(Photograph courtesy of Dana Johnson.)

Left side portion of a typical National built pneumatic stack from piano #x995. The first piano serial number digit is unknown; possibly the number 7. The 3 tiers of stack valves are self-evident, each with a tube that leads into the maze of tubing where they all come together and connect to the tracker bar. For this piano the "tilting yoke" expression device seems to have been relocated forward of the stack's left-side foot. The stack foot is channeled and serves to connect the vacuum pump to the vertical distribution structure standing on top of the foot, which then provides a source of vacuum to each of the three tiers of unit valve assemblies. The leather packing gasket between the foot and the vertical member is partially visible around its edges, and is held tightly together by a metal stud affixed to the foot and that extends a little above the vertical distribution board, where a rugged metal washer and hex nut are tightened down to hold the leather joint firmly air tight.

(Photograph courtesy of Dana Johnson.)

Right side portion of a typical National built pneumatic stack from piano #x995. Both sides of the stack are similar, except that the stack's foot and the vertical structure that sits on it are not channeled, and therefore serve no function other than physical support.

(Photograph courtesy of Dana Johnson.)

Restored National built pneumatic stack, with the tilting yoke expression device mounted on the left-side foot of the stack. The foot of the stack is channeled and is where the tubing from the vacuum pump connects to the stack. The vertical board that rests on the foot is also channeled, and supplies vacuum to each of the 3-tiers of unit valves/pneumatics. A threaded metal stud fastened into the foot extends upward above the vertical board and is capped with a sturdy washer and hex nut, that is tightened down to hold the vertical board firmly against the leather packing between the foot and the vertical member. Although the tilting yoke expression device derives no vacuum supply or other recognized benefit from being mounted on top of the stack foot, this is, nevertheless, where it is usually located. In contrast, however, for National pianos with Simplex stacks the balance-beam type expression control is built into the foot of the stack, and does directly interconnect internally with the vacuum channeling in the stack foot.

(Photograph courtesy of Dana Johnson.)

Side view of a National unit pneumatic. In this side view it is quite easy to see the four distinct wood component layers that make up the unit valve pneumatic: (1) The working valve chamber with a top side adjustable atmospheric vent, (2) the vacuum chamber with an internal brass bleed orifice and cove shaped grooves along the side to accommodate the stud-bolts that clamp the unit to the stack, (3) the pouch board with an internal leather pouch that operates the valve, and (4) the bottom motor pneumatic that plays a piano note. At far left is the end-cap, which serves to connect the tracker bar (via the small brass nipple protruding downward) to both the bleed orifice (in level 2) and to the leather pouch (in level 3).

(Photograph courtesy of Dana Johnson.)

Top view of a National unit pneumatic. This plan view looks down into the valve's atmospheric vent. The white colored leathered valve disk is visible through the adjustable brass ring that serves as both a valve seat and an atmospheric vent. Notches in the top of the threaded brass ring allow it to be adjusted upward or downward by inserting a thin blade or washer and twisting one way or the other.

(Photograph courtesy of Dana Johnson.)

Oblique rear view of a National unit pneumatic. This view from the rear shows how the top three component layers (layers 1, 2 and 3) are aligned to form a flat surface. The square channel in the middle serves as the vacuum supply port for the unit pneumatic. It is this flat surface, with a layer of packing leather to make an air tight seal, that is clamped up tight against the stack. At the end of the motor pneumatic, on the movable lower leaf, is a stepped metal flange, which lifts the push-rod that causes a piano note to sound. At the top of the unit, the four notches in the adjustable brass atmospheric vent are clearly visible. Any thin metal object, such as a knife blade, washer, or coin that is able to fit into and span any two notches can be used to turn the brass ring and thereby adjust the valve clearance.

(Photograph courtesy of Dana Johnson.)

