Acoustic Gtr Q's.. on here?

[Sea Chief]

tedmich Can you do me a favour. Assuming yours there doesn't have a bowed-up top (ie, a problematically bowed-up top).

Assuming your action is good, a bit of neck relief, & your saddle is simply a normal height saddle (ie, the strings exit the pegs > up at an angle > lie over the saddle). Assuming your guitar is normal, it's playable, no nasties.

Ok can you look across the side of your top. A bit like my neck photo above. The same sort of plane.

Now, do you see a ' natural' slight convex uniform upward curve, with your bridge sitting upon the " crest " of it? Sitting @ the highest part of the top's curve?

OR.

Is your top dead-flat ?

I need to go back to my 1st post & get this question answered. I just can't progress unless I establish this.

Thanks, SC

[Helmholtz]

Yes, it is an assumption (as always when I say "looks like" as opposed to "is") based on the observation that the string height above the top seems to decrease towards the bridge.
Further assumption is that the guitar once played fine.

Of course there's also the possibility that the guitar left the factory with a slightly negative neck angle or the neck sitting too low in the dovetail (meaning it's a "lemon").

But a raised top is much more likely than a slipped neck joint.
Depending on initial string height at the bridge some guitars will tolerate more top up-bow than others.

[Sea Chief]

Hang on, hasn't a bridge got a slight natural curve profile to it's base... to match the typical top's curve?

If so, then there must be an " industry standard " top curve to mfr's acoustics, give or take a wee bit of course mfr to mfr.

I see also my bridge has a bit more height to the back of it, compared to the front too (not to be confused with a pulled-up bridge: this is easy to spot, & mine certainly hasn't got this at all).

So might I be right that a bridge actually has a fairly complex shape. Looks immediately like a rectangle of hardwood.

I wonder whether classical guitars, actually don't have this curve to the top: I seem to recall my bridge on mine had a flat top/ flat bridge, much simpler. Before someone stole it, so I can't go & check.

SC

[Helmholtz]

Hang on, hasn't a bridge got a slight natural curve profile to it's base... to match the typical top's curve?

New/replacement bridges for dreadnoughts have a flat bottom.

[Sea Chief]

Yes, it is an assumption (as always when I say "looks like" as opposed to "is") based on the observation that the string height above the top seems to decrease towards the bridge.
Further assumption is that the guitar once played fine.

Of course there's also the possibility that the guitar left the factory with a slightly negative neck angle or the neck sitting too low in the dovetail (meaning it's a "lemon").

But a raised top is much more likely than a slipped neck joint.
Depending on initial string height at the bridge some guitars will tolerate more top up-bow than others.

Well good point (the HH lemon theory). And we all did use to sniff at "made in Taiwan" after all. Especially back in the 80's. Mind you this one (79) looks nothing but well made on the whole, I mean as good as my 05 china takamine ( gs 220, a decent ply job).

This actually seems the most plausible theory, as I cannot pipoint anything.

Thanks, SC

[Sea Chief]

New/replacement bridges for dreadnoughts have a flat bottom.

Ah. Im back a step then. Unless they're thin & bend to fit the top profile. But hardwood won't bend even if so thin.. surely.

Right can you look across your top & do you see a slight uniform upward curve? Can you compare to a classical if you have one?

SC

[Helmholtz]

Judgement needs to be based on precise measurements and skilled evaluation of the instrument in question.

Consult a competent luthier as recommended earlier.
I'm tired of those endless discussions.

[Chuck H]

Most acoustic guitars bow up a little. If the guitar is already at the edge of adjustability WRT it's tolerances then even a millimeter or two more would be enough to ruin the action. This is as Helmholtz said. The guitars I've seen with bowed tops that render the instrument no longer able to be properly set up have all been plywood. It's most noticeable by the bridge seemingly tilted forward a little and the top above the sound hole may look like it's sunken. We do not have any clear pictures of this instrument for even a visual evaluation. And, also like Helmholtz said, measurements taken by a professional are the best way to know. Since you do not know and we cannot measure the instrument and since visual inspection falls short of definitive anyway it looks like we are at an impasse. No?

[Sea Chief]

New/replacement bridges for dreadnoughts have a flat bottom.

