The Hub

I've got a PRS SE A50E acoustic in with complaints of high action. Checked the relief and it was sky high. Tried to flatten it, and it wouldn't go, so I started experimenting.

When there's no string tension and the (single action) truss rod is slack, there's loads of relief. Tightening the truss rod works fine and the nut turns easily until the neck is flat, then resistance goes up but the neck won't go past dead flat. It behaves as if the truss rod is dead straight at that point, so it's only applying linear compressive force, not bending force.

I figured maybe additional compressive force would help when under string tension--the strings would pull the neck back into a high-relief state, giving more room to tighten the truss rod and pull it down. This is not what happens. When you add in normal string tension, the neck will go to .018" relief and then no further. Additional compression via the truss rod doesn't do anything. With lighter string tension you can get a lower relief, but it still hits a spot where it won't go further, and even tuned down the C standard it's .010"--still too much.

It appears that these guitars are built with no or inadequate preload curve to the truss rod channel. So once you get to a certain point the truss rod isn't bearing against anything and you're just adding tension to the rod without changing the shape of the neck.

Anybody seen this before? What do you do?