume and out from the center to the tone pot. So, basically, signal enters in middle on the individual volume pots and on the right side on the master volume, the reverse.

Below is a link to the Gretch schematic of their "tone pot" circuit to demonstrate what I'm referring to.

http://wpc.077d.edgecastcdn.net/0007...ot-Circuit.pdf

Thanks much for any help.

[Chuck H]

What do the outer lugs of a pot do? Or, since I know more or less that they complete the circuit with the middle lug (the wiper) to send signal along, what is the difference between the left lug and the right? The left, looking from the back, has always be grounded to the back in all the guitars I've had and all the schematics I've looked at. If you look inside a pot, and I have, both sides would seem to do more or less the same thing, although the left lug, the one that is usually grounded out and not used, has more bare brass showing than the other side. Any explanation of this would be much appreciated.

No explaination for the greater amount of brass other than to say the MFG has their reasons. But, the reason the two lugs are not interchangeable is that you wouldn't want to turn your volume knob backwards from the standard "clockwise=up/counter clockwise=down". Then there is the issue of pot taper. Pot MFG's know the standard (clockwise, etc...) and build "tapers" into the pots resistance value that may place the pots resistance non linear to the knob setting. As in, at 5 on the knob setting the resistance may only be at 15% of the pot value as opposed to 50%. "Linear" pots are indeed at 50% value at half rotation. But guitar pots are almost never linear. Some sort of resistance taper is used to make the guitar adjust in a more intuitive way. Now the part you asked about... If you swap duties on the outer lugs you'll also reverse the taper. That, and the general rules of which direction things turn down, is the reason pot lugs are not interchangeable.

Next question, and this may be retarded, but what is accomplished by grounding the unused lug to the back of the pot?

The back of the pot is not significant. The back of the pot is grounded to shield the pot innards. But, that does make a convenient ground point for the "circuit". That lug of the pot needs to be referenced to 0V for the circuit to operate. You could reference that pot lug to any 0V grounded point in the cavity OTHER than the back of the pot and it would still work.

Lastly, does it matter which lug receives the signal and which sends it on? Basically, is it important which is in and which is out?

See above. Answer number one. AND... Different guitars are wired differently. there are different isolation needs depending on what a designer MFG wants the pickup switching and volume controls to do. Some guitars have multiple pickups with independant volume controls and some have multiple pickups with shared volume control in shared pickup selections. These different circuits require the pots to be used in different ways (as in wiper vs. outside lug for input). A greater understanding of circuits would be needed for clarity. Not just a greater understanding of pots.

EDIT: Look up "voltage dividers" and maybe even use the word "potentiometer" and/or "circuit" in the search. Lot's to be learned.

[matteee9999]

Great information, thanks for the reply.

But, the reason the two lugs are not interchangeable is that you wouldn't want to turn your volume knob backwards from the standard "clockwise=up/counter clockwise=down". Then there is the issue of pot taper. Pot MFG's know the standard (clockwise, etc...) and build "tapers" into the pots resistance value that may place the pots resistance non linear to the knob setting. As in, at 5 on the knob setting the resistance may only be at 15% of the pot value as opposed to 50%. "Linear" pots are indeed at 50% value at half rotation. But guitar pots are almost never linear. Some sort of resistance taper is used to make the guitar adjust in a more intuitive way. Now the part you asked about... If you swap duties on the outer lugs you'll also reverse the taper. That, and the general rules of which direction things turn down, is the reason pot lugs are not interchangeable.

This was pretty much what I was thinking, though I hadn't thought about the taper issue. Makes sense.

The back of the pot is not significant. The back of the pot is grounded to shield the pot innards. But, that does make a convenient ground point for the "circuit". That lug of the pot needs to be referenced to 0V for the circuit to operate. You could reference that pot lug to any 0V grounded point in the cavity OTHER than the back of the pot and it would still work.

This was my assumption as well. I'm used to working with higher voltages (I'm a contractor and do some AC wiring), so I generally get the concepts, but as I look at it, there's no real "ground" per se between the pot and ground, or what functions as ground (the bridge), so I didn't see how it could ground. When you ground to the back of a pot, where is it going? I'm obviously missing something here. I think perhaps the instincts learned from AC wiring are failing me or getting me in trouble.

See above. Answer number one. AND... Different guitars are wired differently. there are different isolation needs depending on what a designer MFG wants the pickup switching and volume controls to do. Some guitars have multiple pickups with independant volume controls and some have multiple pickups with shared volume control in shared pickup selections. These different circuits require the pots to be used in different ways (as in wiper vs. outside lug for input). A greater understanding of circuits would be needed for clarity. Not just a greater understanding of pots.

EDIT: Look up "voltage dividers" and maybe even use the word "potentiometer" and/or "circuit" in the search. Lot's to be learned.

Yes, lot's to be learned. Can you explain why, in this specific circuit, the individual volume and master volume pots would be wired differently basically the opposite of each other?

