dobro string height vs tension question

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Brian Evans
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dobro string height vs tension question

Post by Brian Evans »

I have recently decided to play my round neck Gold Tone PBR-CA - quite a modern resonator designed by Paul Beard and made by Gold Tone - as a lap style instrument. I know that the tension of a typical resonator string set tuned to high G tuning (GBDGBD) is fully not recommended for a round neck instrument, but I have mitigated this by using quite light gauge strings for the GB pair (from a typical extra-light .010 - .047 set) and only a .014 for the high D string. All the rest are normal light gauge acoustic strings. So it occurs to me that by raising the string height with a nut extender and higher bridge saddles so that the strings are 1/4" or a little less off the fretboard, I've increased the leverage that the string tension has against the neck truss rod and the rotational torque around the neck joint. I calculate that my total string tension is around 167 lbs, and string height won't affect that - but the string height might affect what the impact to the guitar is. Anybody who still retains high school math skills care to try to quantify this for me? I can still add, but multiplication and division send me to a calculator, these days... :)

Edit: action height at the nut is 3/16", and 1/4" at the 12th fret, that would probably help...

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Peter Wilcox
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Re: dobro string height vs tension question

Post by Peter Wilcox »

I don't have the thinking skills, much less the math skills to deal with this, so I think you are over thinking this. :)

If it were me, as long as you're changing the saddles, make them flat rather than radiused, and whatever height they are now is still fine. If you raise the strings at the nut you should be OK up the fret board. The truss rod doesn't matter since the action is moot. I'd use the regular strings from a light set for the upper DGBD strings, and use the A and D strings from a heavy set (or buy single strings maybe .039 .049 ) for the lower G and B.

But that's just me. Others may have better thought out responses.
Maybe I can't fix it, but I can fix it so no one can fix it

Mike Conner
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Re: dobro string height vs tension question

Post by Mike Conner »

Brian: I play square neck dobro and have two instruments, a Wechter-Scheerhorn and a custom dreadnought size. Both are 25" scale. The scale length affects the string tension at pitch, so the 25" scale instruments have a lower string tension compared to a standard 25.4" scale for acoustic guitar. Your Gold Tone is a 25" scale.

The standard dobro tuning is GBDGBD, with the D of the first string one step below the high E on regular guitar tuning. leading to lower tension just due to pitch. (other tunings are fun also - I use a D tuning for some tunes DADF#AD and it's a blast for the keys of D and E). The thinner the strings, the lower the tension but also a thinner, "plinkier" tone with less energy to drive the cone for tone and volume. The lower tension using very light string gages at the dobro tuning does not support the bar as well, which could also lead to intonation challenges.

Standard dobro string sets use a 0.016" high D string. I use a "Scheerhorn" set with a 0.017" D and 0.019" B. The slightly heavier set does have somewhat higher tension, but the trade-off is a cleaner tone with stronger fundamental. (It is also harder to do behind-the-bar string bends ala Jerry Douglas).

My square neck strings are about 7/16" above the fretboard at the nut. Both of my instruments are "fretless" with maple inlay for fret position. You will need about this much clearance above your fretted board to allow for a dobro capo (I recommend the Scheerhorn style capo). It is common to capo at the 2nd fret to play in the key of A for bluegrass and fiddle tunes.

All of the above is more than you asked for ;-)
In the end, do you trust the strength of the neck joint on your Gold Tone? Hopefully your existing nut can be removed intact, and then you can install a new taller nut with no radius and with the string slots appropriate for some heavier strings. Just to clarify, raising the nut height does not affect the string tension since the string length and pitch are fixed, it probably just affects the force pulling on the neck joint. As Peter stated, you aren't really worried about the truss rod and action above the frets. (Nut extenders can mess up the intonation by moving the nut position towards the first fret. It's possible to compensate with bar position but it makes it trickier to stay in tune).

You will need new saddles with no radius also (I'm assuming your Gold Tone has split saddles with a gap for the cone tension screw). The typical 12" or 14" fretboard radius for a standard guitar is just about impossible to play in tune with a bar across the strings in dobro style.

Have fun experimenting! Fortunately, nut and saddle changes are easily reversed should you decide to switch back to standard guitar configuration.

All my babbling on above didn't address your string gage questions, but hope it helps you with some dobro information!

Mike Conner
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Re: dobro string height vs tension question

Post by Mike Conner »

Comparing standard guitar light gage to dobro strings:

0.056 0.045 (D'Addario EJ42))
Dobro: G B
up up
: :
0.053 0.042 0.032 0.024 0.016 0.012 (D'addario EXP16)
Std guitar E A D G B E
same same same :
Dobro: 0.035 0.026 0.018 down
D
0.016

Hope this helps

Brian Evans
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Re: dobro string height vs tension question

Post by Brian Evans »

Thanks guys. You'll note that in my original post I stated the tension from the strings I selected, so I really wasn't asking a tension question. I was more asking a bending stress question from the effect of the tension acting on the neck - the farther away from the center of the neck the strings are, the greater the bending stress, I surmise. And the rotational torque at the neck joint, ditto - farther the strings are away from the neck, the greater the torque. But I've pretty much decided I'm in the window of the design, which is a bolted neck joint (I think) so I'm not going to worry about it.

Mike Conner
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Re: dobro string height vs tension question

Post by Mike Conner »

Brian, seems like you have a path forward. Have fun!

(BTW, my attempt at a graphic on dobro vs guitar was a solid failure ;-)

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Peter Wilcox
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Re: dobro string height vs tension question

Post by Peter Wilcox »

Good luck. Just remember that square necks are built like brick shithouses for a reason! :)
Maybe I can't fix it, but I can fix it so no one can fix it

Mike Conner
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Re: dobro string height vs tension question

Post by Mike Conner »

Even the round neck resonator is closer to a speaker box than a typical acoustic guitar. The Gold Tone is likely laminated wood body. Probably work work out fine. Let us know Brian!

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