Fixed Bridge Question

[Bernie Daniel]

I am putting a new fixed bridge on an '70's flat top that I converted into a mandocello some years ago. The old bridge was a copy of the Gibson ceramic adjustable type.

This is being done because the old bridge cracked. Plugging some of the guitar string pin holes and making new ones for the mandocello weakened it and the increased tension from mandocello strings caused it to fail.

So I made a new bridge plate last night from a rosewood block and deliberately made it larger and more robust. But of course it is significantly heavier (5/8 oz versus 1 1/4 oz). The new one will be a fixed bone saddle bridge.

I can take more wood off the new one but do I need to? Since it will be glued to the flat top does it matter as much how much it weighs?

[Big Joe]

Weight, in itself, is not an issue. You will have more mass and the saddle will be a better choice than ceramic so you will likely have a better overall tone. Just make sure the surface you glue the bridge on is clean and clear of all finish or other chemicals. You may want to score the finish that may be under the bridge very close to the edge, and then scrape the surface where you will glue the bridge. This will give you a clean surface ready to receive the new adhesive. That will give you a better bond and last longer. If you don't have a good clean, raw wood surface to bond with it will come loose when you least want it to.

[Bernie Daniel]

Thanks Joe! Yes I will have to clean off a little finish on the top because the new bridge is little larger in all dimensions -- I did this intentionally so the edge will look clean. Is aliphatic glue OK or would hot hide glue be better? I going to set up a hot glue pot in a few weeks anyway to glue on a fretboard. Appreciate the help!

[Big Joe]

Titebond is sufficient but good hide glue is just as good. It depends if you want to go to the work to deal with the hide glue. It smells like burning dirty sweat socks . Each adhesive has its benefits and deficiencies. You want maximum hold with the ability to easily release the adhesive if needed. Most manufacturers use Titebond or equivalent.

[kkmm]

You need to be very careful positioning the fixed bridge as it has major impact on the intonation.
And once you select string sizes, I would think it's better to stick with it.
My Ovation has fixed bridge and I wish it's floating !!!

[sunburst]

...I made a new bridge plate last night from a rosewood block and deliberately made it larger and more robust...

That sounds like a good idea, but I'm not sure I'm on the same page here. The bridge plate is inside the guitar ('cello), glued to the underside of the top. A robust, rosewood (Honduran, Brazilian, any rosewood harder than East Indian) bridge plate would help keep the new bridge from cracking.
As you continued, however, it sounded like you were talking about the bridge, not the bridge plate. (?)

Anyway, I suggest hide glue for gluing the new bridge, and it is very important to prepare the gluing surfaces immediately before gluing, especially the rosewood (within 15 minutes, optimally).

There is some optimal weight for the bridge, depending on what you want it to sound like, but we can't know what that optimal weight is, so I'd just make sure it is strong enough.

[Bernie Daniel]

That sounds like a good idea, but I'm not sure I'm on the same page here. The bridge plate is inside the guitar ('cello), glued to the underside of the top. A robust, rosewood (Honduran, Brazilian, any rosewood harder than East Indian) bridge plate would help keep the new bridge from cracking.
As you continued, however, it sounded like you were talking about the bridge, not the bridge plate. (?)

Anyway, I suggest hide glue for gluing the new bridge, and it is very important to prepare the gluing surfaces immediately before gluing, especially the rosewood (within 15 minutes, optimally).

There is some optimal weight for the bridge, depending on what you want it to sound like, but we can't know what that optimal weight is, so I'd just make sure it is strong enough.

My apologies! Yes you are correct my conversation is about the bridge base not the bridge plate -- I'll get this terminology right some time -- I hope.

I did make a new bridge plate out of maple when I did the initial conversion and I will reuse that this time. I am replacing the screws that went into through the base and into the plate with small machine bolts with washers. I probably will wait and use hide glue as I want to get some experience with it before I glue the narrowed down fretboard on. Thanks.

[Bernie Daniel]

You need to be very careful positioning the fixed bridge as it has major impact on the intonation.
And once you select string sizes, I would think it's better to stick with it.
My Ovation has fixed bridge and I wish it's floating !!!

The bridge will such that the bone saddle is exactly at 25 inches as it is 12.5 inches from the 0th fret to the 12th fret. But I know this will be a compromise situation. I had thought about converting this to a floating bridge but I think it would look weird on flat top?

