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MMD > Archives > December 1997 > 1997.12.17 > 26Prev  Next


Valve Travel In Reproducing Pianos
By Craig Brougher

Just a little more needs to be said about valve travel and the effects
of narrow gapping.

As has already been pointed out, the surface of the valve leather
facings has a lot to do with the resistance of the valve, too.  Dave
Saul is still speaking in terms of valve gaps, as has been done now for
the last 30 years or more.  I mentioned the fact that I "blew" my
valves to get a resistance measurement, instead of "gapping" them with
a micrometer of some sort, or a shimming tool.

Dave Saul mentioned the Ampico valve travel machine, which also
appeared in some technical advertisements of the day, but like he said,
although it was designed to measure air travel through the valve gap
instead of measuring the gap itself, nothing was mentioned in regard to
how much gap the machine deemed ideal.

I realized that I never explained how I arrived at my "ideal" gap.
Here's how it was done.  Some years ago when I was hiring girls to do
things like this, I would notice, even after rechecking their settings,
that some Duo-Arts didn't play as well as others.  I was using the old
gapping techniques then.  I tried not to let any of those pianos leave
the shop until I had fixed them.  So I would drop the stacks and
retest.

I stayed up nights, sometimes, trying to figure this out.  I finally
hit on the solution to why these pianos could play their zero or first
intensity well, but suffered during full power, and particularly
noticeable in the single pianissimo passages as well.  The mezzo-forte
and average loudness playing seemed to be just fine.  It was the
nuances in the very low end-- just above the set point-- and the levels
at the highest intensities that suffered the most, to my ear.

So I changed the valve gaps and after re-gapping a set of valves
half-way, started "blowing" down through them.  I immediately noticed
small differences that I couldn't gap! I decided that it was the suede
differences, since the gaps were identical.  Then I got the idea of
blowing through an empty valve plate and comparing it to a valve.  That
night was the turning point of valve regulation for me.

I also discovered that valve losses (travel losses) do not somehow
follow linearly with a corresponding increase in gap.  I explain this
effect by saying that a narrowly gapped valve has a bit more proclivity
to touch both seats at the same time as it is being balanced and
hoisted by the pouch-- always at an angle.  So because it is very
slightly "jammed" that way, as some of them travel upward at an angle,
it requires longer to seal.  This time adds to the travel-time and
travel losses.  If that turns out to be incorrect it doesn't really
matter anyway, because at any rate, you are regulating its zero or
first intensity with this set amount of travel, so the extra percentage
of travel loss is canceled out.  Bingo-- it becomes a moot point
anyway!

The Duo-Art is the perfect instrument to prove this fact with, because
its expression regulators do not try to keep stack pressure constant at
all!  Unlike what Dave Saul thought about the Duo-Art regulation
system, these regulators were designed for one thing: To keep the zero
intensity at a specific level _inside the expression box_ only.  This,
of course is a lead-pipe cinch simply by looking at the roll coding of
a Duo-Art.  There is enough pressure reserve inside the expression box
to supply about two or three notes enough zero intensity to play.  If
you want to play 4 notes, you have to add an intensity, so it's better
to add two intensities and put the soft pedal on for safety margins --
which is the same practical idea as what you'd do for the zero
intensity, in most cases.

The reason that narrow valve gaps affect the soft nuances noticeably is
because of the slowing down of very low pressure air.  One might think
that a narrow gap would work for low intensities, but that is only so
for your "ZERO" intensity-- the intensity you have set everything to.
So naturally, that intensity has to be okay.  From that intensity,
however, the piano does not seem to make as noticeable a change from
level to level as it does when there is a bit less resistance in the
valve gap.

Since I cannot tell anyone a "valve setting" this way to use, all I can
do is say that it always gaps greater than .032."   But air resistance-
wise, it is a bit beyond the throttle point of the valve.

If you really want to learn valves, set them with a shim gauge at .032
and listen to the piano perform.  Then pull the stack and re-regulate
the valves my way, replace it, and try it all over again.  You'll see
what I mean! A certain amount of air restriction is still necessary by
the valve.  You need to "feel" that valve face when you blow into it.
But there is a setting, somewhere between .032 and .035 that is just
too restrictive for that reproducer to work well at.  You'll see what I
mean.  So trust your own senses and try it.  You'll like it! You'll see
why Ampico went to all the trouble of measuring air flow instead of
gaps!

Craig Brougher


(Message sent Wed 17 Dec 1997, 15:37:59 GMT, from time zone GMT.)

Key Words in Subject:  Pianos, Reproducing, Travel, Valve

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