Gibson saddle height and top sag

[naoki]

A lot of people have been asking questions about sinking tops of old Gibson. I just got my 1915 A-1 and it seems to have some sinking issues. I'm not too worried about it (or should I?). The single brace between the hole and bridge is not loose. Looking through the hole, I didn't know old Gibson used only this single brace. Anyway, I'm interested in the range of saddle heights (measured from the top of the soundboard). Measured in the middle of bridge, mine has 22mm (28/32 inch), but I need to raise it a couple mm, so it should be 24 mm or so. What is your measurement of playable "stably sagged" or un-sagged gibson?

[250sc]

has the seam between the back and side been repaired or is it loose also.

btw. Nice mando. Once you get everything fixed on it it should serve you well.

[MANDOLINMYSTER]

I have heard about tops being jacked up to restore the arch, but I would think it to be a difficult and expensive repair. As stated above make sure the top brace is secure and all the seams are solid. I also see there is a preety thick shim under your bridge to get proper string height, which exerts extra pressure on the top. I would definately use lighter guage strings for that ole gal.

[Big Joe]

It is not unusual for those mandolins to suffer from top sag. It is not necessarily a problem. As long as it does not continue and the mandolin functions properly it may be normal for that instrument. It is possible to suffer from top fatigue, but that is pretty rare.

We have had to fix a couple mandolins over the year where the top sag was beginning to be problematic. We found the use of carbon fibre with the right epoxy will restore the arch without affecting the tone of the instrument in a negative way. It restores the rigidity to the top and allows it to function as it was intended. I don't recommend this repair unless the mandolin structural integrity is at stake. It does not hurt the mandolin, but it does affect its originality if one were ever to sell it.

[barney 59]

It is not unusual for those mandolins to suffer from top sag. It is not necessarily a problem. As long as it does not continue and the mandolin functions properly it may be normal for that instrument. It is possible to suffer from top fatigue, but that is pretty rare.

We have had to fix a couple mandolins over the year where the top sag was beginning to be problematic. We found the use of carbon fibre with the right epoxy will restore the arch without affecting the tone of the instrument in a negative way. It restores the rigidity to the top and allows it to function as it was intended. I don't recommend this repair unless the mandolin structural integrity is at stake. It does not hurt the mandolin, but it does affect its originality if one were ever to sell it.

-----Carbon fiber and epoxy --used how? I have a mandolin of no great value with a sinking top but I would like to fix it.

[acousticphd]

I can see some flattening or slight sinking of the top arch, but yours doesn't look too different from either of my teens Gibsons, or too much different from some substantially newer mandolins. So long as it seems stable and without a loose brace, I wouldn't think about repair., but rather about getting an adjustable ebony bridge. What sort of action height are you dealing as things stand with this mandolin?

[naoki]

Thank you all for info.
250sc, no the seam is not separated. But the back was probably removed at one point, and the person didn't make sure the side was completely aligned all the way around (about 0.5-1mm mis-alignment in a few regions). The lighting is exagerating the misalighment in the 1st phot, I'm not so worried about it.

barney, Big Joe's carbon repair sounds interesting. Maybe he made laminated carbon sticks following the arch and glued it? It may be something similar to: http://www.frets.com/FRETSPages/Luth...ndotopsag.html

Jeff, I'm following your and Big Joe's recommendation of not worrying about it too much. With the current saddle height (22 mm), the action is too low, and it is not playable. I'm waiting for the adjustable bridge to arrive. Once I know the top is stable, I'll probably make a one-piece bridge by myself. As Michael noticed above, the original bridge already have a big "sim". That's why I'm asking this question of how tall the bridge of other teens Gibson. By any chance, can you measure the height of bridge on yours, Jeff?

Naoki

[Darryl Wolfe]

There is some degree of sag, but not as much as an inexperienced eye might perceive. The top never did take/bear a high point crown where the bridge is located. The top runs fairly flat along the centerline where the bridge is located. My opinion, not a problem and only a small degree of sag

[AlanN]

Yeah, my A40 has muy more sag than your mandolin. Has been that way for 30 years, not a problem.

[Big Joe]

The carbon fibre is a sheet of material and should be cut into pieces about 2x2 or 3x3 inches depending upon how much area you need to cover (you decide the dimensions) and lay them in different angles so the fibres will cover all directions. The first may be straight, the next turned at a 45 degree angle, the next at 90 degrees to the original, then continue like that. You should not need more than about 4-6 layers. Then use a very good epoxy (2 part) and cover the material completely. Use enough to completely soak the material and adhere it to the top where needed but not more than needed. You want to have the top with no pressure and in the shape you want it. Then ensure the fibre is adhered to the top properly. Once the epoxy dries it will give lots of extra support and should stop the excessive sinking. It it is done right you will get all the strength you need and it should not affect the tone of your instrument in any negative way.

