When is a flat top not a flat top ?

[HarrisonGibson]

Has anyone got the definative answer to this one? - At some point in the future I would like a J45 or similar but need to know:

 

When looking at acoustic guitars how much deviation should the top have from flat, I mean at what point does the slight arch on the top become an unstable bulge - whats the difference & how do you check it.

 

When buying a new "flat top" guitar should I accept a slight arch or am I buying something that over the years is going to rise up as the bridge takes a dive for the sound hole?

 

Rightly or wrongly I generally look across the top edge of the top from the playing possition and note that even if the bridge is raised compared with the edge of the guitar as long as its square with the edge and not on a tilt its probably ok - but I'm not sure!

How much lift should it have?

Do Gibson have a tolerance for it on a J45?

 

The best way to ask this question is probably as follows:

If you place a straight edge across the width of the body directly behind the bridge how much (if any) should the gap be at each edge?

 

Thanks folks look forward to some help on this !!

[eltonwce]

Gibson guitars like the J-45 are not really flat they are built with a slight arch to help project the sound.

[rar]

If you place a straight edge across the width of the body directly behind the bridge how much (if any) should the gap be at each edge?

 

Gibson tops have a 28' (= 336") radius. Hence, on a J-45 (which is 16" wide) the gap should be around

 

( 336 - (336^2 - 8^2)^0.5 ) = 0.1"

 

Of course, a little more when the top is under tension is not a problem.

 

-- Bob R

[Jinder]

This is always an interesting topic for me.

 

My SJ200 has a radius to the top, nothing to cause alarm, just a natural parabolic arch to the top. It's a 2003.

 

Lately there's a lot of talk about Gibson designing in a radius to their tops, but my last Hummingbird (a 2008) and my Blues King (built very late 2009, bought '10) both have completely flat tops. Not concave or anything silly, just dead flat. I know most folks will assume this is due to inadequate humidification, but I keep my flat under humidity control and often subject the guitars to humid, sweaty music venues, so I know they both lead stable lives with the occasional subjection to a gulp of juice.

 

Both guitars sound (or sounded, in the case of my recently departed Hummingbird) fantastic...no complaints there.

 

Interestingly, I have never met a J45 without a radiused top. Maybe the other models occasionally fly

under the radiusing radar at the factory, who knows?

[zombywoof]

As I understand it, the benefits of guitar with a radius in the top is it gives you a stiffer top which allows for the top and/or braces to be lighter and thinner.

 

There are those guitar builders though who claim a flattop gives you a slightly warmer sound.

[Buc McMaster]

I figure a flattop is a flattop. Meaning anything other than a true, pinched waist archtop guitar or a classical, nylon strung instrument. In my mind a modern build with a radiused top is still a flattop. A top that rises into a parabolic shape due to string tension is still a flattop. Flattop is a generic term to distinguish them from true archtops. I suppose that traditionally speaking, Gibson's jumbos and Martin's dreadnaughts are what are thought of as flattops. Heck, to me even an old parlor guitar is a flattop simply because it is not an archtop like an L48 or such. Just terminology, really.

[onewilyfool]

This Martin 1933 R-18, I call a "hybred" flat top, built in the same way as a flat top, but top is "bent" over curved braces (about 1/2") but body is same as 00-18. Sound is not archtop, nor flat top......somewhere in between.....VERY unique and beautiful sound...

 

 

 

[zombywoof]

This Martin 1933 R-18' date=' I call a "hybred" flat top, built in the same way as a flat top, but top is "bent" over curved braces (about 1/2") but body is same as 00-18. Sound is not archtop, nor flat top......somewhere in between.....VERY unique and beautiful sound...[/quote']

 

My very first guitar was a 1930s Martin R-18. A friend of the family had it laying around in their closet and gave it to me for like my 8th birthday ors something,

 

For the last couple of years I have been playing a mid-1930s Kay Kraft small body, round soundhole archtop. Also has a bent top and back. Like you said, these guitars don't fit into any mold in terms of tone. They don't sound like f-hole archtops or flattops. They seem to combine the midrange of an archtop with the fatness and woodiness of a flatttop.

 

Since you showed me yours I will give ya a turn around and show ya mine.

 

[onewilyfool]

My very first guitar was a 1930s Martin R-18. A friend of the family had it laying around in their closet and gave it to me for like my 8th birthday ors something' date='

 

For the last couple of years I have been playing a mid-1930s Kay Kraft small body, round soundhole archtop. Also has a bent top and back. Like you said, these guitars don't fit into any mold in terms of tone. They don't sound like f-hole archtops or flattops. They seem to combine the midrange of an archtop with the fatness and woodiness of a flatttop.

 

Since you showed me yours I will give ya a turn around and show ya mine.

 

[img']http://i52.photobucket.com/albums/g5/zombywoof51/Guitars/kay_K-2_Front-1.jpg[/img]

 

 

Zomby....nice