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I've installed a JLD Bridge Doctor in an old Yamaha dreadnought with excessive belly; the extended fret line lands on the top below the bridge. I cranked it pretty tight but there's no noticeable improvement. Will it do any good to put it away for a week while humidifying it and recheck? Or is the next step major surgery? I was hoping to save this one but it may be terminal.

I installed the JLD Bridge Doctor and had very good results right away. I am surprised it didn't make much of a different for you. Are you sure you have good contact on the tail block with the tention post and everything is installed correctly?

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Brad
Avon, OH

It sounds from your description that there are two separate issues. The bellying in front of the bridge and the neck angle. The bridge doctor is supposed to remove the bellying by restoring the bridge from its canted forward position. Did the bridge doctor remove the bellying? You may still need a neck reset to boot.

Belly is just about as bad is before and I have good contact centered on the tail block. It's roughly 5/32" above the height at the side measured at the back edge of the bridge. I screwed the adjuster about as hard as I could without putting a wrench & bit on it. I don't think the neck itself is gone so far it needs a reset; there isn't much "kick" where the end of the fingerboard joins the neck.

You might need to make sure you don't have a loose x-brace or any other bracing that has come undone.

I installed a Bridge Doctor in an old Martin 12 string that had a bit of a belly AND need a reset. The 'doc removed some of the belly, but not all. The tone was terrible (particularly the octave strings which were the ones going thru the heads of the brass pins). The action was still too high - saddle way too low. Took the 'doc out, reset the neck, reglued the bridge, replaced the bridge plate - belly is gone, action is right on, guitar plays like a dream.

I would suggest resetting the neck, but of course on an old Yami that introduces its own issues.

The Bridge Doctor torques the top....it doesn't press the top down directly. The only reason the bridge area usually goes down with the application of the BD is because the X-bracing makes the top more rigid above the bridge than below.
But older laminated Yamahas have lower bracing to enhance bass response, so the top is not as rigid above the bridge area. In that case, the lowering of the top below the bridge is counterbalanced by raising the top above the bridge.....resulting in no net change in the bridge position.

That is irrelevant. A neck reset is needed if the action is high when the saddle is too low. In a perfect world, reducing the belly would be all that is needed. That may require more serious work like a bridge reglue, bridgeplate replacement or even a rebrace.
But if a proper neck angle results in drop-off of the fingerboard tongue, a tapered shim can be installed. Older guitars can have 'roller coaster' tops that sometimes require such measures to achieve a straight fingerboard all the way to the end.

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John