Anyway, I've got some studying to do. I'll check out the info you mentioned. Thanks for getting me started. I've found that just fiddling with stuff and experimenting helps me understand. I've got a beater Tele knockoff I'll mess with and try stuff out on; Teles are so accessible and t's a real chore getting the guts out of a hollowbody. Thanks again.

[Jazz P Bass]

Quote"This was my assumption as well. I'm used to working with higher voltages (I'm a contractor and do some AC wiring), so I generally get the concepts, but as I look at it, there's no real "ground" per se between the pot and ground, or what functions as ground (the bridge), so I didn't see how it could ground. When you ground to the back of a pot, where is it going? I'm obviously missing something here. I think perhaps the instincts learned from AC wiring are failing me or getting me in trouble"
The term "Ground" is a misnomer as far as the pickup goes.
It may be better to call the wires L1 & L2.
The circuit has to be completed, as in any Vac circuit.
When you tie leads to the pot case, it is a common connection.

[Chuck H]

Right. It's a 0V source. And yes, you are the ground. That's why the bridge is ground, because your going to be touching the conductive strings. Of course the 0V point in the guitar circuit is also electrically connected to the chassis of your amplifier. But the circuit works no matter what because it's +/- and not DC.

[Chuck H]

Can you explain why, in this specific circuit, the individual volume and master volume pots would be wired differently basically the opposite of each other?

Yes.

The individual pickup volume controls send the pickup signal to the center lug because if they were connected to the outer lug (and so the center lug to the switch) you would lose the ability to control the pickups individually. If you look at the circuit you'll see why. When the switch combines the pickups, if the wiper of the pickup pots went to the switch either wiper could be turned to ground. Thus grounding both pickups, so individual volume control would be lost. As it is wired the two pickups output does share the other pickup volume controls "load" but that is less important to the circuit function than separation.

The master volume is wired with the wiper to the amp because this allows you to ground the amplifier input. This is quieter than grounding the pickup output and leaving the amplifier input floating at the pots full resistance. If the master were wired with the switch to the wiper and the outer lug to the jack, it would still reduce the guitars volume but the amp would present noise somewhat like there was a cord plugged in to the input but no guitar attached.

[Jazz P Bass]

[QUOTE=Chuck H;233414]Right. It's a 0V source. And yes, you are the ground. That's why the bridge is ground, because your going to be touching the conductive strings. QUOTE]
Just nitpicking, but "You" are not at ground potential.
If this was absolutely true, when you touch the positive input of a guitar cable that is inserted into an amp, it would Not make any noise.
And it does.
If you where "ground" it would be silent.
The same as connecting the tip to the ring.

[Chuck H]

No, it's not "absolutely true" but it is the goal of the circuit. Otherwise it wouldn't be set up like it is. And it DOES get quieter when you touch the strings. More like your a big capacitor maybe. Like a death cap or automotive condesner

[matteee9999]

That makes sense and now I get it. It's the same issue addressed in an article I recently read on the singlecoil.com website concerning how to get the tone of a '59 Les Paul sunburst. Among other more obvious things (wood, pickups, etc.) the original wiring for it was not as it is today and basically the opposite of what we've been discussing. The pickups are attached to the outside terminal and the wiper to the selector switch, with the effect just as you described. The author sees this as a necessary evil to get the desired tone. As I see it, with a Les Paul, with PAFs, this isn't such a big deal since I never play with both pickups on a humbucker e guitar, but with my guitar, with the single coil Dearmonds, I love the sound of both, so this would be a problem. Oh well. Perhaps I'll try it on another guitar.

If anyone's interested, here's what the article has to say about the wiring in a '59 'burst LP and a link to the whole article if anyone's interested. Also, singlecoil.com is a great resource for

http://www.singlecoil.com/docs/paula.pdf

"This is a very important point and what you can find in some guitars today is horrible and has almost nothing to do with a vintage Les Paul wiring. First of all it ́s important to know how to wire the components together. You can find everything here and most shown wirings are simply wrong. The most horrible thing that can be found is the variant with the volume pots wired backwards !!! This is meant to solve a problem that can ́t be solved in a passive system. In the original Les Paul circuit both pickups influence each other in the middle position of the pickup selector (both pickups together). When you roll back the volume on one pickup just a little bit, the other pickup is much louder than the other and if you roll back the volume down to zero, both pickups are silent. This is not very comfortable, but part of the real deal. With the volume pots wired backwards, this phenomenon will disappear but you pay a high price for this: all your treble and high end is killed when using the pots, your tone will loose any color and will sound dull and dead. You can try to compensate this with a socalled "bridging-cap" but all this fumbling is garbage to my ears. The only real solution for this problem is an active system but we don ́t have one here in this guitar. So if you can ́t live with this fact, you can stop reading here - your system will never sound like an old Les Paul, even with original 1959 PAF ́s installed. So please have a look at the drawings below, compare it with what you find in your Les Paul and if neccesary change it into the real thing. Don ́t forget the grounding wire going from pot to pot, this is something often ignored."