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MMD > Archives > April 1997 > 1997.04.07 > 15Prev  Next


Valve Leather
By Craig Brougher

I have run into an "interesting" development in regard to leather for
valves.  In restoring the older "upside-down" Amphion valve Ampico, I
discovered that the stack had half as much power as it should have had
while playing.

Before any notes were struck, however, the tightness of the stack was
exemplary! During play, the pump would drop to 15" when the pressure
was supposed to be 30" or more.

Stuff like this is really disconcerting because you know you've done
everything right. You've tested every step of the way, and only after
the player gets installed do you have problems. I'm telling on myself
because I want others to know that this kind of stuff happens to
everybody, and no one is immune.

So my curiosity gets the better of me. After having removed a dozen
valve blocks and tearing them apart, not finding anything wrong
mechanically, and recovering them anyway, I am beginning to wonder
about the leather.

What I used on this piano is new "Chrome/alum" tanned stuff. It's
white. Much of the skin is too coarse of a suede, so I don't use it.
That's just waste.  Looking at the skin, I'd say that 80% of it is
worthless for good valve leather, but texture-wise, the 20% used was
satisfactory. However, I think that the compression factor and ability
of the valve poppet to slide easily for a good seal was not allowed by
this kind of leather. I substituted a cabretta hide for some of the
poppets and they worked fine.

Now we have a potential problem. The technique was not at fault, but
the selection of leather was. I had used this leather before without
any problems at all. But in every case, the valves it was used in
happened to be every other configuration EXCEPT upside-down Amphion
valves.

In my estimation, about 20% of those valves had leaks when operated,
from faint to gross. The same leather on the inside seat had no
problems at all.  The stack seals as tight as a drum.

The only conclusion I can come to is this: Certain tanning procedures
allow the poppet valve to "adjust" or slide itself slightly to seal.
Other tanning methods do not. The upside-down Amphion valve uses a
tin-plated outside valve seat-- a half-round that would create greater
pressure/sq. in. The annulus would tend to prevent the poppet from
"skidding." But if the problem can be corrected by changing the leather
to something softer with a finer suede, then I thought others might
also like to know.

The bottom line is this: Don't take anything for granted, because
everything you trusted in is gonna change sooner or later! "Nothing is
forever."

Craig Brougher


(Message sent Mon 7 Apr 1997, 21:38:34 GMT, from time zone GMT.)

Key Words in Subject:  Leather, Valve

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