Pianotech

  • 1.  Bass Bridge Down Bearing Increase

    Posted 10-18-2018 10:13
    I had something happen twice last year. After prepping the d/b on a bass bridge, the d/b increased after pulling to pitch. Once on a grand and another on an upright. Both had the plain wire on and up to pitch. This has not happened before. What would cause the d/b to increase as much as 2 mm after the strings are on?

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    Regards,

    Jon Page
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  • 2.  RE: Bass Bridge Down Bearing Increase

    Posted 10-18-2018 20:00
    The bridge does not necessarily deflect in a linear fashion. It often sinks in one area, progressing to a pivot point, which then cantilevers a third part up...seesaw like.  But this most often happens in the high load treble. It does happen in the bass too, but since you were at pitch already in the plain wire I don't think that's your issue.

    The bass hitches, under load, depending on the plate, may deflect up under tension and nose bolt configuaration. Unless you measured the plain wire db with an angle gauge or Lowell gauge, I think this one would be hard to prove. So, I'm not sure I think that this one is it either.

    My bet it measurement error...and I don't say that as someone claiming infallible precision in this department. Rather, if you look at all the variables we are "guessing" at, there is a huge potential for measurement error, setting bridge height. It really is quite difficult, to set up the bridge height test string at the proper height interpolating for possible deflection. By difficult, I mean fraught with inconsistency. For instance, how you hold the test string down with your finger, and what shim system you use at the hitch end, and what shim size per given SL length and back scale length, can impart a major measurement error into to reading. Back scale length as per what shim is chosen is a big potential gremlin here. As well, in addition to this, how much you pre-load the board and where, can give bum readings as the seesaw effect above can also make a hash of an otherwise fine measurement. 

    Setting bridge height is, and continues, for me, to be one of the most fraught parts of belly work...particularly, if you measure your results along every note with a digital angle gauge. If you don't look too closely, it probably seems better than it actually is. Which is why adjustable plate bolts and vertical hitches are so useful.   I say this as someone who pre-loads the bridge with calibrated go-bars, and still has to "fix" unintended DB results. 


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    Jim Ialeggio
    grandpianosolutions.com
    Shirley, MA
    978 425-9026
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  • 3.  RE: Bass Bridge Down Bearing Increase

    Posted 10-18-2018 21:39
    A tricky beast that bass bridge. The answer to the question might reveal itself with a description of what the prepping is.
    -chris





  • 4.  RE: Bass Bridge Down Bearing Increase

    Posted 10-18-2018 23:33
    To prep d/b, I stretch a thread from the agraffe to the hitch area. I doubt the thickness of the thread is a factor with the front termination being so far away. Both pianos had retro-fit vertical hitch pins.

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    Regards,

    Jon Page
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  • 5.  RE: Bass Bridge Down Bearing Increase

    Posted 10-19-2018 00:24
    Yes, so it's tough going from thread reading before, then a bubble reading after the strings are on. Even a with a wixie we're getting variable readings. Such a small angle. After awhile, I'm just glad it's touching.





  • 6.  RE: Bass Bridge Down Bearing Increase

    Posted 10-19-2018 08:36
    Jon,  do you shim the hitch termination to create the db, or eyeball some small amount of db?

    ------------------------------
    Jim Ialeggio
    grandpianosolutions.com
    Shirley, MA
    978 425-9026
    ------------------------------