Illustration of "Unitype" Pneumatic Stack
Possibly Retrofitted in Some National Automatic Pianos

(Drawing from U.S. patent #1,783,319)

U.S. Patent #1,783,319, filed July 11, 1927 and issued Dec. 2, 1930. In the cross-sectional drawings, 40 is the vacuum chamber, 52 is the atmosphere chamber, 47 is the pouch, and 32-33 are the channel from the tracker bar tube to the pouch. The bleed is inside the unit. Unlike a regular valve, the inside chamber between the two valve discs is exposed to atmosphere instead of vacuum when the unit is off. The “bottom”, or right side of the pouch is normally exposed to bleed suction, keeping the pouch and valve in the off position. The area “above” the valve that would normally be exposed to atmosphere is the vacuum chamber. When the tracker bar hole is opened, vacuum pulls the valve to the left, cuts off the atmosphere to the center chamber, and collapses the pneumatic. When the tracker bar hole is closed again, vacuum bleeds under the pouch and pulls the valve to the right, switching the center chamber back to atmosphere. The whole unit is only about ½” wide.

Although not shown n this illustration, a smooth metal spoon is attached by wood screws to the rear end of the movable board. This "spoon" lifts the associated piano wippen when the pneumatic collapses, which then, in turn, causes the piano note to sound.