I Don't Understand Guitar Wiring: Where's the Circuit?

[Steve Conner]

I just posted the first "real" looking schematic that came up on a Google Images search, and it happens to be for a Gibson Grabber G-3 bass, which is somewhat unfair. I wanted to post one for a Strat, but couldn't find anything but those stupid pictorial drawings.

The "wires dangling off into space" are supposed to symbolize metal shields on the pickups, or to specify that the metal cores of the pickups should be grounded. The intent is to reduce hum.

There are 3 pickups shown, but they are in a weird hum-cancelling arrangement similar to the RWRP on a Strat. The middle one is wired permanently in series with the other two. The pickup switch selects bridge+middle, neck+middle, or all three. The Gibson G3 Bass Guitar

Those "super switches" make my head hurt too.

[RobRed]

I appreciate that. The main issue was the schematic, not the particular instrument, but let me mark up the schematic with some questions and put it up as an attachment, because now that I have a little more of an idea of what it is, I am even more confused (fortunately in a little way, but... ). Maybe if you could respond to these particular questions it would help me get a handle on some things.

It is ironically funny that you mention the super switches making your head hurt, because while I have a hard time sorting out how the connections I see on diagrams of use of the super switch in wiring up a particular guitar do exactly what they are supposed to do, at least I can see a bunch of connections and I can trust someone in the know has mapped it all out and it works. On the other hand, in the "preferred" schematic representation here (as my mark-up will make blatantly apparent), there is nothing that gives me even the least clue as to how the switch is supposed to be wired or even work. It is so symmetric in its appearance as drawn that with only 2 connections demonstrated, and those being in series with pickups 1 & 3, I haven't got an inkling how you can look at that and know it is meant to allow you to choose between B+M, N+M, or all 3. If I saw a schematic diagram with a symbol for a super switch that limited in detail, it would be absolutely useless. Are there not places (as for instance with the use of the super switch) where things are complicated enough that a more diagramatic component can be added to a schematic (I will also attach an image of a schematic-like but plainly not purely schematic diagram of the super switch that I took off a Fender wiring diagram. Would one such as yourself see including a diagram like this in a schematic, to allow indication of more complex connections, as acceptable, useful, or even necessary, or would there be some simpler symbol that would just leave you happily to your own devices to sort out how you would make it work?--I'm not being a wise guy in asking this).

Thanks, Rob R

[Steve Conner]

I'm not sure how to answer all of those questions, so in best politician fashion I'll answer only the ones I feel like addressing.

Why does the shield on PU 2 show no ground symbol? It does. Follow the wire that the shield connects to, past point A, and you come to a ground symbol.

How does the selector switch work? This is a strange symbol, not one of the commonly used ones. I guessed that it was meant to represent a Gibson style pickup selector: a single-pole, double-throw switch where both sets of contacts are closed when the toggle is in the centre position. Indeed, the symbol shows the toggle in the centre position, and both sets of contacts closed, and the bass is a Gibson: all heavy hints.

Questions related to the order of the pickup connection: The order of the wires on the coil symbol representing the pickup isn't particularly clear. Normally it would be drawn such that, if the coils were vertical, the connections nearest the top of the page would have the same relative phases. Assuming this convention, the schematic shows the correct hookup for series hum cancelling. This is why the wires appear to cross in odd ways.

[GaryR125]

RobRed - I think trying to equate guitar wiring with a concept like house current, wall switches and table lamps is not a useful analogy.

The video shows a better analogy - the meter in the video works just like your amp. It reads some humanly-undetectable environmental condition, amplifies it to a level we can detect, and shows it to us. The meter in the video reads electrical conditions in the coil and adjusts the position of the needle. The change in the envniromental conditions is the movement of a magnet near the coiled wire. The power to swing the needle in response comes from a battery in the meter.

Similarly, an audio amp reads minute electrical conditions off its input jack and makes a speaker sound. Power in that circuit comes from the wall and is used to power the speaker.

Hold multimeter probe handles in your hand; you'll get a zero reading (resistance anyway). Touch the multimeter probes and you'll get a new reading. Turn on your amp and you won't hear much cause there's nothing happening. Touch the amp cable with your hands; you'll cause some noise to occur as your hands create a (short) circuit at the probe, and change the resistance seen across the probe contacts. The amp will make some sound even though there is no power in, or coming from your body.