HH, sorry but I don't think this is true. It can't be if as Chuck says ( tallying with what I see on all 3 of my acoustics, all almost identical ) acoustic tops bow up a little. Have a look at yours, it isn't flat. Your classical one though, is.

SC

[Helmholtz]

Believe or not: The top as well as the bottom of the bridge base are perfectly flat without strings (at least when a guitar is new).
It's the string tension which pulls up the top behind the bridge (especially with a negative neck angle).
IIRC, total string pull with steel strings is around 70kg.

Classical guitar strings have much less pull than steel strings.

Nuff said.

[Sea Chief]

Most acoustic guitars bow up a little. If the guitar is already at the edge of adjustability WRT it's tolerances then even a millimeter or two more would be enough to ruin the action. This is as Helmholtz said. The guitars I've seen with bowed tops that render the instrument no longer able to be properly set up have all been plywood. It's most noticeable by the bridge seemingly tilted forward a little and the top above the sound hole may look like it's sunken. We do not have any clear pictures of this instrument for even a visual evaluation. And, also like Helmholtz said, measurements taken by a professional are the best way to know. Since you do not know and we cannot measure the instrument and since visual inspection falls short of definitive anyway it looks like we are at an impasse. No?

In the opening post I asked a question, you have just this minute answered it, it's just taken all these replies to get here.

Ideally if this could be concurred by others, heck even my thinking that classical tops -are- flat, then that would be useful.

See what you think here.. the pic taken bang on flat where the neck extends onto the guitar: you can see the top curve:



Thanks, SC

[Sea Chief]

Judgement needs to be based on precise measurements and skilled evaluation of the instrument in question.

Consult a competent luthier as recommended earlier.
I'm tired of those endless discussions.

Not for a £170 guitar HH. Ive said this, & that I cant possibly afford to send to a luthier, & that no luthier is around for hundreds of miles- Ive said this too.

I just wanted to clarify the acoustic top profile. One question. Maybe if someone could see something obvious from the photos too.. but nevermind.

SC

[Sea Chief]

Believe or not: The top as well as the bottom of the bridge base are perfectly flat without strings (at least when a guitar is new).
It's the string tension which pulls up the top behind the bridge (especially with a negative neck angle).
IIRC, total string pull with steel strings is around 70kg.

Classical guitar strings have much less pull than steel strings.

Nuff said.

So you're suggesting, are you, that the curve on an acoustic top ( although only Chuck has agreed they do have curved tops, but you haven't yet concurred) is wholly due to string tension then?

If so I can whip these strings off, they aren't light for sure/ no idea what guage. Put on some light 11-52's. But the fly in the soup is surely the fraction of neck relief I have, will diminish with less tension.

You may be right about the bridge being flat, as I can't see the excessive complication of mfr to make the base curved, very logical.

Ideally we need a luthier just to confirm this is uniformly true. If so, then flat bridges are glued to curved tops ( if as me & Chuck think acoustics do have curved tops. ). Now this, I think you might agree, seems odd. And with a stiff ply top... I can't see feasable the top is pushed flat here for glueing it on, or, the bridge bends a fraction to suit the profile, but this seems too seems unfeasable as a huge pressure surely needed, & ebony for eg is so hard how could it bend? And the glue to keep it flat too??

More connundrums abound. SC

[Helmholtz]

So you're suggesting, are you, that the curve on an acoustic top is wholly due to string tension then?

Yes, solely (except for archtops).
Imagine the weight of person pulling on the bridge.

If so, then flat bridges are glued to curved tops

Nope.

[Chuck H]

Ok... You mentioned the model number of the guitar but I never looked it up. I had assumed this was a steel string guitar until this last page when you mentioned it's a classical guitar (there are no pictures showing the bridge face on or the headstock). Then I thought "Eh, still the same problem." But then Helmholts mentioned the greater string tension of steel strings.

BING! BING! BING! BING! BING! BING! BING! BING! BING! BING! BING! BING!

In post n8 I can clearly see you have steel strings on this instrument. It could well be the extra tension that is the problem. What I'm wondering now is how YOU, who are holding the guitar in your hands and reading along with this thread, hadn't put that together at least by post Projects where Helmholtz mentions string tension. I actually would have thought you knew the lighter tension of nylon strings already just because your a guitar player. have never owned a classical or flamenco guitar and I knew it. Why did this thread even happen???

Put some nylon strings on it. Come back if it's still broken.