Take the back off?

[jerryk]

Hello,
I got my Saga kit put back together with the CA bridge and D'Addario strings, and
a TUSQ nut. Unfortunately, it sounds just like it did - except I think it is louder than
it was with the original Chinese stuff.

So I need to take the back off and do some tuning. As I said before, I put it together
with Original Titebond, so it shouldn't be that big a deal to get it off. Thinking about the details, I realize that I have a great many tools for applying heat to things. Most of them are probably the Wrong Tool:

* A hair dryer ( for drying hair )
* A heat gun - the kind you use to remove paint.
* A fine gas range in the kitchen.
* An electric oven
* Many soldering irons
* An Oxy-Acetylene welding torch
* A miniature Oxy-Acetylene welding torch
* A propane torch
* A McCullough steamer - the kind you use for cleaning and sanitizing
* A thermostatically controlled solder/desolder station for surface mount integrated circuits. This can produce hot air at 400 - 800 degrees F, chosen by a digital control

Right now, I'm thinking to get things generally warm with the air dryer, and then
hit the joint with the steamer.... What do the pros do?

[testore]

I think I'd buy another kit and make your next one. Not sure how many you've made but early instruments are just going to have issues.

[jerryk]

Well, out in Turlock I could just leave the instrument outside in the sun in August, and it would self-dissassemble .

Just getting another kit is surely an option. But I put a lot of work into this one, and don't really want to start over without
a fight.

[Marty Jacobson]

I also think you should just build another. I don't know what it is about the tone that you don't like, but I can't think of a way that removing a reasonable amount of material (which is what I assume you mean by "tuning") could make any significant improvement to the instrument.
But of the items you listed, the steamer is the least inadvisable of the bunch. It's going to make a mess, though. Read through the repairs on Frets.com and see how Frank Ford does that kind of work, that'll get you pointed in the right direction.

[testore]

I hope you didn't read any disrespect on my post Jerry, that wasn't my intention. Realize too that very often, you will be left with other repairs to do once you attempt fixing the initial flaws. Removing a back can cause ribs to split, edges to split, binding to split...... Who knows what else. I know you've put a lot of work into it. But I'll bet you'd have enough time fixing this one as it would take to get 60% through another one, and it will probably turn out better anyway. Just my thoughts.

[Dobe]

You forgot a light bulb as a heat source, assuming it's one of those 'old school' ones. 60W should be plenty. I'd agree with the 'just build another one' camp.

[bart mcneil]

It should not be a problem to remove the back and if he to learn then re-graduating the top and back are the way to go.

[David Houchens]

Get some thin pallet knives and heat them. I turn a clothes iron upside down in my vise and lay them on it to heat. Slowly working the pallet knives into the glue joint. Be really careful not to pry as its not that hard to split the back at the end grain. I've removed several backs this way. The neck block is usually a big pain.

[fscotte]

Another opinion - forget the heat. Just grab a razor and insert it at the button and begin. Slice through as much as you can and then switch to a heavier gauge blade or knife, and make follow the glueline between the rim and back. The back will "peel" right off, perhaps taking a few small slivers of rim as well, or it may not, won't know till you try it.

Once the back is off, glue some sandpaper to a flat surface and sand the bottom of the rim making it smooth and if you have to, sand enough off to get rid of any slivers of wood that may have peeled off the rim.

Sand the edge of the backplate as well and go to town with your graduations.

And next time, if you feel you may want to take the back off, use newspaper between the rim and plate so it comes off easier next time. I've got one experimental mandolin that has had 11 different backplates.

[Pete Jenner]

If you decide to use C4, loosen with steam first and don't use too much.
Warning: Eye and ear protection should always be worn when using explosives.

[jerryk]

OK, I got it almost separated. Everything but the neck joint area. I used the family whistling teakettle and some razor blades. Coming apart very nicely. It helps that the instrument is still white. The teakettle softens the glue and I just tap the razor blade along.

The neck joint is going to be a problem. It's a large area, and I can't really sproing the wood apart to urge it. I hesitate to heat
up the whole area because the back itself is two pieces glued together.

Time to feed my babies anyway. Come back to it later.

[jlatorre]

I usually use two or three putty knives in a pot of boiling water. I alternate them and rely on their heat and moisture to separate the parts.

Since you used Titebond, you must remove all traces of the glue when you're re-assembling the instrument, since Titebond doesn't want to stick to cured Titebond.

I am glad that you decided to take the mandolin apart, despite very good advice not to. When you put it back together after making your changes, you'll be able to actually hear what difference it makes. I think that you learn more from rebuilding instruments than building them, since you usually change only one thing at a time.

One more thing: when you re-assemble it, I recommend that you use hide glue or fish glue. That way, it will be easier to take apart and re-assemble, if you decide that you've learned a little about shaping the acoustics of the instrument but feel that repeating the process would be valuable.

[jerryk]

OK, it's off. Very minimal damage - just one little gouge up at the heel under the neck connection. And the back still fits
perfectly.

BTW, the problem was that the instrument had a resonance at high D - fourth fret on the A strings. It is so active at that frequency that the G and D strings keep producing that note for a LONG time after you leave it. If you are playing say a scale that continues
up the E string, that extended high D muddies everything up.

If I have to, I will remove the tone bars and put on new ones. I have a couple pieces of Englemann spruce that should be ideal.
And I just got a new fine blade for my Makita tabletop saw. And Santa has delivered a nice new 10" bandsaw...

So, as I understand it, I now have three resonant frequencies to set:

* The soundboard.
* The back.
* The Helmholtz frequency of the air chamber.

I was not getting any traction on this tap-tuning thing. What I really need is an
audio spectrum analyzer.... Hmm, maybe I have one. Sort of. The free Audacity
sound recorder has a spectrum analyzer feature.

The two sides of the soundboard have different sounds when I tap them with my
homemade tuning hammer ( a piece of plywood cut in the shape of a hammer, with
an extra piece glued to the "hammer" end to make it heavier, and a piece of thick
felt glued to the end ).

One side has two main peaks - one at 224Hz (A3) and a second at 488Hz(B4). The
B4 peak is about 5 db louder than the A3 peak.

The other side has a main peak at 450Hz(A4) and a second, smaller peak at 509Hz
(C5).

Siminoff recommends G# for the "bass" bar and A# for the "treble" bar for an F mandolin.
I'm guessing that an A mandolin is not that different as far as the sound-making
apparatus is concerned.

So right now I have 488Hz and 450Hz, whereas I should have 466Hz & 415Hz.

So they both need to come down - shave their tone bars.

...Does that sound about right?

[fscotte]

Oh dear...

This is a wonderful place for advice, but when it comes to direct advice on how to tune the plates, it doesn't come often. The tap tuning advice you read is one of those exceptions, but I would completely ignore it. It doesn't work. And it's mostly hogwash.

Deflection does work however. And when Tom Ellis spoke up a few years ago and said "deflection is the key", I took notice. He even gave some numbers he aims for. Spend some time building a deflection jig and forget everything you've read about tap tuning.