Part 4 of 4. Part 3 concluded:

> A puzzling question remains. What explains the presence of
> a #60 bullet-bleed in the C valve in some original Ampico B
> pianos, while others (also original) have the #70 bullet-bleed?

I have a tentative suspicion that someone (in production?) at
American Piano Company may have thought that because the ball-bleed
assembly (whose associated constriction opening is #60) was
intentionally 'removed' from the 'special' valve in the Valve Control
Block, therefore its fixed bullet-bleed should be changed from #70
to #60 to compensate for the 'loss' of the ball bleed assembly.

This is pure conjecture on my part. But then, it is curious that
the #70 bullet-bleed was 'put back in' for (some of, possibly all)
the 'later' Ampico B pianos. Go figure.

One might also ask why a circular (nominally 1" diameter) cardboard
insert (with a pin-prick hole or two) was placed over the conventional
central opening (nom. 7/16" dia.) of the metal cap of the special
valve in the first place. Reducing the effective size of the central
opening of the valve's metal cap serves to suppress externally
perceived impact noise (that is generated internally by the valve
poppet impacting the associated valve seats).

But, could there be another reason? Recall that the intended function
of the special valve is to be operated just one time (at the end of
play) 'during' the playing of an Ampico piano roll. This suggests
that the reduction of poppet impact noise was not the main reason for
the presence of the cardboard insert.

The driving factor dictating the presence of the cardboard insert is
one of reliability. That is, even though the special valve needs to
be activated only once during play, that operation must be extremely
reliable. Consider that if the special valve fails to operate properly
when called for, the piano will not turn itself off after the roll has
finished playing, clearly an undesirable situation.

Examination of the tubing diagram of the Ampico B system (particularly
with regard to the drawer assembly), shows that vacuum (from the pump
reservoir) is supplied to the special valve through two (of three)
7/32" diameter O.D. (11/64" I.D.) elbows that power the Control Valve
Block. Vacuum is also supplied to the Control Valve Block by another
7/32" diameter (O.D.) tube from the pedal regulator. That supply goes
to the A valve, which involves the functioning of the piano roll
tracker mechanism, the music roll brake, and the reroll pneumatic.

This vacuum supply combination appears to have been marginally
effective. We surmise this, because toward the end of Ampico B
production (1941), a small change was made (1937 or so) to the Control
Valve Block. The updated construction accommodated one additional
7/32" tube (from the pedal regulator) which augmented the vacuum
supply to the Control Valve Block, but only to the A valve (not the
special valve, C).

Apparently, this engineering change was implemented to ensure that
the reroll pneumatic received sufficient vacuum to collapse fully with
no 'failures'. If the reroll function did not take place (i.e., the
piano stayed in the play mode), the roll paper would be pulled off the
core of the piano roll. This circumstance would have the consequential
effect of preventing the shut-off pneumatic from collapsing because
the 'pallet control lever' (in the spool box) could not enter the
groove (roll paper was in the way) in the take-up spool.

Remember that the pallet control lever activates valve D (in the
Control Valve Block), the one that initiates the collapse of the
shut-off pneumatic. Incidentally, under this condition, the special
valve (C) would have been activated, thereby providing vacuum to valve
(D). This follows because the 5B port in the tracker bar would be
open (all of the roll paper is wrapped around the take-up spool).

With a marginal vacuum supply, the special valve (when activated)
might not be able to produce a force sufficient to tightly press the
valve's poppet against its upper valve seat. The net result could
be an unreliable seal at the upper valve seat. Then, the shut-off
pneumatic would not collapse fully (piano would not turn off). This
situation could occur if some airborne debris were caught between
the periphery of the upper metal valve seat and the circular leather
seal of the poppet assembly of the special valve.

Incorporation of the cardboard insert in the upper valve cap of
the special valve ensured that the vacuum supply (limited though it
might be) would be channeled directly toward the shut-off pneumatic.
That is, with the cardboard insert blocking the path from the valve
exterior to the interior vacuum, little or no atmospheric air could
leak by the upper valve seat. Repetition function of this valve
would be severely compromised, but remember that the valve needed
to operate only one time.

Furthermore, the presence of the circular cardboard obstruction
prevented debris from entering the otherwise potentially vulnerable
upper portion of the valve assembly. The often-observed (originally
installed) presence of thin (protective) perforated felt covers
(typically black) on the upper exterior surfaces of all three active
valves in the Control Valve Block supports this contention.

If the #60 size fixed bleed orifice were present (for whatever
reason) in the special valve to begin with, the cardboard insert
could have been used to reduce the consequent chattering noise during
reroll. This peculiar situation could be viewed as an example of
'very carefully solving the wrong problem'!

The root cause of this problem is the incorporation of the #60
orifice instead of the #70 in the bullet-bleed. We will probably
never know just what happened in the factory, but the 'puffing'
sounds during reroll can be reduced significantly. Just make sure
that the constriction orifice of the bullet-bleed in the special
valve is indeed #70.

If your rebuilt Ampico B piano makes that chattering/puffing noise
during rewind of an Ampico A or B roll, you may wish to suppress it.
But you only need to do that if it is sufficiently annoying to you.
The 'fix' described above should be music to your ears.

Bill Koenigsberg
Concord, Massachusetts

[ Ref. https://www.mmdigest.com/Gallery/Tech/Ampico/amp34.htm 
[ https://www.mmdigest.com/Gallery/Tech/Ampico/amp34c.gif 
[ https://www.mmdigest.com/Attachments/23/02/02/230202_162836_Control_Valve_Block.png 

[ Bill writes that he is now working on another article tentatively
[ entitled "Last of the Duo-Arts". Let him know your suggestions
[ for more technical/historical articles of this sort. -- Robbie