(Photograph courtesy of Dave Anderson.)

Tilting yoke atmospheric-port bleed type control valve in National piano #7125. This component is typically mounted on top of the left-side foot extension of a National built stack, precisely as pictured above, Here the bleed port is partially uncovered, probably enough so to cause the piano to play at the loud setting. This simple front view is sufficient to thoroughly explain and then understand how this component functions, although how it works may at first seem to be counterintuitive. Although not present here, typically there was a square piece of pouch leather glued to the metal surface behind the tilting yoke, with a hole punched through for the bleed vent. This pliable pouch leather membrane enabled an air tight seal between the two metal surfaces when the yoke's tail end was moved to cover the bleed port.

Structurally the device consists of an die-cast block in which a wide channel has been formed into its topside. On either side of this channel is a narrow pneumatic that faces forward, and on the front side of the movable leaf is a small wood screw that interacts with the "tilting yoke" port type valve. The pneumatic on the left tilts the yoke counterclockwise, closing the atmospheric 1/8" diameter bleed port centered below the pivot screw, while the pneumatic on the right tilts the yoke clockwise, opening the bleed port. This one way or the other action constitutes a simple lock and cancel register, where one music roll perforation turns something permanently on, and another cancels or turns it off. Looking at the tracker scale, tracker bar hole #70 for low vacuum should operate the left side pneumatic, and tracker bar hole #71 for normal (high) vacuum would control the right side pneumatic.

In the bottom or lower half of the die-cast block a small 1/8" diameter hole was drilled from side to side that intersects with the bleed port of the same size, with each side then fitted with a 3/8" brass elbow to which rubber tubing is attached. The tube on the left side is connected to the player system vacuum source; the tube on the right goes to the regulating pneumatic placed directly above the face of the vacuum spill valve. The sole purpose of this so-called regulating pneumatic is to lift open the spill valve whenever the system vacuum level exceeds a certain threshold level. The opening and closing of the atmospheric bleed port on the "tilting yoke" component influences the vacuum level up or down within the regulating pneumatic, which in turn affects how much upward force is exerted by the regulating pneumatic. The more upward force applied the more the spill valve is forced to open, causing the system vacuum level to fall. Conversely the less upward force applied the more the spill valve can approach being closed, forcing the system vacuum level to rise. The rise and fall of the system vacuum level directly determines how loudly or softly the piano notes will be played.

The adjusting screw (with locknut) protruding from the front, and to the left of the bleed port, is used to restrict the amount of airflow moving through the vacuum supply connection, which must be set properly within a narrow range for both the expression system and the accent (Sforzando) hole #1 on the tracker bar to function correctly. When the bleed port is closed the vacuum level applied to the spill valve regulating pneumatic will be at its maximum, more strongly pulling the spill valve open until the system vacuum level drops sufficiently for the spill valve to close off and stabilize, but at a lower state, whereupon the piano notes will be softly played. Then when the bleed port is opened the airflow allowed through the vacuum supply side must be sufficiently restricted so that the airflow through the bleed port partially overrides the restriction to the vacuum supply, causing the vacuum to bleed off enough to bring about a significant drop in the vacuum level in the regulating pneumatic. When this happens the regulating pneumatic exerts less force on the spill valve, thereby causing the player system vacuum level to rise, causing the piano notes to play loudly. To adjust the vacuum side screw the setting should be tightened down a little beyond the point whereby the regulating pneumatic responds quickly and reliably whenever the bleed port or accent hole in the tracker bar is opened or closed. It should also be mentioned that this adjustment also influences, within very narrow limits, the depth or vacuum range between the minimum and maximum levels, with the ideal, if it matters, to be determined through experimentation and individual preference. The more airflow through the vacuum side adjustment the narrower the overall vacuum range; the less flow the greater the range.

The adjusting screw (with locknut) projecting outward on the right hand side (in front of the brass elbow) is used to restrict the airflow going through the atmospheric bleed port. This adjustment only has an effect when the bleed port is snapped open; no effect at all when it is closed. A logical purpose for this adjustment is to dampen any serious resonant and possibly noise producing oscillations that might be induced while the regulating pneumatic, and the system as a whole, is seeking to re-balance itself when the vacuum level is suddenly forced to go to a higher vacuum level.

(Photograph courtesy of Dick Hack.)

Tilting yoke atmospheric-port bleed type control valve in National piano #7352. Here is another example of a stack foot mounted "tilting yoke" expression control mechanism. This one has some dark colored thin pouch leather glued onto the die-cast substrate behind the "tilting yoke," which in this instance shows the yoke's tail piece to be covering the 1/8" diameter bleed port (for piano soft). There is a small hole in the leather to match up with the bleed port orifice drilled into the die-cast block. The pouch leather surrounding the bleed port helps to make the fit between the "tilting yoke" and the bleed port more air tight.

With the end of the stack foot left bare—many are covered with rubberized bellows cloth to seal the end grain of the wood to prevent air leakage—you can see how the lower part of the foot was bored to form an air passageway. The front end of the hole has a wooden plug glued tightly in place, leaving behind an internal channel that serves as a connector between the vacuum pump and the upper main portion of the stack with its three tiers of unit valve assemblies,

(Photograph courtesy of Dan Zelinsky.)

Tilting yoke atmospheric-port bleed type control valve in National piano #7504. Here is yet another example of a stack foot mounted "tilting yoke" expression control, but this time with some ocre colored pouch leather surrounding the 1/8" diameter bleed port. This angled view downward gives a much different perspective, better showing the depth of the unit and how the "tilting yoke" component was constructed. Here it will be noticed that the die-cast block is actually much wider than it is deep, with the two narrow control pneumatics hanging far behind the die-cast part.

