Harpsichord

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  • 1.  Computing brass string scaling

    Posted 11-25-2015 12:11
    Is there any known means of computing the correct scaling for a set of brass strings given: Speaking Length and: Pitch required for that Speaking Length? We have a Neale H'chord  which has many broken brass strings. It has 1 x 16'. 2 x 8' and a 4'. and think the breakage due to over-size gauge stringing in the brass sections. The steels are OK. Then it has a couple of rolled bridges and missing jacks - but we'll contemplate this issue once the scaling is resolved. This 'Neale' came with a pedal board coupled through a roller board - like a tracker Organ. Interesting . . . 
    Michael   UK


  • 2.  RE: Computing brass string scaling

    Registered Piano Technician
    Posted 11-30-2015 21:17
    Michael,
    In general, the breaking point of a given material rises proportionally to its diameter, meaning that if a string is breaking, you need to change material. This is certainly true of piano wire, and also holds for brass as I understand. There are a lot of different brass types, with different strengths, so changing alloy or process (to spring brass, if that isn't what is already there) might help. For maximum strength, phosphor bronze is an option. Reducing diameter may get you a bit less breakage, but only a small amount.

    A lot of instruments are simply badly scaled - or they are scaled to a different pitch. If it was scaled to 415 (whether on purpose or not - they might have copied an older instrument), for instance, the strings might have sort of worked, but so close to breaking point that breakage would be common. I have certainly seen instruments like that. There is no magic formula beyond changing the scale (moving bridge or nut) or changing material or pitch level. 

    There are some scaling spreadsheets out there with harpsichord material specs embedded. I don't have such an animal myself. Maybe someone else does.




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  • 3.  RE: Computing brass string scaling

    Posted 12-01-2015 04:09

    Thanks, Fred.

    One aspect on the 'plus' side is that Malcolm Rose has his string-making-cum-H'chord-workshop just 4 miles from me. So armed with your comment I shall give him a visit to see if he can produce an answer. The rolled bridges are exercising my mind too, brought about by the extreme angle from bridge to hitch in the middle area of the scale and the fact that the bridge(s?) are in two adjoining and continuous sections - thus increasing vulnerability to rolling.

    Michael  UK

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    Michael Gamble
    semi retired
    Brighton
    01273813612

    Original Message


  • 4.  RE: Computing brass string scaling

    Registered Piano Technician
    Posted 12-01-2015 10:18
    I'm sure Malcolm Rose would have a spreadsheet for wire tension and breaking %. Just measure your lengths and diameters.

    Rolled bridges in harpsichords are pretty common due to the fact that there are no ribs under the bridge, but there are ribs stiffening the board in front of it, leaving an unsupported area. The most common fix is to "back pin," meaning you drill for a second bridge pin maybe 1/3 of the way down the back side of the bridge, angling upward a bit, then pull the string past that pin. 

    This serves to pull the bridge back into a more vertical alignment, and eliminates the buzzing of the string against the wood in front of the pin that is often the annoying consequence of the roll.




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  • 5.  RE: Computing brass string scaling

    Posted 12-03-2015 03:23

    Fred

    Thanks for the rolling bridge info - and the photo illustrating a fix. I will look into the possibility of using this. The possibility of the scaling being for other than A=440 hadn't occurred to me - so I'll add this to the melting pot. Again my thanks.

    Michael   UK

    ------------------------------
    Michael Gamble
    semi retired
    Brighton
    01273813612

    Original Message