You can repeat this experiment if you connect your multimeter or amp to a wire, or a resistor, instead of your hands. You'll get a steady reading or sound because there is no change in the conditions in the wire or resistor. If you connect your meter or amp to a guitar (a device that provides an easy way to manipulate the external conditions attached to the audio jack) you will be able to detect changes in certain physical conditions, like vibrating strings or touching a pickup with a screwdriver, when read off the input leads.

To sum up, the amp is a detector of conditions in the input jack. The guitar is just a fancy wire across the contacts of the input jack.
HTH, Gary

[Steve Conner]

No, the amp requires power from the guitar to work. Any "detection of conditions" in any system whatsoever requires an interchange of energy. (In order to read the multimeter, photons must bounce off it and enter your eye.)

Granted the amount of power transferred is minuscule, about 1 microwatt for a Pete Townshend power chord. Your body picks up a similar, probably smaller amount of power from 60Hz EM fields in the environment.

Some "detectors of conditions" (transducers) work the other way round. In a strain gauge bridge or resistance thermometer, the power needed to do the measurement is supplied from the measuring instrument. But a guitar pickup is not one of these.

[RobRed]

So part of that is simple experience and knowing how a Gibson bass would tend to be wired as far as the switch goes?

The thing that threw me on the middle pickup is I don't see a second wire coming off the pickup other than the ground, so I presumed the fact that it was drawn differently than the ground on the other two meant that the ground on neck and bridge was the pickup shielding ground that you mentioned and the ground demonstrated on pickup 2 was the negative wire of the pickup going off to ground. For that reason, I had thought it might have shown a second grown on the middle pickup in a fashion similar to that on the other 2 that would represent the pickup shielding ground.

Now, if I might throw this at you. I cut and pasted the business end of one of my pictorial plans for rewiring my guitar. Then I tried to represent the same as a schematic. I have attached both. Might you take a look and tell me how I made out (I did not dare guess how to represent the rotary capacitor selector on the schematic!)? By the way, in my schematic, whether right or wrong, you will see what I meant about showing one ground for the pickup shield and another ground for the negative lead going off to ground--in my plan the negative wire of all 3 pickups goes to ground, but the pickups have shielding ground wires, as well.

Thanks, again Rob R

[52 Bill]

The thing that threw me on the middle pickup is I don't see a second wire coming off the pickup other than the ground, so I presumed the fact that it was drawn differently than the ground on the other two meant that the ground on neck and bridge was the pickup shielding ground that you mentioned and the ground demonstrated on pickup 2 was the negative wire of the pickup going off to ground. For that reason, I had thought it might have shown a second grown on the middle pickup in a fashion similar to that on the other 2 that would represent the pickup shielding ground.

The middle pickup is drawn the same way as the other two, the left side line representing the cover or shield in just directly connected to one end of the coil and then grounded.

If the original drawing had a dot representing the connection of the two it may have been a bit more clear.

This points up the fact that there really is no single standard that is followed when it comes to schematic symbols and conventions.

[RobRed]

Thanks. I'll be catching on.

[Jazz P Bass]

If you connect your DVM to the guitar output jack, the meter will now provide a continuing path for the voice coil voltage.
We are talking voltage here, not current.
Voltage across a resistance = current.
A good hard strumming will read (depending on the pickup) 100mvAC on a decent DVM.

[RobRed]

Okay. So if you have a short circuit, where there is no resistance, which I guess is what I'm talking about here, you can measure the potential across 2 points developed from strumming, but it does not actually cause electrons to move through the circuit you've created by connecting the tip and sleeve of the jack? Do you have to have a resistance in a circuit where there is an electric potential to also have current? Is that what you're saying? It doesn't seem intuitively obvious to me. I would think without a resistance, electron flow would be faster, not non-existent.

Rob R

[JoeM]

OK, with all due respect Rob, you should spend some time on the earlier link on some basic DC circuits. I feel your trying to absorb this stuff too fast. Getting a handle on basic concepts (Ohm's law etc) would help you immensely.

[Jazz P Bass]

Okay. So if you have a short circuit, where there is no resistance, which I guess is what I'm talking about here, you can measure the potential across 2 points developed from strumming, but it does not actually cause electrons to move through the circuit you've created by connecting the tip and sleeve of the jack? Do you have to have a resistance in a circuit where there is an electric potential to also have current? Is that what you're saying? It doesn't seem intuitively obvious to me. I would think without a resistance, electron flow would be faster, not non-existent.Rob R

Where did you come up with a short circuit?
The ohmeter "closed the circuit"
The pickup coil "is" the resistance.
For now, forget electron movement.
It will only muddy the waters.
The reason that I stated "we are working with voltage not current" is because it is far easier to measure the voltage being produced by the coil.
Plus, the amplifier input circuit is mainly concerned with voltage. Not current.
You can measure the current being produced by the coil but it basically has no meaningful value as far as the amplifier is concerned.

