I was glad to see Phil Dayson's objective measurements of bump in the
curve for air flow vs. valve gaps.  Is that measurement of .035" for
cross valves or round valves?  I would be interested to know at what
value the restriction becomes a problem for both types of valves.

From what Phil and Craig Brougher say, it appears that each stack would
work best if it were tested to account for the type of leather used for
valve facings.

When I rebuilt the Weber stack I was at loss for information.  I read
everything available, and as I remember Kuharski had written an article
suggesting a slightly smaller valve gap than others writers.  I calcu-
lated the area of the valve opening at .032" and then cut holes of
different shapes in a piece of cardboard.  I then blew air through the
holes to see how much resistance using my finger to close off the
opening -- anything to try to understand something about the gaps.
(This was before Phil developed his instrument for objective
measurements).

I was less successful judging while blowing through the valve itself.
Since I lived in Seattle where the humidity is unusually even, I set
the valves at .032" with a fairly thin and smooth leather (as I
recall).  The setting proved to be very useful and I find that it plays
reliably at 4.5 inches, but still too loud in the soft passages,
ignoring the subtlety that I've heard in some DAs.  The need to rebuild
the piano action was immediately apparent.  In England they were using
small gaps.

The original valve gap settings lead me to another conclusion.
Original valves (after 60 years of aging) varied from .028" to .052".
Most were around .040+ or so.  Since about 6 valves were really large,
I tend to think that Aeolian did not think the gaps were so very
critical.  I think that they used softer hammers which allowed higher
vacuum, allowing the soft notes to play as intended without missing
occasional notes.  I noticed that 3 hinge-springs had been loosened--
no tension on the pneumatic.  Anybody else observed springs
inactivated?  My guess is that it was done by a tool after the stack
was put in the piano--used to even out the weak notes.

Some upright DAs have a valve which opens when the hammer rail is
activated.  The valve then opens pneumatic #2 on the Accompaniment.
Ugh.  Not a good way to build a system.

Bill Chapman

 [ Editor's note:
 [
 [ It was advantageous to have the playback instrument compensate
 [ when the loud pedal is on, for otherwise the editor had to reduce
 [ the intensity commands for all the notes struck with the loud pedal
 [ on.
 [
 [ The problem is most pronounced at low hammer velocities.  Consider
 [ that, before the hammer can accelerate, the resting forces of the
 [ hammer and damper must first be overcome.  (In a vertical action
 [ these are spring forces.)  If the loud pedal is already applied,
 [ then the pneumatic force accelerates the hammer too much, and the
 [ note plays louder than it should.
 [
 [ The Ampico B has "pedal compensator" logic which reduces the stack
 [ vacuum slightly while the loud pedal is on.  Evidently Duo-Art
 [ also experimented with this method.
 [
 [ -- Robbie