McCarthy Stringed Instruments

Tone bars or braces, the structural elements on the inside surface of the top, serve two basic purposes. The first is to keep the top from collapsing under string pressure. The second is to propagate string energy from the bridge out to the periphery.

There are two types of structural elements commonly used in archtop guitar tops. In the 1920s Lloyd Loar, the designer of the Gibson L5, the first archtop guitar with F holes, used two tone bars that are roughly parallel, running from near the neck block lengthwise along the top to near the tail block, with the spacing widening slightly towards the tail block. Then in 1934 with the first Super 400, Gibson used X braces like Martin had already been using on its dreadnaught guitars for about a decade; the arms and legs of the X run from the upper bout on one side to the lower bout on the opposite side, crossing just before (on the neck side of) the bridge. Both types of structural elements have advantages and disadvantages. X braces are inherently weak at the joint where one brace is notched over the other, but sound warmer and darker. They propagate string energy over a wider area. Parallel tone bars are strong and project well, but can sound cold or overly bright. They are better at resisting string pressure. Nearly all archtop guitars use either parallel tone bars or X braces. These structural elements have both been in use for the better part of a century now.

For my archtops, I wanted to find a way to get the advantages of both types of structural elements, but without the disadvantages of either. I wanted to create tone bars that would start near the upper and lower bouts, like x braces, but not cross; they would curve to be parallel in the critical area near the bridge, and then continue to curve back out to the lower bout on the same side. I searched for a way to implement this idea without having to bend or laminate the tone bars, or get too extravagant with dedicating a big piece of spruce to carve each tone bar as a whole separate structure. The solution I invented not only does what I wanted, but has another big benefit. I call my design “integrally carved curvilinear tone bars”.

I recalled that some violin family instruments have carved-in bass bars, and I found that the typical height of structural elements used on archtop instruments is within the margin of wood that I would be removing from the inside of the top. Since I would use a CNC router to do the rough carving of both inside and outside surfaces of my tops and backs, I realized that I could add the shape of the tone bars that I wanted to the 3D model of the inside of the top, and let my CAM plugin (MadCAM) figure out how to carve them for me. Actually, it’s a little more complicated than that, but the result is tone bars that have more strength than Lloyd Loar’s parallel tone bars near the bridge, where it is most critical, and also the broad string energy propagation of X bracing out towards the periphery, where it can do the most good.

The additional big benefit of my design is that there are no glue joints or wood grain transitions involved in these structural elements. Aside from the center seam, the whole top structure is one integral piece of wood. To the extent that glue joints and wood grain transitions are barriers to string energy, my integrally carved design is that much better at string energy propagation than conventional designs. I believe this integrally carved curvilinear tone bar design is a big part of why a McCarthy Stringed Instruments archtop sounds like a well played vintage piece, practically from the first time it has been strung up.


The top shown on this page is for a 16” Skylark Spirit in Colorado Englemann Spruce. Some final hand finishing remains to be done.