I'm building a rezo 6 string cbg, have question about the intonation. On all pic's of rezo's I see the bridge biscuit it installed 90 % to the strings, how can that work, and have the right intonation.
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when you look at a FRETTED strings instrument you'll almost always find the bridge installed at some kind of angle and/or carved into a 'zig-zag' kinda shape which makes the strings slightly different lengths from each other.
You'll find that the fatter a particular string is, the more its part of the saddle is set back. This is because of the diameter (cross section) of the string. You see the mathematics you applied to calculate where the frets all go is focused on the centre of the string's cross section, which is where it's oscillations will originate. When you push a fatter string into a fret it stretches more than a thinner one would, hence the set back any the saddle, this is to help the fretted noted sound in tune by increasing the real/actual scale length on that string so that each fret is actually a tiny bit short (flat) from where it ought be for that length, compensating for that stretch. (and so this 'slanting' of the bridge is known as compensating, or a compensated bridge..)
Now this is only for fretted notes, and that's where national and dobro guitars and their clones become interesting. We often forget it now, but in the 30s/40s square necked models outsold regular guitar necked ones, these guitars were mostly used (Hawaiian / western swing / bluegrass style) without fretting...
And even if you had a round neck one, you'd probably be wanting to play with a slide as often as not, and voice chords across the slide, so compensating might be compromised, partway between sounding in tune for the frets and sounding in tune for the slide..
Most resonator guitars can intonate by rotating the biscuit/spider. So you can set the bass side back some, but only with the tradeoff of setting the opposite end forward equally if that makes sense.
Anyway, there you go. When you know how much sliding you're going to be doing then you work out how far to compensate the bridge. Another way to go is deliberately make a thick/wide (bone) saddle, and carve the compensation into it with a dremel after stringing it up. Its tedious, but does look and sound great, see my pics. Best :)