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Double Bass Ply bass top sinking, what is normal?

fdeck

fdeck

Supporting Member
Commercial User
Mar 20, 2004
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21,412
Madison WI
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HPF Technology LLC
#1
I have a ~ 1962 Kay M-1, which may qualify as the world's most beat-up Kay in regular use. It was discarded by my high school in 1982. My dad and I restored it, and it has held up since then. The repairs we had to make included making a new sound post, as the original one was gone. Also, the bass bar was cracked and partially detached. Oh, and the neck block was cracked. And there was no bridge. Other than that, it was in fine shape considering normal wear and tear for a school bass.

But the top is noticeably sunken under the treble bridge foot. I don't think it has subsided very much since we re-assembled it, otherwise I assume that I would have had to progressively raise the bridge.

My only question is: What would be conisdered normal for a plywood bass of this age and provenance?
 
#3
Is there a bulge on the back from the soundpost? My old plywood has a strong bump there and the top has sunk probably about the same amount over the years. In the year + I owned the Upton hybrid it developed a slight bulge and the top began to sink a bit.
I suspect this is a problem on many plywood basses as the back is much thinner in that spot than it is on a carved bass.
Traeger describes a method of flattening the bulge in his book.
Some Kay basses have a round patch back there, I believe to prevent that occurrence.
 
#4
Here are some pics taken with a straight stick of plywood for reference. In the upper picture, the gap that you see is roughly 4 to 5 mm at the widest.

Don't look at the "adjustable" bridge. Rather than install real bridge adjusters, I cut the bridge off at the knees, threaded in steel studs as dowels, and made those fixed thickness spacers out of aluminum sheet metal. My plan was to try different spacers until I was happy. This scheme has worked satisfactorily. Unfortunately it is one of those things where I said to myself: "If this works, I will go back and make it look nice." That was 15 years ago. :rollno:

Lacking the original soundpost, we had to guess as to the correct length, and we simply made it fit the reassembled bass with no force, as recommended in an old violin making book that we found. The square profile of the post is due to needing a way to secure it while we cut the ends to the desired angles using a compound radial arm saw. I think we got it right on the third or fourth try. The wood was a piece of spruce plank with very nice grain, that we think was left over from building the house.

The bulge on the back is barely noticeable. This Kay has the round patch.

bass1.jpg

bass2.jpg
 
#5
Most plywood basses are thin in middle section and are prone to deformation over time. Here is a photo of a belly patch performed on my plywood bass. I got the idea from Traegers book. I used a different method than him but same results. Thats a 1/8th piece of redwood glued down to the underside, then feathered in.

It can also improve the sound of the bass.

Matthew Tucker did a belly patch on his plywood bass not too long ago also.
 
#6
Wow. That looks like an undertaking.

In the process of adding the patch, do you attempt to also restore the curvature of the belly, or assume that the sag is permanent? About the thicknessing... so it's 1/8 inches thick in the middle, down to roughly zero at the edges?

Can the neck stay on when removing the belly? For my restoration, I removed the back, not knowing any better. It was not exactly a cake walk. I damaged and had to replace some portions of the inner ply.
 
#7
Fdeck, yr bridge looks a little too far north to me but maybe that's parallax error. Why do you use the pickup in the treble side bridge wing? usually the other side is most effective.

Yes the neck can stay on when you remove the belly.

Yes you need to try to restore the arching first. That's not always easy.

Here's the belly patch ctregan was referring to. I used pine, chalk-fitted, and probably added 1/4" in the middle. The bass has a great sound now.

1335010335_dae90620eb.jpg
 
#8
It is an undertaking.

I removed the top and made a counter caul or mold to fit the outside profile of the top. This supports and holds the top in position for clamping. You can use plater of paris with plastic wrapped around top plate to protect it.

Here is an example of a mold being used for a violin;

Invalid Link Removed

Here is a picture of my mold; I used Great Stuff insulating foam to fill up the box and produce a negative of the top plate. Then the whole thing went into a vacuum bag to suck down and glue the 1/8th redwood. The foam ended up being to squishy, but it worked.

Treager chalked fit a solid piece of spruce to his belly then used regular clamps, much like a bass bar.
 
#9
Matthew Tucker said:
Fdeck, yr bridge looks a little too far north to me but maybe that's parallax error. Why do you use the pickup in the treble side bridge wing? usually the other side is most effective.
Click to expand...

Thanks for noticing. The pickup is a work in process. I tried it on both sides. But you will notice that there is a little bit of wood glued to the bridge wing on the treble side, because this "Bausch" bridge has odd little curved wings that don't accommodate a pickup. I did the treble side first as a test, before committing to the bass side. There is also a Rev Solo pickup on the way. A new bridge will probably be the ultimate fix, once I figure out everything else.

I definitely want to get things right with regard to bridge and sound post placement. What are the official rules for the bridge feet relative to the f-hole notches, and the bass bar relative to the bridge? I realize these are variable due to tone considerations, but what would good starting points be?
 
#11
Thanks! Indeed, parallax made my bridge look out of whack in those pics, but it appears to be in the right place. I know that I looked up the rules when I reassembled the bass, but that was more than a quarter century ago.
 
