Post
by Trevor Gore » Tue Jun 16, 2015 11:11 pm
I've not had any issues with tops "sinking" but neither do I leave guitars around without a bridge for long periods in varying humidity.
Dave, if there is no bridge plate (as per your earlier photos) there is nothing to hold the transverse arch up in the lower bout until you get the bridge on, other than the induced curvature forced by the primary falcates. When the humidity goes up, most guitars will bulge upwards due to the transverse bracing beneath (includes X-bracing) not expanding. A falcate braced guitar only bulges where there is transverse bracing, i.e. in the region of the secondaries and tertiaries, on a finished guitar remaining flatter in the center held by the stiffness of the bridge and its curvature. This is generally a good thing, as the action doesn't change much with the weather. (Smallman moving to a tilt-neck design was due to the humidity pumping and consequential action changes that lattices cause). If the guitar goes through a few humidity cycles with no bridge or bridge plate, even when equilibrated to its build humidity, it is possible that the single, smooth arch might not fully recover. But you'd want to be sure that you weren't looking at the double arch caused by high humidity. This suggests it may be worth "propping" the center of the top if the guitar is to be left for long periods without humidity control. Not sure it will work, because I've never had to do it. Gluing on the normal classical bridge (as per the book, with CF and design bottom curvature) should smooth it all out again.
BTW, if you do the same with a fan braced classical with no bridge plate (i.e. humidity cycle it) it can and will develop corrugations between the fans that will not disappear when returned to design humidity. I've seen cases where these corrugations have been too deep to sand out, necessitating re-topping the guitar.
So the ways I know that avoid this problem are to complete more quickly and maintain humidity control.