Repairing a 1936 Gibson K-1 mandocello

[Bernie Daniel]

I've finally got around to fixing the top sag on my 1936 mandocello! Actually I finally got up enough courage to tackle it!

After reading the K-2 repair on Frets.com and also the description that Paul Breen (Frank's repair blog) provided for taking the back off a Gibson A-model mandolin I decided proceed and to use a similar approach on the mandocello.

On of the big problems after removing the back plate is that the release of decades of tension can sometimes cause the sides tend to splay out after the back plate is removed. This make gluing the back on a major challenge.

So following Paul's approach I made a clamp to go around the sides to prevent the distortion while the back plate is off.

The clamp is made out of 3/4" finishing ply and has a layer of gasket cork around the inside so that the sides of the mandocello will not be damaged. When the back is off I will also brace the sided so that they do no bend or lean inward after the plate is removed.

My only question is how much tension should I put on this clamp? I think probably it should be very light pressure - -just enough to keep the sides from spreading?

[David Houchens]

I've always used the method of blocks placed around it and screwed to a board. On mandolins and guitars. If the fit is good it shouldn't take any real pressure to keep it in shape.

[Bernie Daniel]

I've always used the method of blocks placed around it and screwed to a board. On mandolins and guitars. If the fit is good it shouldn't take any real pressure to keep it in shape.

Wow, I wish I had thought of that -- would have been a lot less work!

Did you ever have a problem with the back plate shrinking or distorting when it is released? Also do you usually apply pressure from the inside also to keep the sides from deforming INward?

[Bernie Daniel]

Right now I'm thinking that I will go with the approach that Frank Ford used on the K-2 he repaired. He put another transverse brace directly under the bridge to fix the top sag. I have heard a video using this mandocello after the repair and it sounds just great. Of course I never heard the cello before repair.

Another option for fixing the top sag that I am considering is the X-brace that Bruce Weber used on an oval hole mandocello that he built. I've have heard some say that mandocello turned out very well also -- but I've never heard a video of it being played.

Right now I'm leaning toward the single brace under the bridge as that would be least change from the original insturment. How do you suppose a person might decide how heavy to make the brace? I want it light a possible but I want it to work also -- i.e., prevent top sag.

[pianoman89]

To fix top sink in the violin family, a mold of the top is cast directly on the instrument, protecting the instrument with a plastic layer of course. This mold will protrude were the top sinks. The mold is scraped down do give the correct contour (in reverse of course), and hot sandbags are used to apply heat and pressure to conform the sagging to to the corrected mold. This could be an option if you were unsure about adding a brace under the bridge. Here an article explaining the process:

http://christianschabbon.com/article...ng-correction/

I made a simple spreader using all thread and the tops of spool clamps, with the cork facing out, on each end. Cut the all thread just shy of the width you need, and the spool clamp ends can backed out to apply very slight pressure on the sides.

[Michael Lewis]

Since the top has had an issue with collapsing I think the X or 'tone bar' bracing would be more appropriate to prevent it from happening again. However, to make the braces strong enough to do this they may also restrict the top some, so may lose some tone or volume. It's a delicate balance and there may not be a right or good answer. I think Frank Ford would make it closer to original specs, and I have great respect for him and his work. So, here is a circular train of thought to ponder for a while.

[Bernie Daniel]

To fix top sink in the violin family, a mold of the top is cast directly on the instrument, protecting the instrument with a plastic layer of course. This mold will protrude were the top sinks. The mold is scraped down do give the correct contour (in reverse of course), and hot sandbags are used to apply heat and pressure to conform the sagging to to the corrected mold. This could be an option if you were unsure about adding a brace under the bridge. Here an article explaining the process:

http://christianschabbon.com/article...ng-correction/

I made a simple spreader using all thread and the tops of spool clamps, with the cork facing out, on each end. Cut the all thread just shy of the width you need, and the spool clamp ends can backed out to apply very slight pressure on the sides.

Thanks. Yes that is similar to the approach that Frank Ford used to fix the top sag on a Gibson K-2 mandocello! That was an amazing link by the way -- of course with a million dollar instruments no effort should be spared!

But fortunately for me I do not need to do that on this one because when the string pressure is released the top on this mandocello rebounds to its proper shape. I just have to shore it up from sag the next time it is restrung. Here is a link to Ford's repair of the K-2.

[David Houchens]

You may very well get some shrinking in the back when it comes off. When you're ready to glue it back up, dry clamp it all the way around to see and adjust. Sometimes you just have to have a tiny bit off all around instead of a lot all in one place. Then remove a couple of clamps and glue that area, re-clamp, then a few more, etc... until you're all the way around. The worst plate shrinkage I experienced was a 1937 gibson archtop guitar, but it had been loose in two areas for years causing side distortion as well. With the difference balanced out the entire perimeter of the body, it was almost unnoticeable.

