two humbuckers - 2 vol - 1 tone issue

[the rake]

Hi there. Just finished rewiring with two Burstbucker pups in my Gibson. I wired them in the standard 3-way, 2-volume, 1-tone setup. Everything seems to be working well, except when I put the 3-way in the middle or "both pups on." If I turn down either pup all the way, they both turn off completely. They work fine and sound fine when individually selected. Normally, I would expect to be able to just roll off the volume from one or the other pup not the whole circuit. Just wondering if this is a standard problem and I am just missing something incredibly silly!

Thanks for any help and thanks for all the other great advice on this site!

Jimmy

[Down Time]

That is normally how they should work in a Gibson. If you want the volumes to work independently reverse the center and left lug leads at the volume pots.

[Chuck H]

It should be noted that while reversing those leads WILL allow fully independant volume control, it will also make it so you can't present a zero load to the amp even with the volume controls all the way down any more. This means that whatever hiss and noise is coming from the amp with the volumes turned up will no longer be eliminated by turning the volumes down. This can be really annoying for breaks in songs, in between song pauses and when turning down the volumes to get a cleaner tone.

Chuck

[the rake]

Hey, thanks for the great info. Never actually knew that's the way most Gibsons are wired! I appreciate the quick responses. I'll play around and here how it sounds!

Jimmy

[the rake]

I mean "HEAR" how it sounds!

[jakeac5253]

It should be noted that while reversing those leads WILL allow fully independant volume control, it will also make it so you can't present a zero load to the amp even with the volume controls all the way down any more. This means that whatever hiss and noise is coming from the amp with the volumes turned up will no longer be eliminated by turning the volumes down. This can be really annoying for breaks in songs, in between song pauses and when turning down the volumes to get a cleaner tone.

Chuck

This is true, that's how my Jackson is wired, you can hear strumming very quietly while one volume is turned down. Something of note however is that when BOTH volumes are rolled back to zero, no signal is heard. It's like when one is rolled down you are hearing the other one and vice versa.

[Glass Snuff]

It should also be noted that when you turn down the volume with "independant" wiring you're also rolling off the tone, as the resistance the pickup sees is reduced. This is one reason cheap Asian guitars sound so bad. Not a recommended wiring scheme.

[Glass Snuff]

I received a PM asking for clarification, so here goes:

LP copies are often wired with the independent scheme. The pickups are considered flat and lifeless, but it's mostly the wiring's fault. (OK, a pair of Duncan's would help )

There are two things that need to be understood.

1) The pickup only puts out voltage into a load.

No load at all, nothing between the output wires, and there's no signal. Short the wires together, and again, no signal. Only when there's a resistive load between the wires can the pickup have an output, and the resistance at any given frequency determines the strength of that output.

2) The tone control is not an RC filter.

It's an inverse Zobel filter, and a dividing network for the pickup's load. As we continue through this explanation, you'll note that at no time do I refer to anything "going to ground", because that doesn't happen. As noted above, the pickup generates a voltage based on the total resistance of the volume/tone/cable/amp network (caps are just resistors, once you specify a frequency). It does not create 5 volts, 4 of which are then "bled to ground". Throw your simplistic 'DC passive network powered by a battery' concepts out the window, as we're about to jump into the 4 dimensional world of an inductive AC generator loaded with a resonant capacitance.

First, a pretty picture:

These graphs are by "Clorenzo" on The Gear Page, and show the action of a tone control as it's turned down for .047, .022, and .010 capacitors. The model, and thus the values shown, is for an overwound humbucker using 500K pots, but the principle applies to any guitar. Also note the top blue line is a "no-load" pot - that is, there's no tone control at all, just a 500K pot across the pickup.

The vertical axis is labeled in dB, the horizontal axis is the frequency in Hertz. On your guitar, the low E is 80Hz, just off the left side of the graph, the high E string 12th fret is about 660Hz, and that D at the 22nd fret is about 1200Hz, which on these charts is in the middle of the peak to the right. (The resonant peak of most humbuckers is around 4KHz, a Fender pickup peaks about an octave higher.)


It's been stated many times that the value of a volume pot affects the highs, and you can see that clearly here. The bump on the right side is the resonant peak of the pickup, and the colored lines represent a lowering of the value of the volume pot. They do this because the tone pot is in parallel with the volume pot, and turning it down reduces the total resistance that loads the pickup. I mentioned above the cap serves as a divider, and at these frequencies we are above the division, so the values of both pots are simply combined.

When you wire a guitar so the pickup is connected to the wiper, as you roll off the volume, you're turning down the resistance seen by pickup, and you can see what happens. The resonance is reduced, hence the "flat and lifeless" determination. By connecting the pickup's output to the 'top' lug on the pot, the full resistance is always across the pickup (as the pot's lower lug is connected to the other wire of the pickup) and the resonance is maintained when the volume is turned down.

"But I lose highs when I roll off the volume!" you cry. Yes, you do, but that's because when you turn down the volume pot and the wiper 'divides' the resistor, the top bit of resistance is now in series with the output of the pickup, and it does form an RC network with the capacitance of the guitar cord and the amp input's ground. This rolls off more highs the more you turn the volume down (and make that resistance bigger). (you can, of course, solve this by unplugging the guitar and removing the ground, but a few keen-eared players find the resulting volume loss objectionable )

As long as we're here, it's worth mentioning a few things about tone controls. Notice that the value of the cap makes no difference above "5". Also, notice that with a .01 cap, when the tone is rolled off to "0", the output of the pickup is increased above what you get with the tone control on "10". Neither of these results would occur if the tone circuit was an RC filter.

Your tone control, in fact, can be thought of as a pan pot between two resonances - the natural resonance of the pickup, and the resonance of the tone cap. In the middle, the Zobel equation applies and cancels the inductance of the pickup, we get no resonance, and there's the -12dB/oct rolloff of a second order filter. (the T4 line in the graph)


Hope this helps,
GS

[MistaT]

Great post!

Thanks!