And like other expression devices on the piano, it is divided into a bass and treble side. Since it is a single box this means it sits smack in the middle of the pump. The soft expression feature is controlled by two separate buttons, which can be depressed at will by the operator.
When a button is pressed, it opens a small pallet valve under the button, sending a “signal” of atmosphere to the designated side of the unit. This small signal activates a pouch of an inside valve, closing it and choking the amount of air flow to the stack, to a moderate degree. In other words, the soft feature is literally a “choke”. There is a pneumatic on the side of the box, which is in the path of the diverted airflow, linked through two ports. Unlike the valve it closes with a slight delay (it is spring loaded to resist); this is to make the decrescendo less sudden and jarring.
That’s basically it…the idea is that as soon as the button is released the airflow of suction to the stack (and hence the volume level of the piano) returns to normal immediately.
Here’s what the valve box (now with new gaskets and pneumatic cloth) looks like with the panel cover on it.
And finally here it is mounted on the trunk, still open while I do some set up for testing with the pump…
