Stephen Kent Goodman wrote:

> The tracking governor using a single ear sensor (Aeolian Themodist)
> seems to be under constant vacuum, i.e., it is active on the rewind.
> Does anyone have experience that will nullify my observation?

I'm familiar with a number of pianola grands and 65/88n pushups made
between 1911 and 1913 that have single-finger (or single-ear) tracking.
They all

a) have two pneumatics;
b) drive both off the stack supply before the governor valves, so
   both are under full suction during play and relax on rewind.

One pneumatic is vertically mounted and pulls the roll towards the
sprung end until the ear/finger valve destroys the suction in it.
This is the tracking pneumatic.  The action is kept sluggish enough
(by passing the finger valve line through a small bleed) for small
tears in the left-hand edge of the roll not to register.

The second pneumatic is horizontally mounted and pulls a small notched
bar downwards during play.  On rewind a spring pulls this bar up and
the notch captures a protruding screw in the "armature" (moving plank)
of the tracking pneumatic as the roll spring pushes the roll rightwards
and opens the tracking pneumatic.  This movement is also sluggish
thanks to a small suction bleed which prevents air entering too fast.
The notch is positioned so capture occurs when the roll is directly
above the take-up spool.

(View using fixed-pitch font)

  movement         O---------> rod to roll drive cam
  when suction on-> \_________
                     \       | tracking pneumatic (shown
                      \      | fully open)
  movement when        o .   | On rewind, bar moves up and screw
  suction off ^ o========v=======o   slides along the top of it
                |        \   |       until captured by the V at .
                |         \  |
  locking       o--_       \ |
  pneumatic     >   --__    \|
                |_______>    -

It looks crude, but once you've got the springs at the right tension it
works for years without further attention beyond lubricating the roll
plunger.

Dan Wilson, London