[Bernie Daniel]

Since the top has had an issue with collapsing I think the X or 'tone bar' bracing would be more appropriate to prevent it from happening again. However, to make the braces strong enough to do this they may also restrict the top some, so may lose some tone or volume. It's a delicate balance and there may not be a right or good answer. I think Frank Ford would make it closer to original specs, and I have great respect for him and his work. So, here is a circular train of thought to ponder for a while.

Thanks for those comments - yes it is kind of a conundrum. Here is the video of the every mandocello Frank fixed with the transverse brace under the bridge (seen post #4). I think the mandocello speaks for itself -- in the hands of a fine player of course.

[pianoman89]

Since the top has had an issue with collapsing I think the X or 'tone bar' bracing would be more appropriate to prevent it from happening again. However, to make the braces strong enough to do this they may also restrict the top some, so may lose some tone or volume. It's a delicate balance and there may not be a right or good answer. I think Frank Ford would make it closer to original specs, and I have great respect for him and his work. So, here is a circular train of thought to ponder for a while.

I can see your point, but I have to point out that the whole reason the mold and press method was developed was to fix tops that have had issues with collapsing. But I do agree that a brace would be the for sure fix, and while you may loose a bit of tone or volume, I dont think youll actually be able to notice...

You also asked about how heavy the brace should be. Im sure you know this, but you want a tall,thin brace for this application. You get more strength with less mass, which is what you're after..

Good luck with the repair!

[Jim Adwell]

A transverse brace under the bridge will put the added stiffness right where you need. That's what I would do, and I might even add some carbon fiber to the brace to lower the mass. The sound will change, but it may not change enough to notice. I'm not a repair person, though, except for my own experimental instruments.

[resophonic]

My only question is how much tension should I put on this clamp? I think probably it should be very light pressure - -just enough to keep the sides from spreading?

Hi Bernie, I see your coming along with your project. This site is a another great resource, glad to see you poked your head in to check it out.

I made my mold just a tad big around the entire perimeter of the mandolin that you saw pictures of at Frets.net. I applied a fresh mixed batch of PC7 epoxy to the outline cut into the mold and then closed it around the Seran wrap masked mandolin. This resulted in a perfect fit when hardened with NO pressure on the sides. You just want to hold things in place without introducing tension that was not there before. I would post a link if I could to the Frets thread with this discussion but the site seems to be down, maybe I'll try again later.

Paul Breen

[Bernie Daniel]

I can see your point, but I have to point out that the whole reason the mold and press method was developed was to fix tops that have had issues with collapsing. But I do agree that a brace would be the for sure fix, and while you may loose a bit of tone or volume, I dont think youll actually be able to notice...

You also asked about how heavy the brace should be. Im sure you know this, but you want a tall,thin brace for this application. You get more strength with less mass, which is what you're after..

Good luck with the repair!

Thanks! Yes, "tall and thin" was what I was leaning to because you shed mass faster than strength.

But it is reassuring to hear someone else who has been there before say it. I'm thinking that probably Frank slimmed that brace down a lot before close it up again? (referring to post #4)

[Bernie Daniel]

Hi Bernie, I see your coming along with your project. This site is a another great resource, glad to see you poked your head in to check it out.

I made my mold just a tad big around the entire perimeter of the mandolin that you saw pictures of at Frets.net. I applied a fresh mixed batch of PC7 epoxy to the outline cut into the mold and then closed it around the Seran wrap masked mandolin. This resulted in a perfect fit when hardened with NO pressure on the sides. You just want to hold things in place without introducing tension that was not there before. I would post a link if I could to the Frets thread with this discussion but the site seems to be down, maybe I'll try again later.

Paul Breen

Thanks Paul,

I was going to use that PC7 epoxy too but after several trips to Home Depot, Lowes and local harware stores and not finding it I went this way. I know I have seen it somewhere around town before.
I've backed the screw off to the point where the brace will slide off with little effort so I don't think I am adding much outside force to it now. At least that is what I hope.

[Bernie Daniel]

A transverse brace under the bridge will put the added stiffness right where you need. That's what I would do, and I might even add some carbon fiber to the brace to lower the mass. The sound will change, but it may not change enough to notice. I'm not a repair person, though, except for my own experimental instruments.

Thanks for the input Jim.

Yes, if the mandocello in the video (post #9) is a typical example I'd say Frank's repair turned out very well in terms of retaining the typical Gibson K mandocello sound?

[pianoman89]

A transverse brace under the bridge will put the added stiffness right where you need. That's what I would do, and I might even add some carbon fiber to the brace to lower the mass. The sound will change, but it may not change enough to notice. I'm not a repair person, though, except for my own experimental instruments.

Apparently the Larson Brothers were known to have braced with a laminate composed of Brazilian rosewood and spruce... Might be a great application here...?

[Bernie Daniel]

Apparently the Larson Brothers were known to have braced with a laminate composed of Brazilian rosewood and spruce... Might be a great application here...?

