Although I don’t necessarily want to get too far into the weeds on this topic, I do believe it behooves me to discuss valves, given their importance in a player piano mechanical action.

The key to understanding player pianos is fundamentally a question of comprehending valves: air pressure and flow. Conversely if you don’t understand how valves work – in theory and practice – you probably won’t successfully figure out how to rebuild and troubleshoot your project. A helpful analogy may be to think of the airflow as an electrical signal, and the valves as switches.

Let’s focus on some theoretical basics for this post.
There are a number of different types of valves; the most common are inside and outside valves. Outside valves normally function as a “primary” role, while the inside valves are generally “secondary”. This is how they are assigned in a Standard style double valve player action.

Both styles of valves operate in a valve chamber, which needs a constant supply of suction from the pump to operate. Each valve has a corresponding pouch underneath it, which will activate the valve when the pressure builds enough to inflate the pouch. Every valve will have an input and an output as well. Note the following image, and how the output of the primary becomes the input of the secondary:

It is important to bear in mind too that the input signal doesn’t directly come into contact with the valve. The signal determines whether the pouch (below the valve) is up or down, which in turn determines if the valve is opened or closed.

Primary valves receive a small signal (usually via tracker bar), and augment it. They are signal amplifiers.

Secondary valves also multiply the signal, but the signal is reversed. This is an important note. In other words, if the input to the secondary (or any inside) valve is atmosphere, the output is suction. If the valve receives suction (i.e. the valve is closed) its output will be atmosphere.

Here is another line drawing of the signal path in a Standard double valve style action.

There are three discrete stages which occur almost simultaneously.
When a perforation in the music roll passes over a hole in the tracker bar, this admits a small amount of atmosphere and initiates stage 1. This small amount of atmosphere allows the pouch to rise and activates the primary valve. The now open primary valve allows a larger signal of air to enter the connecting channel to the secondary pouch, which is then raised and opens the secondary valve in the same manner (stage 2). The activation of the secondary connects the pneumatic to the valve chamber with its reduced level of atmosphere; in other words the valve outputs a signal of suction. Stage 3 occurs when the atmosphere has been reduced in the pneumatic the outside pressure of the atmosphere forces the pneumatic to collapse. The pneumatic (in this design) collapses upward, actuating the piano wippen and playing a note.

The cycle ends when the paper again covers the tracker bar hole; with no more atmosphere signal the suction in the primary valve chamber quickly equalizes the pressure on both sides of the pouch (aided through a small “bleed” passage next to the pouch). This in turn shuts off the signal to the secondary valve and then to the pneumatic, which equalizes once more with the surrounding atmosphere, and reopens.
This whole cycle may take a fraction of a second.

The valve chambers for the primary and secondary valves are under constant suction (as generated by the pump); in the diagram just above the blue arrows indicate the path of the suction back to the pump. This is to differentiate from the “signal” paths which are colored gold and orange.

I hope you have enjoyed valve theory 101; if you have interest I invite and encourage you to read more on the subject in one of the resources previously mentioned. Don’t feel bad if you don’t understand everything at a glance, it took me hours of study to really get what is going on in these diagrams!

On to materials!