Although not as eye-catching as the motor, the auto tracking device is just as necessary and just as sexy. In fact the tracking device is usually the most mechanically elegant feature of the whole dang piano! In his introduction to the Standard Tracking Device (p 143), Art Reblitz calls the Standard in particular “one of the most sophisticated pneumatic mechanisms in any piano”.
The tracker in my piano is very nearly the same as a Standard, it operates on exactly the same design principles, especially in the valve box (also called shifter box). However in terms of appearance and layout, it is essentially an Autopiano tracker (see image below). Indicators are the bottom bleed chamber, as well as the vertical orientation of the pneumatics, plus another particular feature I will mention later on.
Note that while the general principles of the following information apply widely to most “automatic” pneumatic trackers, there are specific differences which may apply.
The tracking device keeps the paper rolling on the “straight and narrow”. This is vitally important, because if the paper becomes misaligned with the tracker bar as it unspools, the music becomes real bad, real fast. If we continue with an automotive analogy, the motor and the transmission are self-explanatory. The tracker can be considered the steering wheel. Imagine therefore, trying to drive your car, and the wheels did not follow what you were doing as you turned the steering wheel: it would be interesting, but not in a good way!
And so, let’s dig in a little bit and try and figure out how this thing works. I admit it has taken me some time to understand this thing. There is an awful lot to digest here, so understandably it may take you a while to get through it. Alternately you can watch a video by John Tuttle here. In fact, I recommend that you read all this, then watch the video, then do it all again!
Once it’s all taken apart, we see that it consists of two outside (primary) valves, six pouches and seven channels – three of which are almost too small to be considered true channels.
The 2 primary valves are paired with a set of primary pouches. There are two “wannabe” inside valves paired with what are called the “cutout” pouches. These are essentially two thick wooden discs (which look like primary valve caps) glued on top of the cutout pouches. Over head of the of the cutout pouches there are two slightly larger pouches, cleverly called “overhead” pouches. The overhead pouch has the power to override the cutout pouch, due to larger surface area (in my case 1 1/8 vs 7/8 diameter).For channels, we’ve got 4 “inputs”, which is where the four nipples on top of the valve box are tubed directly to the 4 specified holes in the tracker bar. Inputs 1 and 2 both lead to a matched but independent channel which runs underneath both the cutout pouch and the primary pouch. Inputs 3 and 4 are essentially direct feeds to each overhead pouch well (again, independent of each other).On the underside of the valve box (on my model, at least) there is a suction supply, which is just a small hole in the valve well with a nipple outside to hold the tube.Finally, there are two interior channels connecting the primary valves to the pneumatics. The primary valves determine if the pneumatics get suction or atmosphere, so these can be considered the “outputs” of the valve box.
Confused yet? I can’t blame you, after having read all that!A picture is worth a thousand words, so here is what the inside of the valve box looks like:
The first thing that will help simplify matters is to notice that the valve box is a stereo set. In other words, you really only have to understand “half” of what is happening, and the other half does the same thing.
The second thing is that even though the 4 inputs from the tracker bar can theoretically give up to 16 different permutations, for practical purposes it’s actually less than that.
If we work backwards and follow the path, we can see that the tracker linkage (which controls the position of the music roll, via the spool cam) is effected by the movement of the tracker pneumatic. The tracker pneumatic moves (e.g. opens or closes) depending on what “signal” (suction or atmosphere) it is getting from the output of the valve box. This signal is switched on or off by the primary valve. The valve is controlled by the primary pouch. The primary pouch is arranged inline with the cutout pouch, and both of these pouches are fed by the exterior hole of the tracker bar hole set (inputs 1 or 2, as labeled by me). The overhead pouches are fed by the interior holes of the tracker bar, which are again labeled 3 and 4 respectively on my box.
States of play, of one side of valve box (meaning one valve only):
1) Both (inner and outer) holes of tracker bar are open (no roll paper is covering). The inner hole triggers the overhead pouch, which overpowers the cutout pouch. Even though the outer hole is open, the cutout pouch is overpowered, so the valve is not opened, and the suction goes through to the pneumatic.
2) Both (inner and outer) holes are closed. The overhead pouch is not triggered, but it doesn’t matter because neither is the cutout or the primary pouch. Again, the valve stays down (closed), and the pneumatic keeps receiving suction.
3) Inner hole closed, outer hole open. The overhead pouch is not triggered; the outer hole allows air to pass to the primary pouch and lift the valve. The valve opens, closing the suction chamber and letting atmosphere into the pneumatic. This is what could be considered “default” setting
4) Outer hole closed, inner open. Unless the roll was damaged in a very particular way, this situation would never occur. In theory it would mean that suction continues to that pneumatic, as in case 1.
Note that for the first 3 scenarios, as long as the same thing is happening on both sides of the tracker bar (and valve box), the system can be considered balanced, which means the pneumatics are acting equally in concert, and not trying to move the piano roll left or right to compensate.
When there are differences between the sides, we will have an unbalanced system, so we have to revisit our scenarios.
5) One inner hole closed (on one side), other three holes open: atmosphere is fed to the open holes, for practical purposes it is the uncovered inner hole that leads to the overhead pouch, which will expand and let atmosphere pass to one pneumatic. The other pneumatic will still be getting suction, so it will pull the atmosphere pneumatic back into balance.
6) One outer hole opened, other three closed: the outer hole will allow atmosphere into its channel and open the valve, allowing atmosphere into one pneumatic. Again, the system will automatically rebalance.
7) One pair of inner/outer holes opened, other pair (on other side) closed: this would be an unusual situation, most likely resulting from a damaged roll. As explained by John Tuttle here, if both holes on one side are temporarily opened, the tracker does not try to correct, but just keeps status quo. Whether this was a planned design outcome, or just happy accident, I don’t know!
If that is not sufficiently confusing, another key fact is that the inner/outer holes are cross connected to the valve box inputs, so that the inner and outer hole from the same side of the tracker bar are not inline on the same input channel. Why? Because that’s the way it was designed, and that’s the only way it can work! I spent a bunch of time thinking about this, but John Tuttle gave me the above explanation, and really that’s what matters. It just is.
Other related facts:
The outer holes are for standard (or larger) sized piano rolls, standard size being 11 1/4″. The system will still work with slightly narrower rolls (i.e. the system can function using the inner holes only), but if there are rolls which are significantly wider than standard, the system is not well-equipped to respond to roll drift in this case.
Finally, the “particular feature” on my tracker is that there are no bleeds for the overhead pouches. This is related to the fact that my tracker bar scale apparently allows some air into the outer holes at all times, the holes are indexed this way on purpose. So not only is the overhead pouch larger, but due to lack of bleed it really comes down hard and fast, to override the cutout valve and keep the linked pneumatic under suction.
Furthermore, John reminded me that the self-cleaning bleeds are also self-regulating as well; the more that a valve pouch opens, the more the corresponding bleed opens as well, which means the resistance to opening increases concomitantly. Interesting stuff!
Your head probably hurts a little from having read all this, so let’s leave it there, and get into the specifics of rebuilding next time.
Sources: Player Piano Servicing and Rebuilding. A. Reblitz. Vestal Press, Lanham, MA. 1985.
“Player Piano Pointers”. Auto Pneumatic Action Company. New York, 1917. (Note: this is a public domain document, available from AMICA, with membership)