Hi Ed, Very familiar symptoms. I think the bridge notching pattern is something interesting to consider, but not necessarily determinative. The same symptoms occur on pianos where the notching doesn't move on the bridge (generally on those pianos, there is a bend in the bridge at the strut). It is very puzzling to me that Baldwin 243s (Hamiltons) have a reverse symptom at the strut: pitch change gets higher approaching the strut from below, then pitch is closer to standard above the strut. I don't think I have run across any other piano that does that. I've been tossing around possible explanations for over 20 years now, and don't have one that works yet, other than scaling issues (lengths of strings and relative tensions, with a jog in tension over the strut - but this wouldn't seem to affect as many unisons as are affected). One thing I will try to find time for is to check for any measurable difference in DB via the Wixey gauge, which is more sensitive than anything I have measured with before (and easier to use). I guess measuring the relative cant of the bridge top would be good as well, in case it changes relative to front and back angles of the string. The new Mason A I just touched up for a customer this evening (had tuned it three weeks ago) was over 10 cents sharp at the bottom of the treble bridge (plain wires), tapering to pitch at the strut, then a little over 5 cents sharp above the strut, tapering to pitch within about an octave. And virtually all treble side (right) strings in excess of a cent sharp of the left strings, often 5 cents difference. Again, very familiar pattern. RH had risen from about 20% to 45% in the meantime. Piano has a full Dampp-Chaser system. A string cover is on order. Keeps us in business, but it's annoying anyway. Ron N's notions about bridge caps (epoxy saturated) reducing this pitch drift are tantalizing, but don't help with existing pianos. I have thought of experimenting with one of my Yamaha G-2s, trying to saturate portions of the bridges with extra thin CA to see if that might help. My notion is to remove bridge pins, more or less fill the hole with thin CA, wait a wee bit for penetration, then drive in pins. I'd do this when it is nice and dry, so setting up of the CA would be extra slow (as it is here in winter). But too many other priorities have kept me from trying yet. A thought that has occurred to me is that the movement of the SB in response to RH change (the swelling that occurs and subsequent changes under the load of the strings and various other forces) actually shifts the bridges sideways relative to the plate. One piece of evidence is the tendency of bass bichord dampers to miss damping one of the strings (consistently) on new pianos moved to this dry climate. I don't think that the piano sitting on its side during shipping or storage explains it. I suspect the SB moves the guiderail relative to the strings (ie, agraffes and plate) when it dries (well, SB in conjunction with the belly rail and the rest of the wooden structure - in any case, things are shifting positions relative to one another in response to RH change). Regards, Fred Sturm University of New Mexico fssturm at unm.edu On Jun 26, 2009, at 6:43 PM, Ed Sutton wrote: > Here's the pattern I saw on a 5'2' grand piano: > > On the long bridge, in each section, the leftward bridge notches > terminate the speaking length in the middle of the bridge. As the > scale ascends, the notches progress toward the front of the bridge. > Crossing the gap at the plate strut, the notches start again at the > middle of the bridge, and step forward again to the front edge. At > the next strut, the same thing happens. > > The piano was humidity struck. At the leftward end of each section, > where the strings terminate at the center of the bridge, pitch was > 10-15 cents sharp. As the scale progressed toward the front of the > bridge, the pitch drift became less, and was almost at pitch as the > notches came close to the front edge. > > This pattern was repeated in each section. > > Why? > > Ed S > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://ptg.org/pipermail/caut.php/attachments/20090629/8dd0ced3/attachment.htm>
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