Purnell Guitars

On this page I would like to explain and show a bit of the making of my latest guitar which I thought might be of interest. This guitar is a culmination of all the experiments and tests I have made in the development of the sound-post guitar.

As I said on the construction page, the idea to use a post to support the soundboard came about when I became dissatisfied  with the sound of my guitars. The effect of the laminated back and the attempts to make the soundboard lighter was resulting in guitars with to low resonant frequencies, around E, and a “woolly” sound, lacking the definition I was after. So I simply wedged a dowel between the harmonic bar and the back and the result was the raising of the guitars frequency to F sharp due to the stiffening it cause to the back and top. This improved the clarity and added and interesting quality to the sound, I thought. This lead me on to experimenting making guitars incorporating a post and finding the best location for it. Because the backs of my guitars are laminated with no struts on them, I could move the post around, placing it in positions along the bars under the bridge. I found that the best response was when the post is directly under and in the middle of the saddle.

The post make the center of the soundboard stiff without hardly any extra mass being added, and I realized that I could now take a lot of mass away from the soundboard, making everything lighter, compensating for the extra stiffness and hopefully making the guitar more efficient.

The soundboard for this guitar was thicknessed to 1.9 mm in the middle and decreases to 1.5 mm at the edges, roughly 0.6 mm less than usual. The strutting comprises of seven fans with a bridge plate, and fairly standard in the upper bout.

The fan struts are made low and wide with fans 3 and 5 pulled in closers to the center fan to add a little extra support in the middle of the soundboard. I have used a Bouchet bar instead of a bridge plate but I don’t think the extra stiffness the bar create is necessary  with the sound-post. Above you can see the ebony plate inlaid into the center strut where the post sits in a shallow hole.

I construct the neck using the slipper heel method and the sides are glued in with wedges in the heal. Tentalons are used to glue the sides to the soundboard. I like this method for it is controlled and you have time to ensure a good join.

The back for this guitar has a center core of cypress about 1.5mm thick. Ideally I would use western red cedar for its lightness and sometimes spruce. If I could afford it I would use cedar all the time but I have to keep them for soundboards. I use boards with imperfections and in this case the cypress had shakes in but it was fine for the core of the laminate and lightweight enough. The outer Rosewood of the back is about 1mm thick. The inner carbon fiber layer is a 200 gram cross weave material. These are glue together in a two part mould, using epoxy resin, which is shaped to form the arch of the back. The thicker the central core the stiffer the back will be. I could make the back very rigid but I wanted it to move with the soundboard a bit.

I have always carved the neck and heal in situ. This way I can decide what shape heel the guitar can have when it’s all together. Each heel I carve is different and I can strive for that perfect shape.

I have always used french polish to finish my guitars. After years of practice I can now do one in a couple of days. It has got to be the best finish. It is simple and satisfying, once you’ve got it sussed.

Once the bridge is glued on the sound-post is fitted through the sound hole. It is wedged into position and not glued, the down force created by the pull of the strings holding it into position. I suggest to clients that they change the string one at a time to keep the  downward pressure.