LES PAUL PERSONAL
(PROFESSIONAL AND RECORDING)
PAGE 2

NOTE: Before we go any further, let me stress that I'm strictly a Sunday-Techie, with limited knowledge of such matters. Considering that this info might be of interest to the handful of people in the universe who own a guitar like this, what are the odds that someone would be both fully-qualified and willing to blather on for three fun-filled pages? Just a little Reality Check, for your information...

Gibson Les Paul Personal Professional Recording

 

"La guitarra del Capitán Kirk"- This circuit diagram was based on a couple of references that I found on the Internet: One was very faint and nearly illegibly-scrawled, and one was from a German site, about 15 pages deep in Google (so I'll never find it again). It's my attempt to understand schematically what was going on in the circuit, using my own schematic representations of switches. Therefore, it doesn't represent the tricky way that actual switches are often constructed (they do it that way to confound you). I think it's basically correct, but no guarantees. I've omitted the impedance matching transformer (or Ùbertrager) and switch at the output (the Personal and Professional don't have 'em), but you can pretend that they're there or remove them to conform to my diagram. I've also omitted the microphone circuit from my diagram since it's really a no-brainer. It's just an XLR jack + 1K pot circuit which connects to the ring lug of the 1/4" output.

 

THE PICKUPS

I don't have a problem with the pickups, but I was really curious about them; they're such a unique shape and size that you'd never find drop-in replacements for them. Manufacturers simply have no monetary incentive to manufacture replacements for the small number of guitars, still in service, that were produced with these pickups. If one wanted to replace them with high-impedance pickups, would there be any alternatives-- such as having them rewound to retain the nifty original look? Unfortunately not, or not very easily. The pickups are huge, open-bottomed plastic covers, filled with epoxy. If you could grind out the epoxy without screwing up the cover, you wouldn't have much of a bobbin left to rewind. I think the only way you could do this would to make castings of the pickup cover and create the pickup from scratch. Another possibility would be to fabricate a mounting plate out of brass (plated with nickel/gold) to accommodate a P-90, mini-humbucker or humbucker, and cover the gaps to hide the routing. Those pickups are all short enough and width-wise, seem like they would fit within the routing. However, I'm not sure if the polepieces would match up very well due to the slant, and it might look pretty crappy. For what it's worth, I've read references to these being stacked humbuckers with ceramic magnets and individually wound polepieces, but I don't know if this is true. Supposedly these were potted in epoxy to make it more difficult to steal the design (not that anyone actually wanted to, considering Gibson's great success selling 'em).

 

THE PHASE SWITCH

The phase switch is a simple reversing switch that switches the leads on one of the pickups, putting the pickups out-of-phase in the "both pickups on" position of the pickup selector switch. Did you know that if you wire two of these switches together you can turn a light on or off at opposite ends of a long hallway? (However, that does make it difficult to label the "on" and "off" positions...)

 

THE DECADE SWITCH

The name sounds impressive, but the circuitry doesn't appear to be at first glance. It's just a rotary switch that sends the signal to ground through different values of capacitors. I could make a joke about it being an 11-position switch, but I won't because it's already been done. Dammit. Actually, they were thinking about calling it a "Century" or "Millennium" switch, but couldn't fit all the letters on the knob. Yeah, really. (Actually... a Decade Box is an electronics tech gizmo which allows you to step through a range of resistance or capacitance values.)

  .22u .15u .1u .047  
0          
1       X .047
2     X   .100
3   X     .150
4 X       .220
5 X     X .267
6 X   X   .320
7 X X     .370
8 X X   X .417
9 X X X   .470
10 X X X X .507
This is a doozy of a switch. While the circuitry may not seem impressive, the switch and its wiring sure are. I've studied the switch a little more closely now, and although I couldn't see what was going on underneath, some VOM readings told me a few things, so I've created this nifty little table for you to memorize.

You probably recognize this as a 4-bit binary table. It's missing a few possibilities ("X0XX", for example), but I reckon they couldn't find a 16-position rotary switch, and the extra possibilities wouldn't make a perceptible difference (367uf vs. 370uf, for example). I think 11 is more than enough, don't you?

The left column corresponds to switch positions and the other column headers refer to capacitor values. The "X" indicates that the capacitor was in-circuit for that position. Multiple "X"s for a switch position means that several capacitors were in circuit, in parallel (and therefore additive). The right column shows the running total. Nearly all the capacitors in the circuit are 35v tantalium type with an attractive color coding which makes them look like candy (but they taste dreadful).

