Procedures for determining optimum valve travel in reproducing pianos,
especially the Duo-Art, have received much recent attention in the MMD.
Interesting ideas and techniques have been discussed.  However, some
critically important aspects of the dynamic behavior of valves and
their associated action parts have not been addressed.

Setting valve gaps according to test conditions of maximum air flow
often tends to be rather crude and cannot be depended upon to produce
optimum results.  Under realistic playing conditions, air flow at or
near the limit of a valve's capacity rarely, if ever, occurs in a
reproducing piano.  Only during extremely loud play does a valve's air
flow approach a valve's ultimate air flow capacity.   During pianissimo
playing, air flow through the valve is slowest.

The valve is best considered as a switch that connects a pneumatic to
regulated suction for activation purposes, and returns the pneumatic to
atmosphere when switched "off".  While the valve is switched "on", the
pneumatic delivers force needed to activate its associated piano key
and action mechanism.   When the valve is switched "off",  readiness
conditions are restored.  In achieving uniformly even performance, the
important aspect of valve action to be considered is switching action,
as opposed to the extreme condition of maximum airflow capacity.

Consider a pneumatic action in which valves are set with inconsistent
amounts of travel.  Under such conditions, the time needed for each
valve element to travel from one seat to the other will vary from one
valve to another.  Switching action will not be uniform, since valves
having wider gaps will take more time to move  between seats than those
with narrower gaps.  This can contribute to unevenness in playing.

A further consideration is the fact that while a valve element is
traveling between seats, there is a direct path around the valve
element between regulated suction and atmosphere.  Too large a gap
causes "spitting" to occur during switching, which is associated with
a transient drop in stack suction. This can be particularly troublesome
in a Duo-Art, which has a notoriously inefficient expression regulator,
in terms of being able to maintain a set value of stack suction.  When
a chord is played requiring several valves to operate simultaneously,
this transient problem is compounded.

Efficient switching action (and good musical performance) will best
be achieved when valve travel is uniformly set to the minimum gap
consistent with ample breathing under expected playing conditions.
As a general rule, a gap of about .032 in. to .035 in. for any
reproducing piano single or secondary valve will produce excellent
results.  If you are really concerned that your Duo-Art won't play
loudly enough, you can safely go up to perhaps .040 inch!  The value
chosen is not extremely critical, but for best results, it should be
held within a few percent throughout the pneumatic stack to insure
uniform switching action.

There's much more to valve adjustment than gap dimensions, of course.
Ideally, each valve should be adjusted as nearly as possible to be
identical with its mates.  Parts need to be carefully inspected to see
that valves are able seat squarely without leakage due to misalignment.

The relationship of the valve's lifting button to its pouch is also
important.  When this is adjustable, as in the Duo-Art, it's a good
idea to leave about 1/16 in. to 3/32 in. gap between each lifting
button and its respective pouch with the valve at rest.  This is con-
sistent with good switching action and also provides a safety margin to
allow for potential pouch shrinkage.

For the sake of completeness, we need to point out that the Ampico
factory developed a machine to use air flow measurement to set
valve travel, as described in U.S. Patent No.  1,684,015, "Method of
Adjusting Valve Throw", patent applied for in July, 1924 and issued
Sept. 11, 1928.  The inventor is listed as B. Dudley, Jr.

The patent application date suggests that this interesting machine was
developed before Dr. Hickman's arrival at Ampico.  It was probably used
initially to speed up production by automating the adjustment of
secondary valves in later Model A pianos having pressed in, right-side-
up valves. The patent describes the principle of operation, but gives
no details of specific suction levels or other operating parameters.

How well the machine worked, or whether perhaps Dr.  Hickman might have
eventually devised a better adjustment method is, sadly, moot.

Dave Saul