#12
Hi all,

I wanted to share my experience with reshaping a Kay top, as I found it sorta, kinda, relatively easy. When I got the bass the top was really quite flattened around the bridge area. The lower half of the bass bar was hanging loose. I took the top off but couldn't see myself going to the extent of making a mold for the top (this was a $250 bass!).

Rather than making a mold, I decided to see if the plywood had any "memory" for how it was originally formed - and it did, big time! I suspended the top on all four sides using 2x2 pieces of wood around the perimeter (sort of like your pics CT). I left a couple of inches of free space between the finished side of the belly and the workbench top. Then I dampened the mid section with warm water, placed a large wet cotton pad over the area, piled assorted weights strategically in the area (to help it rebound), built a "tent" around the whole mess and blew hot air into it with a hair dryer!

After two or three days of fiddling with this arrangement, the top looked perfectly formed again. The bass bar was lying flat again and was able to be reglued. Okay, it wasn't elegant, but it worked! I had the bass for two years subsequently, and there weren't any signs of it collapsing during that time...

Cheers,

Paul (Eh_train)
 
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#14
Wow. If I only knew about that 25 years ago when the bass was open.

If only I knew that I was going to become a bassist.

My Kay also had the bass bar problem, plus a cracked neck block. Being ignorant of correct practice, I took off the back instead of the front in order to gain access. Now it's all back together again and I am loath to perform surgery for purely cosmetic reasons.

Related to a PM that I received, I wonder if I could progressively raise the sound post. My son's plywood 'cello (1/8 size CCC) has the reverse bulge and I suspect it might be due to the sound post crammed in there real good in order to make the instrument shippable.
 
#15
You could try to raise the top while still on the instrument - you don't really have anything to lose.

I might unstring it and then clamp that dent between an arched 2 x 4 (say) on the outside and something like a 2" x 12" x 1/4" spruce on the inside. Something rigid on the outside to bridge the sinkhole and something bendy on the inside that would progressively distribute the clamping force.

You'd want to try to move it in small increments over several days and some heat would be good too. Judicious use of a heat gun would be great, inside and out, before applying pressure.

Keep us posted. :)
 
#16
Jake deVilliers said:
You could try to raise the top while still on the instrument - you don't really have anything to lose.

I might unstring it and then clamp that dent between an arched 2 x 4 (say) on the outside and something like a 2" x 12" x 1/4" spruce on the inside. Something rigid on the outside to bridge the sinkhole and something bendy on the inside that would progressively distribute the clamping force.

You'd want to try to move it in small increments over several days and some heat would be good too. Judicious use of a heat gun would be great, inside and out, before applying pressure.

Keep us posted. :)
Click to expand...

I am too chicken! :eek: For better or worse, this bass is my player. The last time I worked on it was when I was 18 and stupid. My other bass is an electric. :rolleyes:

But I will have plenty of time to think about it, as the thermometer is pointing to the closing of my workshop until springtime. ;)

Has anybody thought of manufacturing ply basses with that patch in the middle from the git go?
 
#20
I wonder if I can reopen this 3.5 year old thread...

I'm working on an old '39 Kay. I've removed the top to correct a loose and somewhat stubborn bass bar and am in the process of trying this mold-free reshaping trick at the lower end of the bass bar where there's a little sink from the bass bar having been loose for so many years.

I'm also applying the technique to the soundpost area, where the bridge sink / soundpost bulge is not bad, but is noticeable. I'll report back how it goes.

A question for you guys, though - assuming the soundpost area reshaping goes well, would you recommend fitting a patch in the area, similar to what Matthew did earlier in this post?

It's certainly a lot of work. The sink/bulge in the (3-ply) top isn't drastic, but these older kays weren't designed for steel string tension so reinforcement might help for future. I was also thinking about a back patch for the same reason, though there's hardly any bulge there.

Would a patch like this lower the value of the bass? For the kay purists out there, this bass already has an ebony board on it, so it isn't totally stock anyway.

eh_train said:
Hi all,

I wanted to share my experience with reshaping a Kay top, as I found it sorta, kinda, relatively easy. When I got the bass the top was really quite flattened around the bridge area. The lower half of the bass bar was hanging loose. I took the top off but couldn't see myself going to the extent of making a mold for the top (this was a $250 bass!).

Rather than making a mold, I decided to see if the plywood had any "memory" for how it was originally formed - and it did, big time! I suspended the top on all four sides using 2x2 pieces of wood around the perimeter (sort of like your pics CT). I left a couple of inches of free space between the finished side of the belly and the workbench top. Then I dampened the mid section with warm water, placed a large wet cotton pad over the area, piled assorted weights strategically in the area (to help it rebound), built a "tent" around the whole mess and blew hot air into it with a hair dryer!

After two or three days of fiddling with this arrangement, the top looked perfectly formed again. The bass bar was lying flat again and was able to be reglued. Okay, it wasn't elegant, but it worked! I had the bass for two years subsequently, and there weren't any signs of it collapsing during that time...

Cheers,

Paul (Eh_train)
Click to expand...
 

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