Below is a schematic of typical wiring for vintage Gibson-style guitars with two pickups, two volume controls, two tone controls, and a three way toggle switch for pickup selection.

Please notice the comments in the upper-right corner of the schematic.  A lot of people don't wire the volume controls properly on these guitars.  If you wire them the way a Fender guitar is wired then the controls are "coupled" and either volume control will completely kill the output of both pickups when it is turned down all the way.  Actually, Gibson has been wiring guitars this way for at least a couple of decades now, but that is unfortunate.  I much prefer the earlier wiring that didn't couple the controls.  I think you will too, and that is why I present the earlier scheme here.  Another common mistake people make is to wire two or more volume controls as shown here but then try to use a single master tone control with them.  For reasons that are way to technical to get into here, that is not a good idea because usually the volume controls will drastically alter the affect of the tone control.  You can somewhat offset that by leaving the ground end of the volume pots floating (disconnected) but then you cannot turn the pickups completely off when feeding a high input-impedance amplifier or effect.  So, the best solution is to use an individual tone control for each volume control.  Finally, note that decoupling the tone controls, and reducing loading on the pickup when both are selected (which puts the two volume pots in parallel) is probably the real reason Gibson used 500K pots (which then started the "use 500K pots with humbuckers" rule of thumb).

Recently (June 2002) I received this email...

Mayby you can help me with my little problem:
I have a gibson es345 ('69) with original PAF humbuckers.
When I put the switch in the middle position the guitar sound out of phase,
which is very annoying.
When I bring down the level of one of the pickups, the sound fattens up,
clearly a phase problem.
I allready had two repairmen check the guitar but none could solve the
problem.
The wiring seems to be OK.
My repairman told me it has to be a problem with the humbuckers.

Have you heard of this problem before, and do you think It could be a
problem with the humbuckers?

...to which I sent this response...

Does the sound fatten up even if you only back either volume control off
just a little bit, like by 10% or so?  If so, this is normal for vintage 
Gibsons.  With both volume controls at maximum the two pickups are wired
directly in parallel and each applies a fairly high load to the other.  
As soon as you back either volume control off even just a little you throw 
100k or so of resistance between the two pickups which isolates them so 
they no longer load each other.  The 100k of resistance isn't that 
significant to the high impedance amp, so you get almost full volume from
both pickups and the sound is fat.

A "cure" for this is to put a 47k resistor between each volume pot and the
pickup switch.  This amount of resistance will barely reduce your volume at 
all, but there will always be at least 94k of resistance between the 
two pickups, preventing them from loading each other.

Not all vintage gibsons do this because often the cheap pots they used didn't 
go all the way to the end of their electrical travel anyway, leaving a little 
resistance between the pickups even at full volume.

I'm glad you brought this up, I need to make mention of this "fix" on the 
website because I've not seen anyone else note it.

Finally, I should add that a similar dynamic is at work in Fender style guitars.  When you select the neck and middle pickups together, each is loading the other, likewise when you choose the mid and bridge pickups together (or the neck and bridge on a Tele style guitar).  That is why the signal usually gets a little quieter, instead of louder, when you select the combination positions on the selector switch. If you want a different sound from your Strat or Tele try putting about a 20k resistor between the hot lead of each pickup and the selector switch. You'll sacrifice a bit of overall volume (not very noticeable at all if you're feeding a hi impedance amp or effect) but will get a fatter sound in the mixed positions.

I'm a little surpised that aftermarket pickup manufacturers don't seem to have tumbled to this.  It would be so easy to include a little series chip resistor inside the pickup.  I guess it's hard to buck tradition and they have to figure that a lot of people like the Strat mixed positions the way they are.  Once upon a time I loved the neck/mid position until I built a guitar that blended the middle pickup and discovered just how thin the mixed positions are compared to what they can be with just a bit of resistance between the pickups.