In MMD 150406, Joseph Osborne sought advice about a low-frequency
vibration in an Ampico A grand.  From his description, it sounds to
me to be a fluttering valve in one of the two primary type unit valves
usually found in the A mechanism.

In my experience, this sound typically has a "buzzing" quality with a
definite musical pitch, and is usually an issue with the stack cut-out
primary valve, typically mounted on the bass rim of the piano, just aft
of the stack and as close as practical to the pump distribution block
which houses the cut-out pouch which is controlled by this primary
valve.

However, based on Joseph's description of the location of the sound,
I expect the culprit is the automatic cut-out primary valve which is
typically located centrally behind the stack in the vicinity of the
automatic cut-out pouch strip.

Whichever valve it is, their operation and probable failure modes are
the same.  Two factors, either singly or in combination can cause the
fluttering condition.  The first condition is a partially or completely
blocked bleed port in the valve.  The second is leakage in the tube
leading to the valve's pouch.

When the bleed is blocked, it causes the pouch to be super-sensitive,
such that even minute leakage in the pouch tube will cause the valve
poppet to start to open.  But since there is not quite enough
atmospheric pressure in the pouch tube to properly inflate the pouch,
the valve supply suction supply will immediately pull the poppet back
closed, and a steady state oscillation will develop.  Likewise, even if
the bleed is open, if there is more leakage (atmospheric pressure) in
the pouch tube than the bleed can "dump", a similar oscillation can
develop.

More than likely, a combination of these two is at work, a partially
blocked bleed combined with too much leakage in the pouch tube.  The
quickest test is to pinch-off the single tube (labeled "AU" in some
tubing diagrams) going to the Ampico On/Off switch in the drawer.
This is the switch that controls this valve.  If the noise goes away,
the switch surfaces are probably dry, causing leakage into the tube.
Lapping the switch surfaces and reassembling with a light grease should
cure the problem.

If the noise turns out to be emanating from the stack cut-out valve,
pinch off the tube connecting to the port in the transmission frame
that is opened when it shifts into rewind.  If the noise goes away, the
cure is similar: lap smooth the transmission frame sliding surface and
the brass disk which covers it during play, and reassemble with light
grease.

John Grant