June 19, 2018

Double Wedge Bridge

Some flat-top guitar players mention two annoying characteristics of archtop guitars: the fretboard far above the top and the high bridge. That’s why I have tried to avoid both on some of my guitars. The former leads to design changes that, in general, are easily solved. The latter generates a peculiar problem.

Adjustable archtop bridges are made of two parts, the foot and the saddle, separated by two thumbwheels:

When the bridge is very low the strength of the foot, the saddle or both is compromised, and the added thickness of the thumbwheels doesn’t help. Fixed bridges solve the problem, but most players don’t like them. That’s why, before I made the bridge that I’ll describe later, I used a system based on grub screws and metal guides. It worked well and it was simple, but I always thought that there was ample space for improvement:

The most evident alternative was to use a bridge based on D’aquisto’s wedge design, but it is not popular among players because it is not easy to adjust, it has a stick that can be troublesome and it doesn’t allow to make independent adjustments to both sides.  Thinking on how to solve this, I came to the idea of using circular wedges instead of straight. The problem with straight wedges is that, if you pretend to use two for adjusting each side of the bridge independently, you’ll have gaps when they move. There are no gaps with circular sections, and the contact between the strings, the wedges and the top of the guitar can be made continuous. This was my first drawing based on that idea:

It showed the basic ideas already: circular wedges and independent adjustement with screws, no stick. Making a physical bridge from it was much more work but, after making some simple prototypes, I built a bridge that worked well, so I recorded a video:

Later I solved two problems that worried me since the beginning: the internal stiction and the need to have some mechanism to limit the displacement of the wedges. The latter was basic for avoiding damage if the user tightened the adjusting  screws beyond the maximum, or losing them if loosened beyond the minimum.  I could also see that the saddle was in place mainly by its two ends, so I could make a window in the middle of the bridge, put a lower saddle and have a bridge even lower than I had thought. These are the pieces of a later design:

Below left there are several saddles. One of them is rotated, so that you can see that it is compensated for intonation in two sections. The two screws at the center have stops at both sides. Above right, the springs.

This is the bridge inside, without the springs, at the highest position for both sides:

The finished bridge, on its lower position for both sides and equipped with a low saddle:

Below:

And installed on the guitar for which I made it:

 

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