Bridge placement - confusion ;)

[sorengyllstrom]

Hi

I'm about to glue the bridge on my first guitar. I've read about measuring etc. and as far as I understand, the placement for the bridge can be calculated by taking the length from nut to 12th fret and then take the same distance plus a bit, and thats where the bridge should be located. (Pictures attached)

However, I've made a very raw jig (like a clamped on wooden tailpiece in order to check intonation before glueing. The place where the unfretted string is in tune with the same string fretted on the 12th is a bit further from the soundhole. (See picture in comments)

Which way should I go? I'm leaning towards where I can hear that its intonation correct.

Best regards
Søren

[sorengyllstrom]

Jig

[tippie53]

here is a video on this.
I don't go down the fret board like you did I do it follow the line of the string
so check out this video
https://www.youtube.com/watch?v=EJG5Frbu78c
You are in the ball park. The key is this you can make adjustments called compensation to the saddle
once you set up. A few weeks after you string up the guitar you can make the final tweaks as your guitar will settle
in , that means your top rises from the string loan and your bridge will rotate a little.
I would set it up
with about 2mm compensation length to the center of the saddle slot.
That will give you some adjustment to set the intonation.

[ruby@magpage.com]

As amateurs, few of us have the ability to hear the octaves at the 12th fret accurately enough to pinpoint intonation properly. The experts use a sound analyzer to locate the spot where the saddle should be, a little different for each string (look at an electric guitar bridge). And some knowledgeable players even want some of the strings to be off of "correct" by a little bit. And intonation changes with weight of strings, action height, and aggressiveness of play - so what hope do we have of getting it right?

We as budding luthiers have a lot to worry about, so perhaps doing what the big guys do and just locating the bridge by calculation would be close enough. Stew Mac has a fret locator that gives calculated saddle location:

https://www.stewmac.com/FretCalculator.html

Put in "19 frets", "24.9" and "acoustic guitar", and you get all the fret locations measured from the face of the nut for that scale. At the bottom you see "bridge placement". The high e string is at .87 inch longer than 24.9, and the low E is .21 longer. Both of these numbers have an allowable error of .030. So if you use .1 inch longer for the high e and rely in the typical .125 inch slant of the saddle to the CENTER of the saddle, you get very close to the calculated measurement.

Here is a jig I made in about 15 minutes that I have used for about 8 guitars now.

1) Piece of plywood the same taper as the fretboard

2) Glued on piece to hook it over the end of the fretboard

3) This one is for a 24.9 scale, so the 2 measurement are 25" high e and 25-1/8" low E. Since these are to the center of a rounded over saddle and I use 1/8" saddle blanks, the actual jig is 1/16" shorter than that. I hook it over the nut location and butt the bridge-with-saddle up to the end, check for square and centered, then drill my 2 outer pin holes with a 3/16" bit to locate the bridge. I do this before finishing so there is no fooling around later.

Be careful out there

Ed

[tippie53]

there is one issue that is often assumed and can come back and bite you in the but. Not all fret boards are accurate to the true martin scale length.
The one thing is your error become magnified as you go up the neck. One advantage is that as long as you can keep the percentage of error within the footprint of the SADDLE you will be fine.
I stopped using a jig for this a long time ago when I learned that even though the different guys call a fret board martin scale they are off a bit and it can influence the final intonation compensation.

I will use the jig for a rough in measurement but make adjustments to satisfy the players need.

here is a video on how I do it. Again the key variables you face are
A how much top rise
B how much rotation of the bridge after settle in
C How well did you prep and set up the fret plane
One thing I do suggest to my students to learn and understand the subtle differences on adjusting intonation compensation is to make a mock up. Get a fret board , and glue that on a trued piece of wood , use a filler under the fretboard so you have about 3/4 in off the top of this board . Glue a bridge on a board so the thickness of this board set the top of the bride about 1/8 under the fret board plane. This way you can shim it up and see how the intonation is affected but the varying action heights.
This will help you see the cause and effect relation ship to action height string gauge etc. You only need a few frets say at 1 5 9 12 and 15 if you can dial in good intonation at those 5 places and do it consistently you will have mastered saddle and bridge set up.

https://www.youtube.com/watch?v=EJG5Frbu78c
there are many ways to do a lot of guitar things but the saddle location is one that is based on physics.

hope you all find this helpful
Thanks Ed for posting what you have and they can see how your method will get them into the ball park.
There is no one perfect set up for every guitar. You have to learn the players preference. A blue grasser will need a bit more length as they often prefer higher actions than the requirement of the finger player. Also drop and open tuning can also play havoc on a standard intonation so having the fret plane block can help you understand what you need to do to get the best set up for that particular situation .

[ruby@magpage.com]

John is right of course - you should check the actual scale length of what you have without making assumptions. Measure to the center of the 12th fret slot from the face of the nut, then double it, and that is your scale length. However, the .1 inch additional intonation length for the high e, and then the 1/8" angle of the saddle to the Low E will stay the same for all of the scale lengths around 25" because of the .030 allowable error. You can see John uses them in his video.

Wash your hands

Ed

[MaineGeezer]

My last guitar I missed the initial saddle placement by at least 1/8". I had to fill the initial saddle slot in the bridge, extend the front of the bridge by adding a strip of wood, and recut the saddle slot. So I offer proof that even if you really mess it up you can probably recover.

Hopefully, you can hit it so the
pitch is perfect is in the middle of the saddle slot. Then, if need be you can bevel the saddle so the string makes contact on the front or back of the saddle if you need a final fine adjustment.

Don't get too hung up on measurements. Ultimately, the important thing is how it sounds, no matter what the measurements say. A high action will need a different saddle placement that a low action, because when you press down a high action string at the 12th fret the string will stretch and raise the pitch by a bit more than pressing down a low action string.

It's all part of the magic.

[sorengyllstrom]

Thanks for all your great pieces of advice. I think I'll think for a few days. The calculated placement of the bridge is not off by a few cents but way more. Whereas the strung-up placement is close to perfect intonation. Not sure if I trust the calculation on this one ;)

[MaineGeezer]

Can you show what you did for calculations? We may be able to figure out the cause of the discrepancy.

Duih...your first post has pictures of the measurements. Let's see... 32.4 cm from the nut to the 12th fret. 2X that is 64.8 cm, plus about 1/8" (3.175 mm) for the magical fudge factor gives you about 65.1 cm. So if the intonation is correct with the bridge located as shown in the picture, the saddle centerline in the center (left to right) of the saddle, that measurement is 65.4 cm. So the discrepancy between theoretical and actual is 3mm.

Where are the strings contacting the nut? On the front edge, or somewhat back? How high is the action at the 12th fret?

[Diane Kauffmds]

Mainegeezer is right. My calc came out to 65.1 cm too, looking at your excellent photos..

Lay your ruler against the nut, down the middle of the fretboard, right down the center of the bridge. 65.1cm will land in the center of the saddle slot. That's where your bridge will go. You want the bridge to be square to the fretboard.

Remember that your saddle slot is angled for compensation, so each string length is slightly different. When you make your saddle, you dial in intonation by filing the angle on your saddle. You can file slightly different angles for different strings to zero in the intonation.