Fret Scale, Intonation, Bridge Location - What's the connection? - Forums

I have built two CBGs, one slide w/no frets and one with fretted fingerboard. I'm currently working on a a fretted model with my son.

I found, especially with the fretted one, to get the thing tuned up, the bridge location is not where it should be. It is built 25.5 inch scale, so the bridge should be at 25.5 inches from the nut. But to tune it properly, I had to move the bridge to about 25.75 inches at the 1st string and a bit longer at the 3rd string. Is this common? I know there is always some adjustment for intonation, but how do you design a guitar hand mount a bridge if its going to be off that much? I assume the only way is to string it up, tune it, then mark the final "tuned" bridge position, and then mount it. Or, is a floating bridge the way to go and hope the player understands this when he restrings the CBG later.

Replies

The Capo Kid November 21, 2013 at 10:48am

What type of bridge are you using ? , what's the action height ? , the higher the action the sharper the note will get as you head up the fretboard hence you're having to move the bridge back , post some pics that might help .
- Dale > The Capo Kid November 21, 2013 at 1:31pm
  
  I'm good with adjusting the variables until I find the magic set up on each build. Now I understand why its not just a simple measurement. Other factors are at play. I was just happy my first fretboard came out with playable accuracy, other than a couple of high frets - easily fixed..
  
  I just wonder how the masters get it right and construct their CBGs with a fixed bridge, fancy or decorative set ups, like one I saw that had a carved recessed cavity in the top surface and a bottle inlayed as the bridge. From all the explanations, no doubt there must be quite a bit of experimentation and plain ol' expertise and experience to craft something like that. I'll keep at it
  - Ron "Oily" Sprague > Dale November 21, 2013 at 2:47pm
    
    Yep. Lots of experimentation, some luck, and attention to detail. Like they say, it's just a stick in a box ;-)
The Phrygian Kid November 21, 2013 at 4:13am

It's very very difficult to formulate,(because there are many variables besides just individual string guage, eg fret height, playing action, even the individual players grip must be accounted for) so it pretty much must be done by ear. Another factor that many players and even builders are not aware of is that the string does not actually sound from its entire length anyway because of a factor called 'elasticity' a relatively stiff material like music wire needs a certain amount of length to transition into shaping a wave, it can't just occur at an arbitrary point like the contact with the saddle, so when you sound a note you're actually hearing a little less length of the string than you might think. Strange but true. (This is why even the thinnest strings still need some compensating)

The way it's done on flat tops etc is because that model has been prototyped and tweaked in the design phase. Also don't forget that a saddle can be carved away after it's set to play in tune, which is a little of a chore, but with a dremel you can buzz away a little bone one string at a time until it's just right in an hour or two.
darryl kernaghan November 21, 2013 at 3:15am

dale ,i,ve found a floating setup works for me,your intonation can be affected by many things including action height,particularly with a high action,the differnt string guages will sharpen or flatten at different rates ,so to compensate i generally set the middle string of a 3 er,then rotate bridge as needed,others set outside strings and middle should be pretty close,hope this helps,more advice coming from others who know more than me i,m sure
Ron "Oily" Sprague November 20, 2013 at 10:51pm

Dale,

Moving a bridge by 1/4" here in CBG land is definitely not uncommon. Angling the bridge to get reasonable intonation is also not uncommon; it is done on most commercial acoustic gits. Typically, when one sees a VSL of 25.5", that measurement only applies for a particular string diameter, usually either the 1st (thinnest), or the 6th (or last, thickest) string. If for the 1st string, you have to string it up, test open then at the 12th fret to make sure you're getting the same note, mark that location, then do the same procedure for your last, thickest string. You'll note (no pun intended) that the bridge position for the last thickest string is somewhat tailward compared to that for the first string. All other strings in between will be close, not perfect, in terms of intonation. Then there are adjustments for compensation, which are related to bridg and saddle height, as well as micro adjustments for intonation. You may have to try different saddle heights to get things closer to intonated. That's why most people use a floating bridge.
- Dale > Ron "Oily" Sprague November 20, 2013 at 11:41pm
  
  Thanks Professor Oily for learning me something new. I have observed what you're talking about and I think I understand the concepts. It is only through building CBG's and learning to play that I have been able to grasp music. Thanks to everyone here!