The loud pedal valve block (LPVB) in the Ampico Model B system is
a clever and sophisticated pneumatic device.  Its description and
functional characteristics are reasonably well explained in the 1929
Service Manual for the Ampico B, published by the American Piano
Company.  The LPVB was also incorporated in some late model (1927-1929)
Ampico A pianos (which did not contain the full complement of Ampico B
components).

Having seen, repaired, and restored a fair number of Ampico B piano
mechanisms, it appears that all the LPVBs are not identical.  One
expects and sees the usual instrument-to-instrument variations in
LPVB construction regarding external tubing layout, nipple and elbow
configuration and placement, and different hold-together clamps (solid
versus butterfly type).

Aside from these accommodative and/or evolutionary differences, there
is a more subtle variation among the LPVB units.  The size of the bleed
constriction associated with pouch D (see pages 27-28 of the service
manual) apparently depends on the manufacturer and size (length) of
the piano in which the LPVB mechanism was originally installed.  This
constriction is usually placed in one end of an ordinary 5/32-inch OD
elbow that is mounted on the exterior of the LPVB.  The constriction
passageway is either drilled through the center of a small brass plug
(soldered in place) or through a slug of solder that fills in the end
of the elbow.

The bleed hole constriction sizes for pouch D that have been observed
to date (sample size = 8) span the range from wire gauge drill bit size
#55 to #69.  Drill bit size #55 is .0520" in diameter; drill bit size
#69 is .0292" in diameter.  To illustrate by example, the smaller size
(#69) is associated with larger instruments (e.g., 6'6" Chickering
Ampico B) and the larger size (#55) with smaller instruments (e.g.,
4'8" Marshall & Wendell Ampico B).

These dimensions reflect a D pouch constriction diameter ratio of
about 2 to 1.  Although this might seem insignificant, the ratio of
the extremes of the constriction areas is about 4 to 1.  The diameter
of the constriction passageway can exert a substantial influence on
the performance of the LPVB.  The pouch D bleed constriction governs
the speed with which air can flow back into the loud pedal actuator
pneumatic.  This activity occurs after the loud pedal function is shut
off by the absence of certain perforations in the Ampico roll that is
being played.

The correlation of constriction-diameter with piano-length makes sense
in connection with the suppression of the inevitable "thumping sound"
produced when the dampers fall back onto the piano strings as the
damper pedal is released.  The mass of the damper mechanism in larger
pianos generally is greater than that found in smaller pianos.

The pouch D constriction effectively exerts a pneumatic braking force
on the falling of the dampers so that their aggregate impact velocity
is reduced.  If the constriction for pouch D is too large, the
Ampico-actuated thump may be objectionably loud.  If the constriction is
too small, damping may still occur but later in time than intended or
not at all.  The latter outcome is not likely because of air seepage
through wood and gasket material, but it is theoretically possible.

This is musically relevant because Ampico B mechanisms are commonly
reinstalled in "gutted" pianos or retrofitted into Ampico A pianos.
It may be that the Ampico B loud pedal valve block was, in fact,
originally selected at the factory to "match" pianos of different sizes
and manufacturers.  Arbitrarily retrofitting the LPVB from an orphan
Ampico B mechanism into a piano of "different than original" size or
manufacturer could introduce roll-played damping problems such as
objectionably loud damper thump or delayed/quiet damping.

I have not yet measured or studied the constriction size(s?) associated
with pouch C (which controls the exhausting of air from the loud pedal
pneumatic) because I do not think that the effect is as important there
as it is for pouch D.  The 1929 Ampico B service manual addresses this
pneumatic situation with just two relevant lines of text:

  "Constrictions are placed in the tubes leading to pouches C and D to
  control their speed of operation.  This mechanism has been designed
  to obtain rapid and quiet damper operation."

When an accomplished pianist plays a piano, he controls the motion
of the damper pedal with his foot, using information fed back from
his ears to minimize inherent damper thump.  This learned procedure
realizes "rapid and quiet operation" of the damper mechanics for that
instrument.  The Ampico B loud pedal valve block attempts (and
succeeds) to anthropomorphically duplicate the nearly noise-free
operation of the damper pedal mechanism by the pianist.  All hail
Clarence N. Hickman, the inventor of the Ampico B mechanism!

Bill Koenigsberg
Concord, Massachusetts

 [ The Ampico B loud pedal valve is the sort of ingenious
 [ mechanism that Charles Fuller Stoddard might have invented.
 [ See http://www.mmdigest.com/Gallery/Tech/Ampico/amp27.htm 
 [ -- Robbie