Oblique front view of a National unit pneumatic. The large circular opening in the top is the atmospheric vent for the unit valve. It is fitted with a threaded, notched brass ring that, by twisting it one way or the other, can be adjusted upward or downward to set the optimum valve travel. At the center of the front facing end-cap is a rubber button plug, which can be removed to clean a fine-mesh brass (tracker bar filter) screen located inside the end-cap. The semi-circular impressions on each side of the rubber plug are from the metal washers on the stud-bolts used to secure the unit pneumatics to the stack. The cove shaped grooves alongside the sides of the vacuum chamber block (level 2) accommodate the same stud-bolts. Curiously, the rubber plug is nothing more than a standard off-the-shelf rubber bumper button, such as might be used on a grand piano lid, to prevent damage when hastily lowered to its flat rest position.

(Photograph courtesy of Dana Johnson.)

Front end view of a National unit pneumatic. With the end-cap removed, the faint outline of the ocre colored packing leather for the end-cap is visible. Here all four layers of the unit valve assembly are easily discerned, with the pouch board and pneumatic underneath glued together making layers 3 and 4 act as though a single item. What may appear to be the upper hole is actually the brass bleed orifice that has been pressed into a hole that connects with the (layer 2) vacuum chamber. The bleed has been stamped into a cup shape with a flattened rim to prevent it from being pushed too far into the hole. This rim also facilitates removal of the bleed by gently prying it loose with a tool, such as a knife blade. At the bottom of the bleed cup is a tiny orifice, just barely large enough to quickly equalize any air pressure differential in the pouch, allowing the valve to close once the perforation in the music roll passes over and seals the opening on the tracker bar. The lower hole (in layer 3) connects directly with the leather pouch, which, when inflated, causes the valve to rise, opening the valve chamber to the vacuum source while simultaneously closing off the atmospheric vent.

(Photograph courtesy of Dana Johnson.)

Inside the end-cap for a National unit pneumatic. The so-called end-cap is channeled by boring one shallow flat-bottomed recess, and then by boring another intersecting hole above it that goes clear through to the other side. This hole is then fitted midway with a circular fine-mesh brass screen. The brass tracker bar tubing fitting at bottom slips into a hole drilled through to the top horizontal hole containing the brass screen. On the backside of the screen (front side of the end-cap) is a rubber plug that seals the hole from atmospheric leakage, and that makes it easy to open and clean out lint and other debris trapped by the screen. Because air impulses coming from the tracker bar must pass through the fine-mesh screen (which in this instance is quite dirty with trapped particles) the onrushing air is filtered before entering into the recessed interconnected area on the other side of the screen. When the end-cap is mated with the unit valve assembly the brass bleed orifice will lie behind the screen, and the lower recess will mate with a hole that communicates with the leather pouch. Notice that there are no screw holes to attach the end-cap to the unit valve assembly. It is held tight against the unit valve assembly by the same stud-bolts that clamp the unit valve to the stack.

(Photograph courtesy of Dana Johnson.)

The valve chamber (layer 1) of a National unit pneumatic. This is an upside down view of the uppermost wood layer of the valve assembly. In the center of the chamber is the adjustable brass valve seat that serves as the atmospheric vent. When the valve disk is pushed upward it is unseated from the layer 2 board, opening a passage (alongside the valve stem) into the vacuum chamber. Simultaneously, as the valve moves upward, it seals off the atmospheric vent. The series of bored recesses at left make up channeling that mates with the two holes that go all the way through the lower layers and into the motor pneumatic at the bottom level. The four small screw holes located around the vacuum chamber are for the long wood screws that hold all layers of the unit valve tightly together.

(Photograph courtesy of Dana Johnson.)

Top side of the vacuum chamber (layer 2) of a National unit pneumatic. At center is the actual valve body, with the wooden valve disk leathered on both its top and bottom surfaces. Surrounding the valve are six metal pins that constrain the horizontal motion of the valve disk by keeping it functionally centered. With the valve in this rest position no air can flow through the valve opening and into the vacuum chamber underneath, hence the valve is said to be closed. The two larger holes at left connect with the valve chamber (level 1), and make up the vertical channel for airflow in and out of the motor pneumatic at the bottom of the unit valve (level 4). The four smaller screw holes are for the long wood screws that hold all four layers of the unit valve tightly together.

(Photograph courtesy of Dana Johnson.)