[ProfChris]

You probably need it a little further away than the 0th to 12th fret distance to provide some compensation. Others here will advise how much, but a little is always needed.

[Bernie Daniel]

You probably need it a little further away than the 0th to 12th fret distance to provide some compensation. Others here will advise how much, but a little is always needed.

Thanks for the comment. I am making the center of the saddle 25" or 2 times the 0th to 12th fret. The treble end of the saddle will be a little less and the bass side a little more -- so there will be "compensation" --- but it will not be adjustable so I will have to live with what I end up with.

[John Arnold]

Yes you are correct my conversation is about the bridge base not the bridge plate -- I'll get this terminology right some time -- I hope.

Just call it the bridge. The bone part is the saddle (I know, it is counter intuitive).

I am making the center of the saddle 25" or 2 times the 0th to 12th fret. The treble end of the saddle will be a little less and the bass side a little more -- so there will be "compensation" --- but it will not be adjustable so I will have to live with what I end up with.

If you do that, you will end up with sharp intonation across the board. You need to add at least 1/16" to the 25" measurement on the first string, which is the shortest.

[Bernie Daniel]

Just call it the bridge. The bone part is the saddle (I know, it is counter intuitive).

If you do that, you will end up with sharp intonation across the board. You need to add at least 1/16" to the 25" measurement on the first string, which is the shortest.

Ok thanks for the input. Just to be clear cause once it is glued..........

The "original scale length of the guitar was 25" scale with -- 12.5" from the 0th fret to the 12th fret and the same again from the 12th to the bridge (measured down the middle of the fret board).

When I glue the new bridge on you recommend that:

the 0th fret to saddle on the A course side be 25 1/16"?

This will make the 0th fret to C-course side of the saddle 25 1/8" because the bone saddle is slanted on the rosewood bridge.

Do I have the right?

[ProfChris]

Yes, that's how compensation works. I don't have the numbers because I only build ukes, but 1/16 treble side, 1/8 bass side sounds about right.

The reason it's needed is that when you fret the string you stretch it - thus it plays fractionally sharp. To compensate for this the saddle is moved towards the tail very slightly. Different guages of string stretch differently, so ideally required different compensation,so with a fixed bridge you set it at "thereabouts".

[Bernie Daniel]

Yes, that's how compensation works. I don't have the numbers because I only build ukes, but 1/16 treble side, 1/8 bass side sounds about right.

The reason it's needed is that when you fret the string you stretch it - thus it plays fractionally sharp. To compensate for this the saddle is moved towards the tail very slightly. Different guages of string stretch differently, so ideally required different compensation,so with a fixed bridge you set it at "thereabouts".

Yes I kind of understand that part.

But if you look at the bridge base that I made you will see that it is already "compensated in that the 0th fret to 12th fret distance = 12th fret to saddle in the middle of the fretboard.

But because of the slant of the bone saddle (relative to the edge of the bridge) it will be longer on the bass side and shorter on the treble side already. So the bridge was designed with compensation built in.

I guess the question we are dealing with here is deciding exactly how MUCH compensation one needs?

I have heard that for the mandocello since all strings are wound that generally there is a linear decrease in scale length from bass to treble. On ukes you have the reverse -- none are wound!

Anyway I am going to glue the plate such that I get the 1/8 to 1/16 compensation that has been recommended. Thanks for the help!

I'm going to glue it tomorrow and then leave it clamped for the five weeks we are in the motorhome -- I'll guess the glue will be dry by then! But a mandocello at tune has close to 300 lbs tension so you want it to be bonded!

[Bernie Daniel]

Would you guys make the two mating surfaces sandpaper smooth before gluing? Or would it be better to rough them up to with a file or a rasp to give the glue something to "bite into"?

[ProfChris]

Bernie, compensation requires the 0-12th distance to be shorter than the 12th to saddle distance for *every* string. So your original plan to fix it with the 12th-saddle distance = 0-12th at the middle means *negative* compensation for the top strings. I think you're right to go with John Arnold's numbers. On a soprano uke (13.5 inch scale) I use about 1/8, with a 90 degree saddle, but the nylon strings are stretchier and the action is higher than on a mandolin.