This is not a repair for all instruments, only those whose top is severly weakened. If your mandolin is a rare vintage instrument worth lots of money you way want to think twice, but it will restore the instruments funtionality. This does not seem complicated, but it is really a bit more than most DIY ers should do. It requires the right fibre, the right epoxy, and the right appication to work well. I almost hesitate to discuss it on a public forum because the last thing I want to see is the work being done wrong and ruining instruments. Like any repair, if done right it can work wonders. However, if done wrong it can ruin an instrument. Because this is done with an epoxy it is not really a repair that can be undone.

While there are a couple epoxies that are acceptable for instrument repairs and can be released very carefully with heat and moisture, they are not usually available on the open market and are very expensive. Any epoxy repair on a musical instrument is very dangerous if you really do not know and have confidence that you are competent with this kind of repair. It is NEVER the right adhesive for neck joints, bridge repairs, etc. It does have its place in a luthiers bag of tools, but only for very special applications. It is not for amatuer repair jobs.

Ok, I'll stop. I just worry I may have opened a can of worms I'll be sorry about down the road. I remember when crazy glue first became popular that we begin to see people bring their guitars in for bridge work only to have them tell us they tried to fix it with super glue. Now the top, bridge, and bridge plate were soaked with the stuff and they would not come apart. I won't even begin to discuss how we had to work with some of those repairs. I certainly don't want to see a rash of that again because of this thread. Enough warning. Just have a good evening!

[Rob Gerety]

You mention you believe the top was off at some point - what sort of repairs would typically require removing the back?

[John Kinn]

I also have a late fortries A40 with a lot of top sag. In addition, the top appears much thinner than the average mando at some places. But structurally it seems fine. The last luthier who set it up adviced me not to use the thickest strings though..
John

[naoki]

Thank you for describing your carbon method, Joe. It is good to know that there are someone who can fix this problem. I hope people will use their common sense when reading this info. Rob, I'm not sure what they did by removing the back. I'm just speculating that they removed the back, but it could be simply bad seam repairs.

After leveling the frets, humidifying for a while, fitting a new bridge (used Brekke Original), setting actions, putting light gauge strings, the Gibson is working pretty well. The saddle height ended up 23.1mm from the soundboard (in the center). The action is not as low as it can get (comapred to our Weber with super low action), but it is workable and not too bad. Eventually, I'll need to refret and flatten the finger board to get a lower action, but I should be happy for now.

It is pretty loud and has that unique round/dark voice (maybe from the construction of lateral brace?).

[Zigeuner]

All of this talk of action height raised my cutiousity. I got my '17 A-3 out and measured the height of the center of the ebony bridge. It's just over 13/16. That's near 2 cm (20 mm). The strings are set into the notches a bit so they would be slightly lower than that measurement at the bridge. The G strings are about 1/16" over the fret at the 12th fret with the E strings just a bit lower.

I wouldn't want to say that the height of my bridge is a standard height. When instruments are built, there are often slight differences in neck angle. Though small, these differences will be amplified when the bridge height is calculated. Martin guitars, for example often will have variations in the thickness of the bridge as well as the height of the saddle between instruments of the same model. The makers will have a selection of bridge heights in order to do a proper setup when the instrument is first completed. My 1962 D-28 for example has a bridge that is 5/16" thick whereas my 1967 D35 bridge is 3/8" thick.

The question was asked as to why a mandolin back might be removed? Well, I understand that some folks have removed them to be able to graduate the top more easily to gain some difference in tone. Another reason, obviously, would be for repairs. It's not unusal for violins to have this sort of work done. It's a good argument for hide glue, as well.

In either case, this is for the experts among us. It's a little bit like making sausage. The results are nice but the process is intimidating.

If the top on your mandolin is sinking a bit, that's apparently not uncommon. After 90 years or so, it likely won't get too much worse. On the other hand, I like light strings anyway.

Good luck!

Here's a website where a fellow did a repair on a mandolin in order to replace missing tone bars inside. The back was removed to facilitate the work. This is not for the faint of heart. LOL

The link to the mandolin repair is partway down in the list.

http://www.bryankimsey.com/

[Big Joe]

Back removal is not that big of a deal...if it is done carefully. We have some methods of repair today and tools and techniques that allow us to do things from the outside or other methods so back removal is not always needed for some jobs that it was considered standard for many years ago. I think most of us feel back removal is acceptable only when absolutely needed and other options do not appear available. We have a network like the cafe and associations that we can gather information from today that was not always available in past years as well. I certainly wish we had access to information when I started working on them that we do now.

If a back is removed carefully and put back on properly it can be very difficult to tell it has been removed. However, in many cases it is pretty easy to tell. Some of the Loars had the backs removed at the factory before being sold to install the Virzi, then some were sent back to have the Virzi removed, hence the back was again removed. They way they did it then was to cut it off. You can see some of these mandolins are noticeably thinner in the rims than they were originally. It certainly did not seem to hurt the tone of the mandolins though. How it altered the tone is pretty hard to tell 90 years after the fact. None of were there to hear it before the sides were thinned.