(Photograph courtesy of Dana Johnson.)

The regulated spill valve component in National piano #7823. With restoration in progress, the stack has not yet been reinstalled when this photograph was taken, and so the normal interwoven tangle of rubber tracker bar tuning is not present to hide the spill valve mechanism. The spill valve assembly is supported on a die-cast base or pedestal, the top surface of which forms the non-movable face of the valve seat. The movable part of the valve is the bottom leaf of the regulating pneumatic, to which is loosely attached a thin metal plate covered on its bottom side with a suitable valve leather. This metal plate is about one-half the length of the pneumatic and is attached by two tiny loose wood-screws on its backside (about midway on the length of the pneumatic). The regulating pneumatic is suspended above the fixed valve face surface by a metal bracket screwed to each side of the die-cast pedestal. When the pneumatic's lower movable leaf is raised the spill valve is gradually opened as the metal plate tilts upward starting from its back end. Conversely, when the lower leaf is lowered the spill valve is gradually closed as the metal place levels out and creates an air tight seal against the spill valve face.

Sticking up on the top side of the regulating pneumatic, toward the front edge, is a round brass appurtenance made up of two cylindrical brass parts (a smaller hollow stem piece inside a larger knurled cap-sleeve) and a machine screw that serves to adjust the tension on a compression spring inside the pneumatic. The brass stem is partially drilled out to accommodate this compression spring, with the topmost portion having a threaded hole for the machine screw, which can then be used to adjust the tension on the spring. The cap-sleeve is also hollowed out, except for a threaded hole in the top end for a machine screw. In this photograph the machine screw (with a hex-nut next to the rounded screw head, is first screwed fully into the top of the cap-sleeve and the nut then tightened to lock the machine screw from turning. This cap-sleeve assembly is then inserted over the brass stem and screwed into the threaded hole in the top of the stem. The knurling on the cap-sleeve makes it easy to grip and twist it for the purpose of adjusting the tension of the compression spring located inside the brass stem, and that pushes down on a plunger that fits into a dished-out area inside the lower movable leaf of the regulating pneumatic. The more spring tension pushing down the higher the system vacuum level necessary to open the spill valve, and, conversely, the less the spring tension the less the system vacuum level will be. This simple adjustment is probably best used to set the "at rest" lower or soft vacuum level, which must be set for a vacuum level that is high enough for all stack valves to function reliably, while maintaining good repetitive accuracy without any dropouts.

There are two brass elbows on the top stationary side of the regulating pneumatic. The 3/8" fitting to the rear connects to the "tilting yoke" component. The small brass elbow to its front connects to hole #1 accent (or Sforzando) on the tracker bar. The large diameter opening in the fixed spill valve face is internally channeled to a 1" hose fitting out the backside of the die-cast mounting pedestal, and is teed into the main hose connection between the vacuum pump and the stack.

The spill valve is used to not only regulate the vacuum level, but it also serves as a vacuum dump valve during rewind and roll changing operations. The dumping operation is controlled by a cam that is an integral part of the Rewind/Play mode control system, the single lobe cam working against a lever that rocks on a center pivot point and that mechanically lifts the front end of the spill valve wide open. Doing this immediately dumps all system vacuum, so that no piano notes sound during certain non-musically oriented system operations.

(Photograph courtesy of Dick Hack.)

The regulated spill valve component in National piano #7352. In this example of a regulated spill pneumatic, the outer cap-sleeve is missing or was never present, making more clearly visible the partially hollowed out brass stem that accommodates the upper portion of the internal compression spring. Here the machine screw (with a hex-nut used to lock the screw) is merely threaded into the top of the brass stem—without the usually present cap-sleeve. But whether, or not, there is a brass cap-sleeve is probably immaterial, other than perhaps providing some bit of convenience, in as much as no tools are necessary to adjust the inner spring tension when a knurled cap-sleeve is available. Without the cap-sleeve covering the brass stem its threaded exterior is readily apparent. The threaded end seems to be used to screw the stem securely into the wooden leaf of the pneumatic, thereby fastening it more or less permanently, yet still making it possible to easily remove it if ever necessary.

The cam follower lever arm, and its pivot point, which lifts the bottom leaf of the regulating pneumatic, are clearly visible in this photograph. At the lever's tip is a screw-eye for the attachment of a tension spring that pulls the lever arm away from the regulating pneumatic, so that it does not influence the vacuum regulation process during the Play mode operation. Through the top side of the lever arm is a threaded hole for a machine screw, which can be used to carefully adjust away any lost motion between the lever arm and the spill valve surface.

(Drawing courtesy of John Perschbacher.)

Simple ink cross-sectional drawing showing a side-view of a National spill valve assembly. At the base of the drawing is a representation of the wooden "keybed" area shelf, with the airway hole though it being the vacuum connection to the die-cast spill valve body. Next up is the die-cast body of the spill valve, its topside being the spill valve face. Resting on the spill valve face is a leather covered thin metal plate, which is loosely attached to the movable lower leaf of the pneumatic by two tiny wood-screws—the screws located about midway down the pneumatic's length. The loose fitting screws allows the end of the metal plate to swivel somewhat about its attachment points when the lower leaf of the pneumatic is raised. This lets the metal plate gradually peel away from the spill valve face when the lower leaf is pulled upwards. On top of the pneumatic is an adjustable brass cap assembly that allows the spring tension pushing down against a small plunger to be varied. The plunger rests within a small dished-out area in the movable lower leaf of the pneumatic, and by adjusting the spring pressure pushing downward it can be used to set the "at rest" lower or soft vacuum level.