Dobro Mandolin's Internal Organs Exposed

[JimKo]

This is a follow up thread to a post in "Looking For Looking For Information on Mandolins " section. Thought it was more appropriate here.
Took the instrument apart. It was made in 81/82. The intonation had been sharp up the neck so, based on advise I had gotten, I slid the spider down to lengthen the scale. Looking back, probably could just have loosened strings and did that without taking it all apart. I wanted to lower the action by sanding the bridge bottom but it was in there pretty tight so i didn't mess with it. The instrument isn't mine. Cured about half the intonation problem. Now I might either tune it slightly flat so it is in tune when played up the neck. (Open stings can be really overrated.) If that doesn't work I might file the legs on the spider so that it will slide back a tad further. Actually had no parts left over when I put it back together. Amazing.

[Randolph]

Great selection of tools Thanks for showing the innards. I have a new National and now I have a better sense of how the insides work.

[MikeEdgerton]

The inside of the mandolin isn't all that different than the inside of the guitar. Sometimes searching for the more popular bigger brother can give you some insight. I do love it when we have pictures of the inards of an instrument shown. Thanks for posting this.

http://www.4stringbanjos.com/Restora...r/DobroTG.html

[Oliver A.]

I noticed that the saddle seems to have a chamfer at the top edge. If the angled part is currantly towards the tailpiece, you coud turn it around the other way to lengthen the scale. 'just asking. You probably already know that.

I understand the sledge hammer and the hunting knife but why then would you need a screw driver?

[JimKo]

Screwdriver? I was wondering what that tool was for.
Actually, I was thinking about turning the saddle around but it is in there pretty tight.

[Oliver A.]

Can you rotate the entire spider 180 degrees?

[pops1]

I don't think you should rotate the spider unless it was in backwards. The maple saddle should pry out without much effort, yes it is tight, but it will come out. In my past post I wanted you to move the cone, spider and all, back to improve intonation. Now that you have taken it apart I would not change dimensions of the spider. Also the tension of the screw that goes thru the spider to the center of the cone has a lot to do with the tone of the instrument. Tension on this screw is important to not have too much, but be tight enough. Once back together small adjustments will improve or worsen the tone.

[JimKo]

Oliver, I don't think it can be rotated 180 because of the limited opening in the hearts and moon top piece. I am not sure, but that might be too much of an adjustment too.

Pops, I will go back to work on removing the saddle. Have some new light strings on order and when they get here I will tear into it again. I won't mess with the spider dimensions. Appreciate you saving me from myself.

Thanks to both of you for the suggestions.

[John Kelly]

As the others have said, I love your precision sledge hammer and Bowie knife, Jim. The screwdriver is only for those who can't handle the real tools! Interesting pictures of the innards. Amazing the power that cone can generate.

[pops1]

Jim, I am not sure about the light strings. While flat wound strings may tame it down some, these are usually made for heavier strings. The heavier strings would put more pressure on the cone and lower it some making the action better and intonation better. I have had one of these mandolins from the 30's a long time ago, and worked on several of the national guitars and a dobro guitar or two, all used heavy strings.

[jhowell]

One fairly easy thing you can do while it is apart is to 'fit' the feet of the spyder. In guitar world, there are usually small casting abberations on the feet. Tape 220 to something really flat like a table saw wing and gently, with consciously even pressure, polish the feet. Also, make sure that the cone rim is flat by checking it against said flat surface. Stew-Mac has a pretty decent tutorial for setting up reso guitars...99% should apply to a reso mando. And as pops1 mentioned above, the heaviest strings you can fret usually give the best reso/slide tone.

[John Morton]

That saddle/spider configuration is something I've never seen - the saddle is off center by maybe 1/4". Is this something peculiar to mandolins? I believe the normal spider bridge sits on center, with an opening in the middle for the screw. In the latter case, rotating it 180 deg. produces no change except for whatever bias you put in the saddle slots.

If there is some way to change the offset, that would be the cure for any intonation problems beyond what you can correct by filing the saddle.

[pops1]

You are right John, I have never seen that either. It looks like it is cast into the spider tho so I don't think it will move. My dobro mandolin was in the center as you said and other dobros I have seen. Not sure if this is an 80's thing or not. Wonder if OP could get a spider that was centered it would definitely change the mandolin playing sharp.

[allenhopkins]

That saddle/spider configuration is something I've never seen - the saddle is off center by maybe 1/4". Is this something peculiar to mandolins? I believe the normal spider bridge sits on center, with an opening in the middle for the screw. In the latter case, rotating it 180 deg. produces no change except for whatever bias you put in the saddle slots...

I also noticed that the bridge doesn't sit right in the center of the spider. My '30's Dobro mandolin has a two-segment bridge saddle, with two string courses slotted on either side of a center gap. The gap accommodates the screw that goes through the spider and connects to the cone. Thus, the bridge is exactly centered on the spider and the cone -- allowing for "slippage" caused by Dobro's somewhat rough-and-ready quality control

This instrument has a flange that accommodates the screw, and a solid one-piece bridge saddle that's somewhat offset from the center. Therefore, if the bridge placement is slightly "sharp" (too close to the neck) as it is, reversing the spider would make the bridge significantly "flat," since the bridge would be placed quite a bit closer to the tailpiece.