The values seem awfully high compared to those found in most guitar tone circuits... in fact, the circuit may be a bit more ingenious than I'd initially thought. Yes, those values were selected to work with the low-impedance thing, but I believe they also form a filter network with the inductance of the pickups. I think. Maybe. In low-impedance mode, the effect of this switch isn't simple treble-cut as you might expect, but affects the circuit's resonance. Very weird. The frequency band narrows and the center shifts downwards you increase the capacitance, and around the middle you begin to get some very Strat-like tones. If you connect this circuit on the high-impedance side of the impedance matching transformer, it acts more like what you'd expect, and you get some deep sea jazz tones at maximum capacitance.

For this info's relationship to the real world, refer to the picture of the switch shown above (the white round thing sprouting blue wires). Yep, this switch is a humdinger. The blue wires on the right side are the switch input from the rest of the circuit (connects to the pickup switch). The four contact lugs along the top each connect to a capacitor, with the other ends of the capacitors tied to ground. The first capacitor lug (left) is soldered to a lug at the (picture) bottom of the switch. It's hard to tell from the picture, but the switch has two halves; I think the bottom lug connects between both halves of the switch. Otherwise, nothing is soldered to the top half's capacitor lugs. You can see that the lugs' contacts aren't all the same length. To add to the confusion, the contact track also has a little extended tab which is hidden by the second capacitor position's lug in the picture. Confusing yet ingenious little switch, huh? (Or confusing paragraph, ingenious switch?) Clearly, the Gibson engineers weren't sitting around twiddling their thumbs. I only mention this stuff in case you're considering other innovative uses for this switch. Or to hint at your prospects for finding a replacement at Radio Shack.

 

THE TONE SELECTOR

Generally speaking, the tone selector switch acts as a selector between three preset tones. The weird thing is that one position offers all the adjustable parameters, one position offers a smaller subset of the adjustable parameters, and one position offers the smaller subset of adjustable parameters plus its own fixed parameters. Got that?

This is one of the most bizarre and unintuitive aspects of the circuitry, guaranteed to elicit a plaintive "ehhh???" if you try to figure out what's going on solely by flipping the tone switch. It's one of those things that you have to understand intellectually in order to truly appreciate: This ain't no Stratocaster circuit. In these three diagrams, for the sake of clarity, I've eliminated the parts of the circuit which aren't active in each of the three switch positions. I've supplemented the diagrams with commentary, just in case the diagrams prove to be equally puzzling.

TONE POSITION 1 In this position, both pickups are selected and the tone-shaping capacitors of the Decade switch are supplemented by the fixed-value capacitor and resistor at the top of the the circuit diagram which are tied to their respective pickups. This is also known as the "Ehhhhh?" mode.

TONE POSITION 2 This is what I consider the "Why Bother With The Other Positions" mode of operation; The Bass and Treble cut sections are in-circuit, along with the other stuff that's always in circuit. The only components that aren't in circuit are the fixed-value resistor and capacitor. It's kinda nice to be able to use the pickup selector.

TONE POSITION 3 You could consider this the "economy" mode; it's similar to position 1, except the pickup selector works and you don't get the fixed-value capacitor and resistor. This is also known as the "Ehhh?" mode.

I believe that it's a good thing that the designers didn't have a five-position switch at their disposal; there would probably be modes involving the volume switch ("no sound mode") and pickup selector ("no choice mode")... oh sorry, they already got that one!

 

UPPER BOUT CIRCUITRY

This is one of the coolest things about this guitar. To the best of my knowledge, no other guitar has this feature...It's that special. The upper bout is routed for the XLR microphone jack (upper left), the pickup selector switch (left center) and the microphone level potentiometer (right)-- functionally, it's one of the best-placed knobs on the guitar IMO, even if it does look a little weird. The microphone circuitry is pretty simple: the jack connects to 1 lug of the approximately 1k pot (137 7018 - CTS manufacture, 18th week of 1970); the center lug goes all the way through to the stereo 1/4" output jack.

While some may dismiss this feature as "whacko" or "demented", this could come in handy if your PA system died in the middle of a gig and the singer needed somewhere to plug his low-impedance microphone. Hey, it could happen and it never hurts to have contingency plans. And remember-- no matter how new your car is, always carry a set of jumper cables (because new batteries do die and when you really need some jumper cables, almost no one admits to having 'em).

--10/29/03

 

NEXT: DEFILING THE GUITAR (Not for the faint-hearted!!!)

 

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