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Broken tone bars and the Gibson
ES-175 Sinking Top
© Frank Ford, 12/10/99; Photos by FF, 12/99

Well, here's yet another monument to "expedited factory production technique" as practiced by good ol' Gibson. That company has definitely made more changes in design to facilitate production than any other guitar maker. From injection molded flat top bridges to ceramic adjustable guitar saddles to microwave cooked glue joints, the hits just keep coming.

In this guitar, the big idea was to eliminate the hassle of fitting tone bars to an arched top. "So, let's just cut most of the way through 'em and then we can simply mash 'em down to the curved top as we glue 'em up." The only problem with that is that they tend to break. . .

On the positive side, though, there is the fact that the Gibson company was entirely responsible for the development of the archtop jazz guitar. Arched top, cantilevered fingerboard, elevated fingerrest, adjustable truss rod, cutaway, adjustable bridge - all Gibson innovations. In fact, all their great competitors: Epiphone, Stromberg, D'Angelico and the rest actually didn't invent any of that stuff. It was all done by the R&D team at Gibson. Does that mean we have to forgive them their corner cutting? I dunno.

I suppose you could call it a "hollow body electric" but because it has a full-depth body, I think of it as an electrified acoustic jazz guitar. The ES-175 has a 16" wide body, with laminated maple top and back.

It's difficult to show the sinking top in a photo. Under the load of string tension, the bridge presses the top quite low in the center.

I think it's easier to visualize the depressed top by noticing how high the pickup and bridge must be raised to accomodate decent string action. With the bridge adjusted any lower than this, the strings would lie right on the frets.
  I used the Insight Video Camera for the interior black and white shots.

With the pickups removed, and dropped down inside the guitar body, I can reach the video camera in through a pickup mounting hole. Check out these "kerfed" top braces, or "tone bars." They run the full length of the top.

Now, in theory, these tone bars should be as strong as solid ones. The tensile strength of wood is extremely high, and if they didn't have grain "runout" and if the cuts weren't quite so deep, I wouldn't have a job to do in here.

That treble bar in the picture above has quite a little crack in it, but this bass bar is broken even more obviously. No wonder the top is caving in!

Another view of the brace shows how loose it is and and about how much the top sags, even with the strings removed.

In order to reinforce the tone bars, I'll have to jack up the top so it will resume its original arch. Beneath the bottom pads, you can see a plate of 1/8" scrap ebony I've used to spread the load a bit.

Here's the reinforcement plate. It's 3/32" thick spruce (this time with no runout) the same width as the brace, and quite long. No reason not to bridge over the other potentially weak areas, in addition to grabbing as much gluing area as possible.

It takes a bunch of clamps, but thankfully there is access through the various holes in the top.

Inside, it's a forest of clamps and jacks. I'll put this aside for a day, and repeat the process with the bass side tomorrow.

Now, both braces are done, and you can easily see the spruce "cap plates" I've installed. The braces are really rigid again now!

Compare this "after" photo with the third photo at the top. You can see that the bridge and pickup are back to normal positions, now that the top doesn't sag.

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