The different bridge construction, I guess, is an "improvement" of the later (1980's) Dobro instruments. My suggested "cure" for the intonation problems would be to make a new bridge saddle, compensated and with its apex as close to the tailpiece as possible, as well as lowering the action (saddle and nut height adjustment) to as low as practicable, avoiding fret buzzing, to reduce the "stretching" effect of fretting higher-action strings.

That, and "squeezing" the spider toward the tailpiece, without reversing it. And, perhaps, learning to love slightly sharp intonation on the higher frets...

[John Morton]

I suspect this spider was made that way to correct for an error in scale length. Manufacturers often used mismatched parts to use up old stock - they probably had a bunch of these fingerboards and necks on the shelf, from other projects.

The two choices in this case are off center in opposite directions. I suggest stringing it up to measure the intonation in both positions, to get and idea where the saddle should be. If it wants to be between the two options, make a dummy saddle and prop it on the shelf that has the hole. If that brings it in, make a block with a groove to accept the saddle, and glue it in place.

[pops1]

Another thing I have done when it's not easy to adjust saddle enough is remove and re-position the fingerboard. Usually it doesn't take a whole lot and is fairly easy thing to do.

[JimKo]

Pretty tied up today but will post a better picture of the spider first chance I get. Sounds like the consensus is to use medium strings instead of lights on these newer models. Is that right?

[JimKo]

Just in case anyone is interested here are some closer shots of the spider.

[JimKo]

And another.

[pops1]

That is not like the spider's from older Dobro's. Yes use heavier strings.

[allenhopkins]

As you can see, the bridge doesn't sit in the center of the spider; it's offset, with an attached flange which accommodates the threaded bolt connecting the spider to the center of the resonator cone.

If the mandolin's designed the way the older Dobro mandolins were, the center of the spider was at the point where the bridge should be located for proper intonation. My '30's Dobro, as a mentioned above, has the bridge attached to the center of the spider, with a "split" bridge saddle -- one segment for the first two courses, the other for courses #3 and #4 -- with the connector bolt in the gap between the two halves of the saddle.

I assume that the makers of the later 1980's Dobro mandolins moved the sound well, resonator etc. closer to the tailpiece in the instrument's body, leaving the offset bridge in the proper "sweet spot" to intonate accurately. If not, and the body's geometry is unchanged from earlier instruments, the bridge is going to be too close to the neck, and the instrument will intonate sharp.

Measurement of the reassembled mandolin might provide a clue as to whether the bridge is installed closer to the neck than it should be. In most Dobro instruments, as discussed elsewhere, there's a certain amount of "slop" in positioning the spider in the sound well, so there may be room to finagle the spider closer to the tailpiece, if that's needed for proper intonation. However, there clearly was a design change between when my mandolin was made, and when this one was manufactured 50± years later.

[Mark Gunter]

I mess with resonator guitars and as of yet no resonator mandolins, and prefer spider bridges to biscuit style. This is the first picture I've ever seen of an offset saddle in a spider bridge. Thanks for opening up your mandolin and sharing pictures. I would have just assumed that all spider bridges have the saddle in the center before seeing your picture.

[michaelcj]

The bridge is off center from the center point of the spider, unlike a dobro where it is centered and split with the cone tension screw down the center between the two halves of the bridge.

Were it mine I would certainly install the spider at 180 degrees from where it was [or so that the bridge is behind the center tension screw] and string it up and see how the intonation is. Of course if it was originally behind the center screw and you were sharp, you've accomplished nothing. And if it was only a small bit sharp and ahead of the screw…. it may be more than you needed.

Only a matter of a bit of your time with a screwdriver and strings….. Nothing ventured, nothing gained.

The spider and cone don't care about their orientation as long as the bridge is perpendicular to the strings…. you can even fudge with a bit of a slant… bass back, treble forward, as I do with my round neck resonators to try and dial the intonation in much like the saddle slant on an acoustic guitar.

[JimKo]

Just thought I would kind of cap his off. I tried 3 adjustments that might have all had incremental effects to correct the situation. 1) Put on new heavier gauge strings, 2) installed a new, lower bridge, 3) slide the spider back as far a possible. I think the 3rd one had the most effect. When I started out the strings all measured about 4 bars sharp, at the 12th fret, on a black Snark tuner. (Yes, I know that isn't possible the most accurate tuning device on the face of the earth but you gotta dance with you brung you.) After adjustments, everything was about 2 bars sharp at the 12th. Most importantly it now sounds OK when playing out of D & E positions. Took it to a gig last night and it mics really well. I like playing it on the country blues type stuff. Sure was fun to work on and again thank to everyone for your ideas.

[Ginridge]

I recently restored a mid 30's Regal spider cone resonator. It also has the same intonation issue. Even by pulling the cone as far back as possible, the strings were sharp. After trying to file off a bit of the cone lip to allow it to go back further (didn't make enough difference), I hit upon a trick that worked great. I took my Dremel router and cut the tail end of the rim that the cone sits on back about 1/8 ". In this instance, it was just the amount needed to slide the cone far enough back to play in tune. The cover plate covers well beyond this minor adjustment in the rim edge. Seems like an elegant solution.