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Message 119
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| Subject: | Re: [RESOGUIT-L] bridge inserts | | Date: | Wednesday, September 28, 2005 14:36:38 (-0700) | | From: | Betty Wheeler <bettywheeler @.....com>
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Here is Ken Brown's post on replacing bridge inserts, originally sent
to the List on July 4, 2003.
--Betty
To: resoguit-l@topica.com
From: Ken Brown <kbrown@mail.utexas.edu>
Subject: [RESOGUIT-L] Changing bridge inserts: 16 easy steps
Date: Fri, 04 Jul 2003 22:08:29 -0500
At 10:17 AM 07/04/2003 -0400, you wrote:
"I am contemplating changing my bridge on my '76 Dobro to a maple
ebony topped bridge. Can anyone give me any assistance as to degree of
difficulty, tools needed and etc.?Is this something a novice should attempt?"
=> Dick Deneve should answer this instead of me, but I guess maybe he's at
the Dopyeras' right now, or something. So I'll jump in. I agree with much
of what Jim McNab has said, but I'll add a few things.
Yes, it's something a novice should attempt. Why? Because reso-luthiers
are scarce, and if you pick a dobro, you need to know how to do basic
repairs. Bridge inserts are often somewhat brittle, and when one chips,
you need to know how to fix it. Here are some observations, based on
personal experience.
1) The new insert needs to be fitted with a very tight friction fit. I
don't recommend using any glue because A) the glue might deaden the sound
and B) someday you'll probably want to get that insert out again after it
chips.
2) The new insert must make perfect contact with the base of the slot.
Even the tiniest air space underneath will cause one or more strings to go
dead.
3) Just about all the #14 spiders I've seen have slots that are not very
precisely milled. It's often the case that the bottom of the slot is
uneven, and the lower part of the slot may be slightly wider than the top
part. If you can find a machinist with a precision milling machine and get
him to mill it out to an even configuration, you're better off. It isn't
essential -- you can work around it -- but if you can manage it, you'll
have made a good improvement in that spider. If you mill the slot, you'll
widen it slightly, but bridge insert blanks are always extra-wide to start
with anyway, so it shouldn't be a problem as long as the widening isn't
overdone. If milling isn't feasible, then don't worry about it -- just use
it as is.
4) When working on the spider, make sure the center part is supported
underneath by a stable block of wood. In most (not all) spiders, the legs
extend slightly farther downward than the center part, so if you rest the
legs on a table and press on the center, you risk bending the legs. Always
support the center and keep the legs free.
5) I think you're better off buying unslotted inserts and cutting your own
slots, because then you have control over the string spacing and slot
width. If you do it this way, you fit the inserts into the spider first,
then slot them after fitting.
6) Before you disassemble your dobro and while it's still tuned to pitch,
look under the palmrest and measure (at least mentally) the clearance of
the strings under the palmrest. Do you have enough clearance? Do the
inserts need to be taller? Fitting your own inserts gives you the chance
to make any needed adjustments here.
7) Tap the old inserts out with a hammer and a small piece of wood. Try to
get 'em out intact. Use the old inserts as a guide to marking the blanks,
adjusting height if necessary. Now would also be a good time to check the
spider legs to make sure they're level. You check it by laying the spider
on a surface known to be absolutely flat (like a mirror) and tapping or
looking closely for any gaps.
8) You'll have to remove some material from both the sides and bottom of
the new insert. Cut or sand the extra material off the bottom of the
insert first. Make sure you leave a level, right-angled cut on the bottom.
The thickness is absolutely critical, because if the inserts move at all,
even the tiniest bit, when inserted in the slot, you've got to throw 'em
away and start all over. You thin the new insert by sanding it slowly and
carefully on fine grit sandpaper (perhaps glued to a flat surface). Sand
a bit, check the fit against the slot, then sand some more, reducing the
grit size as needed. Reverse the orientation from time to time, to make
sure you don't get one end thinner than the other. Stop sanding when the
insert will slide into the slot from the side, but only with a certain
amount of force. This is the most critical part of the whole operation,
and it's essential to be meticulous here.
9) Slide both newly thinned inserts into their slots, make sure the tops
line up level, then take a flat block of wood and place it on top of the
two inserts. Use a small hammer to lightly tap the block, ensuring that
the inserts are forced all the way to the base of the slot. Don't use
excessive force, and make sure the base of the spider is well supported,
but the legs are free.
