Why not a wafer?

Discussion in 'Luthier's Corner [DB]' started by GretschWretch, Dec 12, 2021.


  1. GretschWretch

    GretschWretch Supporting Member

    Dec 27, 2013
    East Central Alabama
    There has been more than a little discussion recently about collapsed tops, occasionally with carved/hybrid basses but mostly laminated, and corrective measures often are prohibitively expensive. Which got me to wondering, why not support the top with an interior wafer?

    The National N850 electric bass and the KustomCraft Bone Buzzer both use internal wafers as their top-support brace, eschewing trestle bracing, soundposts, or the insufferable center block favored by semi-hollow electric basses. These wafers are about 3/16" thick and support only that part of the top that must resist direct down pressure from the bridge. Of course, on a DB one would have to be careful to not run afoul of the bassbar and soundpost, but a wafer running maybe an inch and a quarter in both directions from the center notches of the bridge should be all that would be required. And it certainly would bolster the top.

    I do not suggest this for carved or hybrid basses because the wafer would inhibit top vibration, but for plys I wonder if wafers might be worth considering.
     
  2. dhergert

    dhergert Gold Supporting Member

    Jan 17, 2018
    Blue Zone, California
    Did you mean something like this?

    full.jpg full.jpg
     
  3. GretschWretch

    GretschWretch Supporting Member

    Dec 27, 2013
    East Central Alabama
    Yes, only without the auxiliary soundpost and hatpeg supports. And for a wood bass, the "patch" would have to be glued down.
     
  4. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Often nice old carved basses develop problems and require a glued in belly patch which must be carefully chalk fit and blended in. That takes time and money.

    On plywood basses with significant problems, I’ve had to pull the top and do something similar but I straighten the best I can and glue an patch made from 1/8” birch plywood. Using a clamping caul with the proper arching it holds the shape and has held up over the few years I’ve done it. Traeger recommended spruce on plywood basses but also didn’t recommend much on basses with much deformation due to value considerations. Maybe Dr Condino will contribute to this.

    For plywood basses with a slight bump or where pulling the top wasn’t feasible, I’ve simply glued the patch in through the ff hole using thick hide glue or occasionally Titebond (for longer working time and hoping that any repair person would forgive the sin in the context of the other issues in the mess on his bench).
     

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  5. GretschWretch

    GretschWretch Supporting Member

    Dec 27, 2013
    East Central Alabama
    We're on the same page, but I was thinking more along the lines of, why not put in the patch as the bass is built? That ought to prevent tops having to be removed later for the repair.
     
  6. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Lots of plywood’s have them, although some Kays don’t. I’ve seen them on the back but not the top. I owned a KC Strings bass which had a proper carved in soundpost patch. Might be overkill. On carved basses, I think problems are due to overly thin tops.
     
  7. dhergert

    dhergert Gold Supporting Member

    Jan 17, 2018
    Blue Zone, California
    So, the background on this is that sometime before I owned it, my Alcoa had suffered a ~2" - ~3" top-tear running across the top just inside the treble F-hole. It was probably a result of some sort of accidental abuse, most likely involving a fallen sound post under continued normal string tension. If not addressed, this tear would eventually have traveled across the top and would have meant the end of this instrument. (Worth noting, this repair work was the reason that the access panel was installed in this Alcoa.)

    The person originally doing this welding repair didn't realize that the 1930s aluminum alloy had a much lower melting temperature compared to today's "normal" sheet aluminum. The resulting original efforts at welding repair work were a disaster which can be seen with the melted aluminum scars inside the DB. So, an aluminum plate was welded on the outside of the top, over the whole mess, and this repair, despite being a visual catastrophe, was technically successful...

    After significant cleanup, the whole DB was powder-coated, for the most part hiding the repair related destruction that had occurred. But, even before I owned this bass, it was noticed by the previous owner that the powder coat finish near the bridge was showing flexing marks along the welding of the aluminum plate. With that in mind, once I owned the bass, I sought a better way to support the top under bridge tension, spreading the support along the whole path of the bridge feet.

    Being aware of mandolin and violin design Virzi tone plates, I combined that concept with the sound post to produce both added resonance and wider support around the bridge. My original efforts with 6 wooden feet under the platform resulted in rattling; in testing I found 4 feet were much more efficient and self-balancing, eliminating the rattling. The assembly is fully adjustable for length (or height) at the sound post area as well as independently at the feet, and with further testing I found that increasing the length (and in doing so, the internal tension) dramatically improved volume and note clarity.

    This combined Virzi / sound-post prototype has been in place for over 3 years now. The instrument has been stable regarding past repairs and it continues to produce excellent volume and note clarity.

    Edit: It may be worth mentioning that using 4 independently sized feet for top support means that this approach does not require time consuming shaping of an appliance to the inside top of the instrument.
     
    Last edited: Dec 12, 2021
  8. Steven Ayres

    Steven Ayres Supporting Member

    Mar 11, 2007
    Northern Arizona
    Kay started with a transverse brace on the back supporting the soundpost on some gambas, and shortly whittled that down to a 3" disc covering the general area of the post position. I've found that often the disc isn't well positioned, and limits the post position. I've installed patches of both fine plywood and clear spruce to reinforce and mitigate distortion of the sound table. To the question of why not build it in, I agree with Greg's hint that this is best handled with proper thickness of the top. Proper fitting of the post — not too tight — can also reduce damage in ply basses.
     
    Greg Clinkingbeard likes this.
  9. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    I didn’t state this very clearly. I’ve occasionally seen basses which have the disc on the back but not on the top and basses with one on each plate. Never seen any other type of bracing on a Kay. Recently, I had a King with a short wedge on the back. It was about 2-3” long by maybe 1.5” wide (horizontal) and allowed a 90 degree cut on the post. There was nothing on the top and it had collapsed under the foot so badly that the inside layer of laminate had grain separation. It was close to having the foot go through the top. I fit a 7” x 4” patch through the ff hole and the arching stayed under tension.
     
  10. Steven Ayres

    Steven Ayres Supporting Member

    Mar 11, 2007
    Northern Arizona
    I'd be interested in hearing about your clamping strategy for that.
     
  11. stefaniw80401

    stefaniw80401 Supporting Member

    May 18, 2004
    Evergreen, Colorado
    Earlier this year, at my local luthier shop, I inspected a DB made by Maple Leaf Strings. All laminate with solid wood belly and back patches. They call it "hand graduated". That bass sounded great! I think this idea def needs to catch on with modern ply builders.

    https://www.mapleleafstrings.com/products/p/110b
     
  12. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    I’ll add pics later. If I’m fitting a small circular disc through the ff hole I may just use one C clamp in the middle of the disc and a clear acrylic caul on top right on top of the soundpost bump. For a larger one that requires reforming the arch, I have a both concave and convex cauls. For the King, I used the convex caul and lightly moistened and warmed the patch before clamping overnight. Using several clamps and viewing through the endpin hole I can confirm full contact underneath.
     
    stefaniw80401 likes this.
  13. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    The patch I used was a bit larger than the outline on the top. I use China marker which is easily removed with oil.
     

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