So far as the glue joint is concerned you want a close mating surface, so don't rough it with a file. 120 grit sandpaper or thereabouts is where I stop.

[Bernie Daniel]

Bernie, compensation requires the 0-12th distance to be shorter than the 12th to saddle distance for *every* string. So your original plan to fix it with the 12th-saddle distance = 0-12th at the middle means *negative* compensation for the top strings. I think you're right to go with John Arnold's numbers. On a soprano uke (13.5 inch scale) I use about 1/8, with a 90 degree saddle, but the nylon strings are stretchier and the action is higher than on a mandolin.

So far as the glue joint is concerned you want a close mating surface, so don't rough it with a file. 120 grit sandpaper or thereabouts is where I stop.

Understood! Thanks. I had a bit of a logic breakdown in how I was describing it anyway. I think I've measured it about a dozen times now!

On the gluing surface preparation I was thinking about how to get paint to stick on plastic or you have to rough the surface a bit but this is a bit different in that the glue is between two wood surfaces. So I'll sand them nice clean then make sure all the dust is gone. I will have three deep throated clamps on so should be in good shape in getting to well mated up.

[kkmm]

I had thought about converting this to a floating bridge but I think it would look weird on flat top?

Maybe you confuse floating bridge and adjustable bridge. Flat top can have floating bridges and they usually do, but the saddle height is usually NOT adjustable for flat top. Yesterday, I just sanded a floating bridge (one piece) for my flat top to lower string height. Floating means you can move its position on the sound board, it's not glued in place.
The Ovation mando has a glued in place bridge(like a guitar) !!! It's hard to adjust intonation with a glued in bridge.

[Graham McDonald]

Bernie,

I would put the front of the saddle slot at least 25 3/32" from the nut, and preferably use a 3/16" wide saddle (but it might be too late for that). With 3/32" additional scale length, you might still find the optimum contact point is towards the back of the saddle.

cheers

graham

[martinedwards]

I made a bridge spotter jig. it hangs on the end of a guitar and then I run the two E strings through it and hold the bridge in place like a floater. this allows me to slither it about until the intonation is spot on before glueing it in place.

saves mistakes in measurement.....

here it is on an acoustic explorer shaped bouzouki.

really!!

[Bernie Daniel]

Maybe you confuse floating bridge and adjustable bridge. Flat top can have floating bridges and they usually do, but the saddle height is usually NOT adjustable for flat top. Yesterday, I just sanded a floating bridge (one piece) for my flat top to lower string height. Floating means you can move its position on the sound board, it's not glued in place.
The Ovation mando has a glued in place bridge(like a guitar) !!! It's hard to adjust intonation with a glued in bridge.

Hmmmm. After many years I have become pretty familiar with the terms fixed, floating and adjustable as to guitar and mandolin bridges -- and what is glued, what is adjustable and what is not -- so no I am not confused at all thanks!

As to the idea that flat top guitars usually having floating bridges? No I don't think so -- in fact I would say in my experience the opposite is true and the vast majority of flat top guitars do not have floating bridges.

I just gone putting on the hot hide glue and clamping up a fixed bridge on a flat top tonight.

I would like to see a picture of your floating flattop bridge though -- for a flat top mandocello a floating bridge might be a great idea. Thanks.

[Bernie Daniel]

Bernie,

I would put the front of the saddle slot at least 25 3/32" from the nut, and preferably use a 3/16" wide saddle (but it might be too late for that). With 3/32" additional scale length, you might still find the optimum contact point is towards the back of the saddle.

cheers

graham

Thanks Graham -- I ended up with between 1/8" and 3/16" more on the 12 to saddle distance compared to the 0th to 12th. If I need more to get compensation I could do like you suggest and widen the saddle slot and put in a thicker piece of bone. Glued it up tonight actually -- first time working with hot hide glue -- definitely an art not a science!!!!!!!!!!

[Bernie Daniel]

I made a bridge spotter jig. it hangs on the end of a guitar and then I run the two E strings through it and hold the bridge in place like a floater. this allows me to slither it about until the intonation is spot on before glueing it in place.

saves mistakes in measurement.....

here it is on an acoustic explorer shaped bouzouki.

really!!

That is really a great idea!

How have you ever used a floating bridge on a flat top as was suggested by kkmm in post #18 above?