back plate and upper rib angle

[Adrian Levi]

Firstly happy thanksgiving to all the American folk on the site.

I have a question that maybe someone can answer ...

I am almost ready to put the back(round-back) onto my French bass after replacing the linings . Now the upper ribs have been cut back slightly , I think that this was simply part of the standard building plans and not done at a later stage in the basses life .
When I clamp the back plate on, the plate will have to bend to meet the ribs on the upper bout seeing that the back plate inside edging is ruler flat.The widest part of the gap is about 20mm.
Its easy to simply glue and clamp the plate back on BUT surely its not a good thing to have the upper back bout pulling against the neck block etc . It wont come off but Im worried about the seemingly unnecessary tension on the instrument. Any insight would be great ..:)

[Adrian Levi]

I found this pic .Is this the way to go perhaps , cutting an angle into the back plate ? The angle on my bass is not quite so steep.

[Thomas Erickson]

So is your back bent (like the one in your photo) and it just isn't a steep enough angle to mate up? Or was it made flat? I'm wondering how it was attached before... or was it? :confused:

[Adrian Levi]

So is your back bent (like the one in your photo) and it just isn't a steep enough angle to mate up? Or was it made flat? I'm wondering how it was attached before... or was it? :confused:



Thomas yes the bass was built like that , the back is not bent (cut) to the angle of the upper bout ribs ,but it does not require any major torque(due to the flexibility of the thinnish maple back) to get it flush with the ribbing and the button . Once glued properly especially to the back of the neck heel it will easily stay in place and be a perfect 'visual' fit.... BUT there will always be that bit of pressure pulling away from the heel and thats what I'm interested in / can this actually inhibit the sonic potential of the instrument ....???

My understanding is that an instrument does not want to have any unnecessary structural tension related to it's build ?!?

[Adrian Levi]

Happy Thanksgiving Adrian

you are putting the same back back on? yes?

have you intalled the front plate already, or was it already on and you never took it off.

if you are putting the same back plate back on and it is not fitting the ribs --it seems reasonable to think that:

the front plate was installed incorrectly- or adding new edges or shaving the top ribs, or placing the top too far back thus raising the upper ribs - all or any of which would throw off the back to ribs

I would check the top plate very closely first to make sure that the ribs are not drawn up for some reason

if the top is not correct, then I would remove and reglue

Maybe others will disagree, but I would start with the back plate in place first- I have only assembled one where both the back and front were off and gluing the back first worked out okay

take your time- you have a very good instrument there- well worth getting it totally right

the top would not have to be off much to cause a 20mm gap between the back and upper ribs. I would not want to spring it in. or force clamp it

The top is on and sitting flat and really nicely . I can sit the back on really well and its perfectly in line with rib outline but simply comes up away from the gluing surface of the upper ribs and the neck heel.

[Adrian Levi]

Adrian, sounds like to me that the back lost its shape a little while off. Can you reshape then glue it.

If the bass is laying on its front the weight of the neck will pull down the upper ribs unless the neck is supported. This will, of course, make a gab between the back and the upper ribs.

I would probably draw it down with clamps and let it set for a few days before I glued it. You will need to make sure that the neck projection height and alignment are correct before gluing.

Wayne,no neck pull as the neck is off and the top is perfectly in place with no tension at all ...

There are 3 possibilities
1. as you say , the back lost its shape
2. the ribs were cut down at some stage
3. the maker didn't mind 'springing the back'

I have attached pic with a torch shining in the gap to highlight the gap...

[Arnold Schnitzer]

Make sure the back makes good contact at the upper bout linings and neck block. Of course you want to size the blocks. Then glue the back on starting at the tailblock, and working your way up. (You'll need to keep it temporarily clamped at the button area to maintain alignment.) This method will spread the "spring", just like the original maker did. Hopefully, you have already flattened any obvious high spots, and made sure the glue surfaces are true. The small amount of spring in the back will actually tend to counteract the string tension, so don't be concerned.

[Adrian Levi]

Make sure the back makes good contact at the upper bout linings and neck block. Of course you want to size the blocks. Then glue the back on starting at the tailblock, and working your way up. (You'll need to keep it temporarily clamped at the button area to maintain alignment.) This method will spread the "spring", just like the original maker did. Hopefully, you have already flattened any obvious high spots, and made sure the glue surfaces are true. The small amount of spring in the back will actually tend to counteract the string tension, so don't be concerned.

Basically I'm almost ready to get the back on then , so a bit of spring is not a worry -great , thanks Arnold :)

[Adrian Levi]

Adrian, you are the cautious one, no doubt.

No more gap than what you have, i would glue it - will it not draw down fairly easily?

I had a seam pop open on a flat back and the back and there was a gap of about 2 inches. This was several years ago and it was a bass that I had made. I called the maker and he said to just glue and clamp it. It worked.

You don't have anything to be concerned about--from the picturre.

good to play it safe and ask some questions, however

If you are concerned about it you could add a couple of strip support(just a short piece of wood added to the ribon both sides)which will give you more to glue to. Probably not needed but it might make you feel better about it

You could also use rabbit hyde glue. I have been told that it is the best ie strongest- get it at International Violin, or let me know I can send you some

2 inches and I was worried about a few millimeters lol::) !! I just need to get everything as perfect as possible. I have so many basses lying around that have been badly 'repaired' that I never want to be someone that adds more messed up basses to an already messed up planet :D

[Adrian Levi]

Hey, where's my thanks?:) I've been up all night by your side with this

I was typing your thanks when you where typing your 'wheres my thanks post'

I was doing it in order of thread ...:)

BTW how did that 2'' sprung flat-back ,back sound and play when you were done with it ?

[Ken Smith]

Ok, I waited to reply and didn't see anyone mention this.

French basses are often built with a gradual bend from the upper corner block to the neck button as opposed to the obvious upper back bends as seen on many German, Viennese, Italian and English basses.

This is normal for French basses. Here are the links to 3 of my French basses (past and present) with gradual bends to look at and compare. It is quite common on French basses.
http://www.kensmithbasses.com/doublebasses/jacquet-gand/
http://www.kensmithbasses.com/doublebasses/vuillaume/
http://www.kensmithbasses.com/doublebasses/jacquet-pillement/

Follow what Arnold has told you. He is the most experienced restorer here on the Forum and knows his stuff. Thank's Arnold for chiming in.;)