Recent discussions here about sponge neoprene that led to the
exploration of the Duo-Art and its characteristics have not told the
whole story.  There is more.

When Robbie first advanced the idea that perhaps bleeds be introduced
at each end of a Duo-Art stack, to compensate for the valves being too
tight, it elicited remarks from Craig who cited a well-known anecdotal
story concerning the late Molly Yeckley's piano.

Molly did much to promote the field of mechanical music.  She collected
reproducing pianos and rolls, served in numerous positions in AMICA,
and ultimately served as president.  Additionally, she hired at least
four technicians to work on her Steinway style AR Duo-Art.

Unfortunately, most of the technicians, who were all competent, were
never retained to do a complete restoration.  Some of the earlier work
was experimental in nature, thus the installation of patent leather
valves.  This piano literally became the most well-known test bed in
the field!  To put things in perspective, some of the restoration work
was being completed in the early 70's and the field of total
restoration as we know it today was in its infancy.

The patent leather valves in that piano, as cited by Craig, seemingly
created the problem of the stack being too tight.  But was it really?
The Duo-Art expression regulator -- the throttling device -- is a knife
valve.  The type of knife valve used in the Duo-Art has a huge flaw.
That flaw is, if the valve leaks, the result can be a divergent
departure from the desired steady state equilibrium.  If the stack
is not airtight the leaking knife valve problem is not noticed.
Conversely, the tighter the stack, the more the severe the divergence
becomes.

In other words, the pump vacuum helps "seat" the knife valve.  If the
knife valve leaks, there can be a decrease in the pump vacuum and thus
a decrease in seating effect of the pump.  If the stack is extremely
tight, the vacuum leaking around the knife valve now starts to build
on the back side of the knife valve.  Now the effect of the diminished
pump vacuum is exacerbated by a drop in the pressure behind the knife
valve.  The result of all this is divergence.  In a no-demand situa-
tion, a small leak will lead to near full vacuum at the stack, when it
should be at minimum.  The next note played will be loud regardless of
roll coding.

If the stack valves leak a moderate amount, the divergence does not
occur.  But leaking stack valves are really not what anyone wants.
The best valve is one that does not leak.  Aeolian knew of the
divergent knife valve problem.  To counteract that problem, later
Duo-Art expression boxes have an atmosphere bleed introduced directly
into the chamber of the expression regulator.  This is similar to the
modification Robbie suggested, but much simpler.  The bleed is a 1/16
diameter hole directly through the frame of the expression pneumatic.
The hole is covered with loose felt.

Because knife valves slide across the inlet opening, and because the
Duo-Art valve has a double pivot point, the possibility of a leak in
the valve is much more probable than in disk valves.  The Duo-Art knife
valve can be very tight, almost 100%.  Testing and achieving the 100%
tight condition is elusive.  The most revealing tests are the ones at
very low vacuum levels, as the higher levels tend to aid in sealing the
knife valve, thus masking a problem.

The bottom line is, tight stack valves are only a problem if the knife
valves leak.  Leaking valves degrade performance.  Tight valves enhance
performance.  If both valves, stack and knife, are tight, the
performance is breathless, in several ways.

Bob Taylor

 [ Thanks for further information, Bob.  I'm learning from many
 [ experts, and it doesn't bother me at all that the explanations
 [ differ.  I'll hold my breath, though, until the final regulation
 [ is complete!
 [
 [ I'm amazed at the number of poorly-performing Duo-Arts in homes, and
 [ I'm equally amazed when I hear those few special Duo-Art pianos which
 [ really "sing".  Thank goodness there are still some collectors who
 [ will seek _and_ hire the thoughtful experts to restore and regulate
 [ their instruments.  -- Robbie