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I am building a j-200 sized jumbo out of Koa, and I am wondering on how thick my top should be, or a good starting point to go from. I guess my question is, is it like Spruce?, is it like Redwood?, is it like Cedar? Does it need to be thinner, or thicker. Maybe someone who has built a guitar out of Koa could give a few ideas on how Koa works or does not work as a top.

Thanks,

Tim

It depends on your particular top. I like to leave koa maybe 5 or 10% thicker than most spruces. While koa or mahogany tend to have more crossgrain stiffness, longitudinal stiffness is not as great as spruces. For a big guitar I wouldn't go under .125", but that's in conjunction with the way I brace, bridgeplate thickness etc.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Is this when deflection testing may be usefull? - I hope so, I've just started doing it with my Englemann build as tonally it seems to differ remarkably to Sitka and Eurospruce.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I just finished my second all koa guitar. I do deflection testing and targeted the same deflection as other tops. I ended up under 0.100", but I do build a pretty floppy top and them brace accordingly. When my tops are thinned to my deflection value they have a tap tone something like cardboard.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Thanks for the replies, I have not done any deflection tests, so I guess it's about time. Since I have no data to go from, what would be my target deflection if I was to use an 18" spred for the base, and what weight do I use? It would be great if some people could post some pictures of their set ups so I can have an idea of how to build one.

Thanks,

Tim

Can't find the link just now (video), but I remembered 0.200 inches with a 5 lb weight, 18 inch scan, 18 inch width of top from a video by a member of OLF
Average out both directions (and edges if measuring there like I have with a ruler - bit crude, but just a start)
I tended to go by feel and tap before with Sitka builds, but was confused by the different sounds of types of spruce, so started testing.
Remember to test side to side deflection too (14 inch span I believe-?), this can vary especially with hardwoods I understand. Can't say what the ratio is for "typical" boards,
Targets vary a lot according to build style/bracing preferences.
Target deflection will also vary according to the guitar size (width and length).
I am using this for a starting point on my current SJ build, and it turned out thickness was very similar to recent recommendation by Todd Stock for and SJ/OM build for this type of wood when he kindly stuck his neck out.
Searches for deflection testing will turn up lots of discussions and info, e.g. http://luthiersforum.com/forum/viewtopic.php?f=10101&t=30784&p=408321&hilit=deflection+testing#p408321 and pics of set-ups, including a multi - part one in the tutorials section http://luthiersforum.com/forum/viewtopic.php?f=10117&t=24469

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

From my experience Koa varies in stiffness alot from piece to piece. The ratio of stiffness from across the grain to along the grain varies alot too. For my approach the stiffness both directions matter, but the stiffness along the grain is most important for durability. I thickness my tops by feel, and tap tone, then do deflection tests.

For my Koa topped guitars, usually the Koa is stiffer across the grain/not as stiff along the grain than most Spruce. My final thickness varies, but I normally use a slightly larger bridgeplate, and spread the tone bars a little to compensate for the weaker strength along the grain.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Thanks again for the replies, this has given me a much needed starting point for building this guitar and for deflection testing in general.

Thanks,

Tim