String tension for vintage Oahu lap-slide guitar

[Rodney Bates]

I recently acquired a vintage Oahu lap slide guitar, with very old strings on it. Can anyone give me an idea of the maximum total string tension I should string it with? I tend to like heavy strings for more pitch bend, but am wary of overstressing it.

It has a thinnish square neck and a nut raiser that looks like pot metal, and apparently
factory installed. The sound I can only describe as "primitive", but I do like it. The only
markings are "Oahu publishing company" and "Cleveland Ohio". No serial number, model number, etc. There is a patent number stamped on the metal bridge, but I will have to
take the strings off to read it.

I miked the top thickness at 0.122 inches near the edge of the sound hole. I don't know how to evaluate the bracing.

A photo is attached.

[Mark Swanson]

First, how do heavy strings give you more pitch bend? That is confusing to me.
Since you are not fretting any notes then you can use any gauge strings you like. That neck sounds like it's plenty strong, and as long as the top stays flat you are ok. Just check and make sure your bridge isn't lifting and the top doesn't belly or pull up, and even if it does a little you'll be fine.

[Greg Robinson]

Mark, with lap guitars, because you aren't bending side-to-side like on a fretted instrument, but instead are bending down (behind the slide, the range of the movement on a pedal-steel, or whatever other technique), the range of movement is limited. Because a heavier string tuned to the same pitch is closer to its elastic breaking point, you get a larger pitch change for a given range of movement.
I guess this is what Rodney is referring to.

Rodney, as Mark says, I'd be pretty comfortable putting any gauge string set on that, up to mediums.
Also, your photo is not attached, did you remember to click the "add the file" button after selecting "choose file"?

[Jeff Highland]

What tuning are you planning on using?
That is the major influence on string guage selection.

[Rodney Bates]

Yeah, getting more pitch change in a limiited space is what I meant about bending and heavy strings. Actually, all my intentional bending is sidewise, keeping contact (hopefully ) with the underside of the bar, but I still start to stretch the adjacent string a bit, in order to get a whole step. I am aware that heavy strings make for harder bending, i.e., more force required.

I have a Weissenborn I have strung with total 188 lbf, biased a few pounds heavier on the low strings and lighter on the high ones. I am the happiest with this of anything I have tried. I worry that that much tension would be abusing a vintage instrument.

I will initially tune (lo) D A D F# A D. These days, I am starting with target tension and then choosing string guage based on that and pitch.

I don't know what happened to the picture. I did click "add to file", and it took several minutes uploading. (My connection is a slow satellite one.) This before I clicked submit on the post.

[david frassetto]

Check the top bracing. I have two Oahu acoustic lappers and both have ladder bracing on the tops. Over the years they both have developed quite a belly bow and one is so bad that I am going to have the top pulled and replace the bracing with an X type of bracing. I tune them down to open D and use D'Addario 11's. This has kept them from getting worse. I used to use 12's and tune to open E.

[Rodney Bates]

OK, I did some homework. Assuming David's string sets are brass wound sets for standard tuned fretted guitar, using D'addario's tables and accounting for scale length difference, I get
129 pounds for David's older set/tuning and 96 pounds for the current one. Then I miked the strings on mine and got 152 pounds total. It shows no distortion of the top, and the bracing is ladder-style.

The person I bought it from had had it for over two years and not changed strings, so they are at least that old. It goes terribly out of tune almost instantly.

I guess I'll try maybe 140 pounds for now and watch it closely.