Might be a bit beyond my skill set although I have both woods here. Do you have an idea how that might be done and are the advantages greater strength of the rose wood? Of course it is more dense. I the idea that a slimmer piece of rosewood would give more strength and offset the greater mass (due to its greater density)?

[Jim Adwell]

Might be a bit beyond my skill set although I have both woods here. Do you have an idea how that might be done and are the advantages greater strength of the rose wood? Of course it is more dense. I the idea that a slimmer piece of rosewood would give more strength and offset the greater mass (due to its greater density)?

http://larsonbros.com/

There's a video on their home page that has the laminated bracing. The rosewood strip in the middle looks pretty thick. It seems to me that solid spruce would be stiffer for the same mass. Perhaps the sound improvement (if there is any) is worth the loss of volume. I dunno.

[pianoman89]

Might be a bit beyond my skill set although I have both woods here. Do you have an idea how that might be done and are the advantages greater strength of the rose wood? Of course it is more dense. I the idea that a slimmer piece of rosewood would give more strength and offset the greater mass (due to its greater density)?

I think if I were to do this, I would laminate say two thin strips of rosewood between three thin strips of soft spruce. Yes, I think the idea is to get a very rigid brace with less mass.
When I prepare thin strips of wood for lamination, I use a large drum sander chucked into a drill press. I use a quality drum with a very true surface. I then clamp a guide on the table of the press. Using one clamp as a pivot, you can adjust the amount of material you would like to take off, then anchor with a clamp on the other side. Run the material through several times until you achieve the desired thickness. I would personally go to 1-1.5 mm on the rosewood and 2.5-3mm on the spruce.

All this said, you would be just fine with a solid spruce brace. I would only go with the laminate if you were very concerned about adding mass to the top.

[Bernie Daniel]

Well the back is off!

It came off very grudgingly as it was apparently glued on very well at Kalamazoo back in the 1930s. The neck block took a long time as I kept hitting the nail they used to center the back. But it is off.

The top did not always cleanly separate at the glue line but instead took along part of the kerfing. I don't really see this as a big problem as this will provide a nice clean surface for gluing it back on. You can see that in pic #2 of the back plate - a lot of kerfing is still attached!

I think perhaps the kerfing wood was dry and that is why it tore instead of the glue releasing. Not sure why it should have been though. The instrument has been in its case in my basement and generally the relative humidity does no drop below 36% down there.

You can see the efforts of a local repair guy to tried to re-enforce the top in 2008 with a popsicle-stick spruce brace glued under the bridge. This fix worked for about a year and then started to fail just like before without it. At that time I did not have much experience with repairing instruments and did not question his choice to use a thin wide brace. It was obviously the wrong fix.

This time I'll add a much stronger brace. The "popsicle brace" is really glued in well -- I'm sure how he did that without removing the back. I am thinking about just fitting my new brace over top of the old one?

Right now I'm thinking the new brace will look much like original brace under the sound hole just longer.

BTW the FON is 4246 0 (possibly A246 0) and the SN is 94279

[Jim Adwell]

This time I'll add a much stronger brace. The "popsicle brace" is really glued in well -- I'm sure how he did that without removing the back. I am thinking about just fitting my new brace over top of the old one?

Right now I'm thinking the new brace will look much like original brace under the sound hole just longer.

I would try to remove the brace, if possible, and put a whole new brace on in the same place, more or less. You can use the old brace as a template to shape the new brace to the top.

That brace near the sound hole looks exactly like the braces I have always made, with a very few exceptions. Can you guess what power tool I use to shape them?

[Bernie Daniel]

I would try to remove the brace, if possible, and put a whole new brace on in the same place, more or less. You can use the old brace as a template to shape the new brace to the top.

That brace near the sound hole looks exactly like the braces I have always made, with a very few exceptions. Can you guess what power tool I use to shape them?

I probably should be able to guess but right now I am coming up blank -- but please let me know because it the end I have to make one too!!

BTW how would you best choose to remove that old bridge if you did? It is heavily glued down with what looks to be HHG. Not sure it would be much of a template. It looks to me like the reason it was left so thin is that the repair guy fitted it to the top with pressure -- that is bent it to the top contour not carved it in?

If I were to take it off I was considering just sanding it off as one option? Maybe just stick a hot palette knife under it?

[Bernie Daniel]

Hey Jim, this morning when I looked at that transverse brace again I'm going to say you make them with a small drum sander on your drill press!

[Jim Adwell]

Hey Jim, this morning when I looked at that transverse brace again I'm going to say you make them with a small drum sander on your drill press!

Close, I make them with my belt sander. The curves at the end are the same diameter as the drums on the belt sander.

[Bernie Daniel]

Close, I make them with my belt sander. The curves at the end are the same diameter as the drums on the belt sander.

Very convenient. Well I'm glad you brought the topic up because now I'm going to shape mine on a drum sander on the drill press.

What would be the best strategy for removing the old brace do you think?