A tenor bridge conversion

[Ron Overs]

Hi André,

Nice to see you in print again.

>I have a question :
>
>You cut off the treble bridge in the tail. Did 
>you do that because that last part of that 
>treble bridge is so close to the rim?

No, the proximity of the bridge to the rim in 
this case was actually OK (although the bass 
bridge is too far in from the rim as usual), it 
was the tension and stiffness of those large 
diameter wires at such a low tension which is the 
problem.

>I worked on a brand new Bechstein C and I 
>noticed that those first treble notes (nr 21 to 
>appr. nr 30 sounded weird and were difficult to 
>voice correctly, as usual).

That's the problem (the first plain wire 
tri-chord is note E32 in the modified piano), 
they can't be voiced because the inharmonicity is 
way up, the Z is way down and the % break is way 
low. There is nothing that anybody could hope to 
achieve with voicing needles. The original 
scaling is an utter mess.

>I also wonder what kind of string thickness you 
>use on that new bridge you built. Do they get 
>thicker in size because they are shorter?

No they don't. On the standard piano they do but 
it isn't the right way to go (from my 
perspective). That is part of the problem. If the 
tension is dropping because of the hockey stick 
bridge, increasing the wire diameter will only 
make a bad situation worse.

We actually use thinner core wire with a wrap to 
achieve a balance between inharmonicity, Z and % 
brk. But such a redesign can't be achieved 
without also using a tenor bridge. There is a 
golden rule when crossing from plain tri-chord 
wire to wrap, there must be a reduction in 
speaking length of between 10 to 14 % for the 
first covered bi-chord note (tri-chord covers 
also should be confined only to history books). 
The ideal reduction in speaking length depends on 
the choice of core and wire wrap diameters. You 
can't re-configure the string sizes while keeping 
the hockey-sick end. I realise that a number of 
manufacturers do just that, but it doesn't work.

Here are some reduced size graphs of %brk, Z and 
Inharmonicity for both the original scale and the 
new tenor conversion.

Here is the original scale. % brk is the top 
graph, Z the middle, and I(4) the lower two.



The tension drops to around 20% in the low bass 
as well. We revise the diameter of both core wire 
and the cover wrap in the bass also. Notice also 
that the inharmonicity turns and heads upwards 
before the long bridge crosses to the bass. This 
is typically what happens with hockey stick ends. 
Overall tension 16850 Kg.

And here is the tenor bridge conversion.




Overall tension 16785 Kg. While the tension per 
unison has risen considerably, there are less 
strings overall with the bi-chord tenor scale, so 
the overall tension is reduced slightly.

To answer Calin Tantareanu, he asked about the 
problem of strike point with the tenor. That's a 
good question Calin, and yes it does create a 
problem, because we've moved the bridge position 
and it will result in a strike ratio which is 
further out on the speaking length. Ideally, we 
should also shift the agraffes to keep the strike 
ratio correct, but in this case we'll reduce the 
hammer position along the shank about 5% to help 
offset the negative effect of the strike ratio 
change. It won't be ideal, but it won't have 
nearly as bad an effect as the original poor 
scaling had on the tone. We'll be able to do some 
voicing to help blend the tone. It won't be 
perfect, but nevertheless the net result will be 
a considerable overall improvement.

In the case of this piano, the client wasn't 
prepared to spend enough to allow us to move the 
agraffes as well. We're considering doing an S&S 
B next year by moving the agraffe line to keep 
the strike ratio correct. A model B with a tenor 
and moved agraffes is the ultimate fix for that 
piano.

The soundboard was stripped, sanded and 
refinished. The original bridge had nothing 
physically wrong with it. It was just a matter of 
scaling.

Feel free to forward my posts to the Bechstein 
Yahoogroup, but I'm not prepared to get into a 
major debate defending the work we do.

Hope it all makes sense.

Ron O.
-- 
OVERS PIANOS - SYDNEY
    Grand Piano Manufacturers
_______________________

Web http://overspianos.com.au
mailto:ron at overspianos.com.au
_______________________
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