The top would of the guitars with presently most popular mechanical design may significantly bend over and increase the action and affect the intonation if high tension strings are attached to the guitar. I have had this problem with 14 gauge thin E string through 63 gauge of the thick E string set with tension of the rest of the strings in the set just as much as the tension of the thin E. I have heard of people putting gauge 16 on the thin E and using the tension for the rest of the strings with strings for a base guitar for the thick E. I wouldn't be surprise if there are people who, at least on occasion, put even higher tension strings.
The bridge of the acoustic guitars is glued and or screwed to the top wood, on accession, to a plank under the top wood which distributes the string tension force onto a greater area. High tension strings are preferred by some guitar players in order to perform fast solos without the pick being caught on the bending strings. As a gross generalization, Spanish music players take advantage of high tension strings with even higher gauge strings than 14 for the thin E and 63 for the thick E.
High tensions strings exert a huge force on the top wood and, even with a plank, the top wood tends to bend moving the bridge upward and thus increasing the action. The increased action not only affects the intonation but also defies one of the advantages of high tension strings: lack of fret buzz at lower action.
"Doctor, I hurt when I sit," says the patient.
"Then don't sit," solves the problem the doctor.
So, if your guitar can't take high tension strings, don't put high tension strings on your guitar tension nylon strings.
No solution exists with the present design. Selection of a guitar with strong AND THICK top wood or a guitar with a metal string holder positioned on the side of the guitar with a movable bridge may help.
Compensation of the saddle and or the bridge is the easiest patch. Put the high tension strings and play on for a while until the top wood bends and hope the top wood would bend just as much and would stop at a given position. Then, remove the saddle and file the bottom side of the saddle to make the saddle lower. In case the saddle needs to be lowered a lot and the string touch the portion of the bridge which is towards the hole, then file this portion of the bridge. The saddle has to be filed evenly and a bit accurately while the saddle can be roughly filed off to just not be close to the strings. Also, only the parts of the saddle under the strings can be round filed or trimmed, the rest would add more stability. Note: the bridge may be expensive, made out of ebony in some guitars. Better dont destroy this. The saddle is usually made out of synthetic materials such as plastic and is inexpensive. Experiment with the saddle as much as you wish. You can always get another one for $5. Filing the saddle is the best way to adjust the action of any guitar rather than adjusting the truss rod tension which may lead to breaking the neck body assembly which is usually glued thus unstable.
If the bridge is bolted onto a plate which is under the top wood, remove the bolts, remove the plank and replace with a bigger plank if you can fit one through the hole. Otherwise, the guitar body has to be opened and a bigger plank inserted through the back. The bigger plank, which reaches the corners of the top wood may solve the problem at the expense of the sound quality and volume.
A very good approach would be for the manufacturers to build metallic guitar body frames. Titanium alloys would be the best yet the most expensive. Aluminum alloys are much less expensive and would do fine. Stainless or rusty steel are welcome. Graphine or graphite composites too, whatever they may be. Once the frame is built, the neck can be bolted as well as the truss rod into a plate of the frame and the top, tone and side wood can be glued and or bolted onto the tiny frame around with the bridge bolted on a plate of the frame. Accessories such as humbuckers, microphones, tuners, preamplifiers, rechargeable batteries, jacks, etcetera, can also be secured to the frame. This may improve the sound of acoustic electrical hybrids because the accessory would not stay on the wood reducing the resonance (sound reflection) qualities.
A guitar frame would make everyone happy: the metal supporters and the wood lovers. The frame would take an insignificant area, mainly in the corners of the wooden area and would have theoretically negligible effect on the sound, practically none.
Top wood bending on guitars with high tension strings has been observed and patches as well as manufacturing ideas for a possible solution of the problem have been proposed.
By Steven Stanley Bayes