[RobRed]

Hi, Joe:

The whole thing came about because I misunderstood you to mean there was no current and that you could only measure a voltage. I was trying to explain that to myself, but because I misunderstood you in the first place and what I was trying to explain to myself was wrong, the explanation could not make sense. Part of the problem is also that I am just digging into studying electronics in this context. When I'm learning something, I have a habit of just putting what I'm thinking on the table--it seems that's a good way to find out if I'm getting something straight or not.

As far as the short circuit goes, I did not recognize the coil represents a resistance--I thought as part of the pickup it was just part of the source of potential. As such, it had me picturing the typical basic image of a short circuit with a battery (here the pickup) connected at both ends by wires without any intervening resistance. I was wondering how you could have that and not generate a current--useless as it might be. I was wrong on two counts. From what you've said I know there is a resistance that I just did not recognize as such, and you did not mean to say there was no current but only that it would be more difficult to measure and a less useful measurement than the voltage.

I did not mean to muddy the waters by bringing up electron flow. That concept may not be particularly useful in thinking about guitar circuits, but that's all a current is, and I was thinking out loud, again simply trying to figure out how you could have a potential connected from end to end by conductors and not have a current.

My initial question to which you responded was meant to be a much simpler one (though on the same line) than the one you answered. It had just occurred to me, after the drawn out discussion of whether or not the amplifier was part of the completed circuit of a guitar's wiring, that if you otherwise completed the circuit by simply wiring together the two poles of the jack, without the amp in the picture, you would likely generate a current (again, admittedly small and useless) by strumming on the guitar. I just threw the question up to make sure I was thinking clearly and it made sense. It was not meant to be so much a question of how easy it would be to measure the current or if it was even a significant measurement. I just wanted to make sure what I was thinking made sense. When I misunderstood you to say there was no current, that threw me for a loop.

Anyway, sometimes misunderstandings help you get a better or more complete understanding than you had or were looking for in the first place. What you offered will have me thinking about the relationship between potential and current, about why it would be easier to measure potential than current in this context, and why it is that the amplifier is more influenced by potential than by current (that really surprises me!) as I continue studying this.

Thanks for your input. Rob R

[Steve Conner]

What you offered will have me thinking about the relationship between potential and current, about why it would be easier to measure potential than current in this context, and why it is that the amplifier is more influenced by potential than by current (that really surprises me!) as I continue studying this.

I think this is a good introduction to the concept of "impedance". Guitar pickups and amps are high-impedance devices, which means they are, as you say, more influenced by potential than by current. The reason is that the first guitar amps were made out of tubes, which are naturally high-impedance devices. It takes a fairly large amount of voltage to actuate the grid of a tube, but practically no current, as the grid is pretty much just dangling in space inside the tube and hence more or less an open circuit.

Therefore, the first guitars were designed as high-impedance to match the tube amps. Pickups got thousands of turns of really thin wire to produce a high-voltage, low-current output, and the pots were all made large values such as 250,000 or 500,000 ohms.

There are also plenty of low-impedance devices in existence, examples being a dynamic microphone or a moving-coil phonograph cartridge, where the signal consists of a relatively large current accompanied by a much smaller voltage than you would find in a guitar pickup.

It is a fundamental fact of electronics that potential is easier to measure than current, because you need to break open the circuit to measure current, but you can measure potential by just poking it with a meter or oscilloscope. This makes circuits that can be understood in terms of potential more popular with designers. However, for every circuit in existence there is a "dual" where the roles of current and voltage are swapped. Some of them are very useful.

[big_teee]

OK, with all due respect Rob, you should spend some time on the earlier link on some basic DC circuits. I feel your trying to absorb this stuff too fast. Getting a handle on basic concepts (Ohm's law etc) would help you immensely.

I think he's doing great.
I can tell he's got the wheels up there a turnin. lol
He's trying to keep it in a guitar text, so he can build guitars.
Rob Draw us some more pretty pictures!
And, BTW what program do you use to draw all those fancy diagrams, and Schematics?
T