10) The configuration the top edges of your newly fitted inserts depends
on your particular needs. If you have little clearance under the palmrest,
you may need to round off the outside corners. Tim Scheerhorn does this
his instruments, because they have very little clearance. He also rounds
off the top edge -- some luthiers taper theirs. It's up to you. Anyway, if
you decide to do it, that's the next step.
11) Now use the old inserts as a guide for marking where the new string
slots will be cut. When cutting the slots, as a general rule the wound
strings are buried only half their diameter in the top of the insert. The
two unwound strings are, or necessity, seated deeper. And if, like me, you
pick the treble D string pretty had and have trouble with the first string
jumping out of the slot, you may want to bury it a little extra. It
shouldn't be deeper than about two string diameters at the very most, usually.
12) As a general rule, the tops of the strings should be level across the
fretboard. That is, if you lay a straightedge (or your bar) transversely
across the strings, say above the 12th fret, you shouldn't have any really
big gaps. But fortunately, since we're dealing with a fretless instrument
here, you have quite a bit of latitude here. Normal bar pressure should
accomodate any slight variances in string height.
13) How to cut the slots? You can rough out the slots for the four unwound
strings with an Xacto knife and then saw the slot into the proper
configuration using an old string. I keep a set of four old strings (cut
down to one-foot lengths) on hand for this. When sawing, keep the end of
the string toward the nut level, but put a little bit of downward break on
the part of the slot toward the tailpiece, because that's how your strings
break over the bridge when it's strung up.
14) So much for the wound string slots. What about the two unwound
strings? Here I really recommend buying a couple of nut slotting files of
the proper size, because you just can't find "sawing strings" of the right
gauge for cutting these two slots. Even the lightest gauge banjo or
mandolins strings are too big. If you customarily use a 0.017 and 0.016
string, buy slotting files of that size, or as close as you can come. As
Jim pointed out, Stew-Mac sells a couple different kinds of these things.
I recently bought a set for about $11.95 per file. Yes, they're expensive,
but you can probably afford at least one or two for the unwound slots.
Here's a link for the kind I bought:
<www.stewmac.com/cgi-bin/hazel.cgi/hzpi/u/HzSt010n17>;
Why not just use the Xacto knife for those two little slots? Because,
believe it or not, the configuration of the slot has enormous effect on
how that string sounds. If you cut a narrow V-shaped slot, the string will
be pinched and really dead. If you cut the slot too wide, it'll buzz in
the slot. The configuration of the bottom of that slot is fairly critical.
On the other hand, if you cut it just a little bit too narrow, string wear
will eventually open up the slot until the string seats itself properly,
so it's better to have a slot too narrow than too wide, probably.
15) After you get the slots cut, put everything back together again,
string it up, and tune to pitch. If you're lucky, you won't end up with
the tops of the inserts crowding the underside of the palmrest. If you
have very little clearance, you may have to back off the coverplate screws
and pull the coverplate up a bit until you get all the strings up to full
pitch. Scheerhorns are a good example of this -- they have so little
clearance under the palmrest that if you detune all the strings at once,
you have to back off the coverplate screws to get enough clearance. This
is often the case with instruments that have the Quarterman QC-1 cone,
because the rim profile is so high.
16) After you get it tuned, play it. Do any of the strings sound dead? if
so, there are two possible causes: A) the string isn't yet properly seated
in its slot, or B) you've got an air gap between the spider slot and the
base of the insert.
If it's "A," then the cure may just be to keep playing it for a few hours.
If it's "B, then you'll have to take it apart and tap the offending insert
down more firmly (taking care because now the top edge has been slotted).
If you have a dead string, my advice is to play the instrument heavily for
a day or two before doing anything else. The problem may solve itself
through normal string wear. Of course, I'm assuming you're stringing it up
with new strings, not old dead ones! If that doesn't fix it, then don't
send it to me!
Now, having said all this, Dick Deneve will probably sign on and point out
all sorts of transgressions and errors in my essay. Well, what th' heck --
he's a professional and I'm not. You get what you pay for (and also "a
preposition is a bad thing to end a sentence with").
Ken Brown (AAFOUF #00072; member, IBMA)
Austin, Texas
The Blackland Prairie Boys
Bluegrass from Central Texas
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