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Flattening a warped Silver Bell resonator

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Dec 13, 2019 - 4:30:31 PM
8 posts since 12/24/2018

I’m working on cleaning up and putting back together a first year 17 fret B+D tenor banjo, and the resonator is badly out of flat. All of the later silver bell resonators I’ve seen are laminated or at least have a veneer on the outside, but this one is the early style. It’s solid all the way through (happens to be two flatsawn boards joined slightly off center) and most of it is about 1/4” thick except for the raised rings. There’s at least 1/4” of cup in it, and the flange is bent significantly as well, and the screw holes are torn out and split.

How would you go about restoring this resonator to its original shape? Weights and time? A go bar deck for a month? I’m also interested in suggestions for repairing the flange. I’m assuming replacement is out of the question as the round hole flanges don’t come up every day.


Dec 14, 2019 - 12:34:46 PM

6556 posts since 8/28/2013

The flange is probably repairable with careful brazing. It would have to be replated afterward, though.

I don't know about the wooden part. Fixing any warpage requires that the item be bent back beyond where it's straight due to the tendency for the wood to go back to where it was. It would be very hard to figure out how much overcompensation would be required, and it might take ten or twelve tries and still might not be flat enough to work. My own solution would be to have an entire new wooden piece turned by a really good lathe operator.

Dec 14, 2019 - 1:24:23 PM

12486 posts since 6/29/2005

A resonator like that is bound to warp (boat) unless you are lucky enough to live in  place where the humidity never fluctuates.  It's caused by the fact that the wood shrinks and expands across its width but not along its length.

It's probably what caused the flange to get "out of flat" and crack—the movement of the wood with humidity change is an unstoppable physical force like trees cracking concrete sidewalks.

I don't know of a foolproof permanent way to make the resonator stay flat. You may want to flatten the flange, get it into round, and silver solder the crack with high silver content silver solder, like 56%, which will match the nickel plating pretty well, but as has been said, the brazing will darken the area around the braze.

Then put the resonator back in there, but don't screw it in tightly because it will just pull the flange out of whack again.

Several years ago, I made a resonator for one of those, but did it 8-piece so it wouldn't "boat".

Dec 14, 2019 - 2:40:46 PM

286 posts since 5/29/2015

Heat the resonator to around 130 to 140 degrees and then clamp it flat. Let it cool. Or clamp it flat in the summer and leave it in your car for a week or two.

Dec 14, 2019 - 5:54:35 PM

6556 posts since 8/28/2013

As I stated earlier, (although I forgot to mention heating it first) you'll have to clamp beyond flat. When necks are straightened, they are always tweaked to an opposite curve, so that the tendency of the wood to return to its old curve is counteracted.

Ken is absolutely correct as to why these early B&D resonators warp, although this one is a rather extreme example. I'd guess that an experienced wood worker at the factory pointed out this"grain expansion," and the design was changed pretty early on.

If you do have a new wooden piece made, I'd suggest you refrain from absolute authenticity and use Ken's 8 piece idea, or at least use a laminated structure of some sort. The edges of the layers could probably be finished in such a manner as to make them nearly invisible, and you should have no future worries about your newly soldered, re-plated flange.

Dec 14, 2019 - 6:10:14 PM
Players Union Member



12127 posts since 8/30/2006

I would repair the flange and have a new plate turned. Community college project?

I think the wood plate is too small to bend back and beyond

Edited by - Helix on 12/14/2019 18:10:56

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