The underside of the vacuum chamber (layer 2) of a National unit pneumatic. The large circular recess bored into the wood serves as the vacuum chamber, with the pouch button end of the valve stem at its center. This component flipped over sits atop the pouch board, with the valve button dangling directly over the leather pouch. The square channel at left connects to a vacuum supply when clamped to a stack horizontal support board. The black colored material is packing leather to seal off atmospheric leaks when this level 2 block is flipped over and attached to the pouch board (level 3) below it. The two larger holes at left connect with the valve chamber, and allow airflow in and out of the motor pneumatic. The four smaller screw holes are for the long wood screws that hold all four of the unit valve layers stacked tightly together.

(Photograph courtesy of Dana Johnson.)

Operational valve body component removed from a National unit pneumatic (or unit control valve). At top is the wood valve disk with packing leather glued to both the upper and lower surfaces. The valve stem (or spacer dowel) connects the upper valve disk to the lower button, also of wood. The lower side of the button is also faced with thin packing leather, lessening any wear and tear on the delicate leather pouch that lifts the valve component when a piano note or control function is to be activated. The valve stem fits loosely in the larger hole bored between the valve chamber (layer 1) and the vacuum chamber (layer 2), and must be loose enough to allow enough air volume (from the quickly collapsing motor pneumatic) to pass alongside the valve stem unhindered. The valve disk and stem is kept functionally centered in the hole by means of six metal pins that encircle the valve disk.

(Photograph courtesy of Dana Johnson.)

Top view of a stack pneumatic with the pouch board glued in place. Once the motor pneumatics were covered with rubber cloth the next step was to glue the underside of the pouch board to the top side of the pneumatic, thereby combining layers 3 and 4 into one integral component. The outer edge of the ocre colored pouch leather has been glued to the pouch board, and in the center of the pouch is a thin cardboard disk. This simple disk not only protects the soft pouch leather, but it is larger than the button on the lower end of the valve stem, and so it increases the force applied when the valve is pushed upward to seal the atmospheric vent. The two larger holes at center connect with the valve chamber (on level 2), and allow airflow out of the motor pneumatic (level 4) when a partial vacuum is applied. The four smaller screw holes are for the long wood screws that hold all of the unit valve layers stacked tightly together.

(Photograph courtesy of Dana Johnson.)

Side view of a National unit (control) valve. There are normally only six of this type of unit valve assembly installed on the National built stack, with two such unit valves occupying the two left-most positions on each of the three stack tiers. In this side view it is easy to see the four distinct layers of the valve assembly: (1) The valve chamber block with its top side adjustable atmospheric vent, (2) the vacuum chamber block with a brass bleed orifice, (3) the pouch board block with a leather pouch, and (4) the bottom wooden distribution spacer (covered in black rubber cloth) with a 3/8" hole drilled through to its back end to accommodate a brass nipple or elbow. This long spacer is extended to get its back side even with or slightly past the horizontal structural shelf to which the unit valve is clamped, thus giving unrestricted access to the tubing fitting from the back side of the stack. Once the bottom distribution board (level 4) is internally sealed and covered with rubber cloth it and the pouch board (level 3) are glued together for a more or less permanent bonding.

The wooden end cap (shown here at the unit valve's left side) is connected to the tracker bar via the tubing nipple protruding downward. The end cap is channeled to communicate with the bleed (connected to the vacuum chamber—level 2) and the flexible leather pouch (pouch board—level 3), and to provide space for the little circular brass screen strategically placed to filter out dust and paper particles entering the system through the tracker bar. The black rubber button that plugs the clean-out hole for the circular brass screen is not visible. The end cap is held tightly in place by the stud bolts used to clamp the entire valve assembly snugly up against the stack shelf.

(Photograph courtesy of Dana Johnson.)

Oblique view of a National unit (control) valve. The upper portion, consisting of three easily visible layers, has a central square channel to supply vacuum to the valve assembly (level 2). The partial outline of a white colored leather packing gasket is visible, and it is this part of the valve assembly that butts up to and makes an air-tight seal against the channeled horizontal stack shelf. The bottom layer (level 4), covered in rubber cloth, extends back from the valve portion in order to clear the tier's horizontal support shelf, with a hole drilled in the back end to accommodate a 3/8" brass nipple or elbow. Rubber tubing would then be connected to either a control device or some other accessory function.