Michael Pawelek wrote:

> I can see how the four small bellows inside the player work in
> concert to create the vacuum beneath the reeds, but what is the
> purpose of the one large bellows beneath the four smaller ones?

Howdy Mike,  Never having seen the particular instrument you describe,
it's still evident from your description that the larger bellows is
the reserve, or reservoir.  The purpose of the reserve is to bring
stability to the air supply, or in this case, the vacuum "supply".

The term reserve, or reservoir, makes a bit more sense if you think
of an organ that runs on air pressure, as opposed to vacuum.  In that
case the reservoir has air under pressure "in reserve".  However the
principle behind the reserve works equally well for maintaining a
stable pressure or vacuum.

A pressurized air reservoir starts out closed, held closed by weights
and/or springs, is opened by pumping air into it.  Think pipe organ.
Conversely, a  vacuum "reserve"  works by being held open by springs,
then pulled closed by evacuating the air inside.  Think player piano.

All organs of any size, whether operated by pressure or vacuum will
have a reserve of sufficient size to help regulate and smooth out the
pressure or vacuum available to the system.

The fact that you say you can push the reserve bellows up from the
bottom, makes sense that your system runs on vacuum.  It's probably
spring loaded inside to keep it open.  As the vacuum is created by the
4 smaller (exhauster) bellows, it will be pulled closed.  It may even
have a regulator valve, so that when it closes almost all the way, air
is allowed into the reserve to keep the system from "topping out", and
overdriving the reeds.

The larger surface area and volume of the reserve evens out the vacuum,
which might otherwise be a bit jerky if only supplied by the 4 smaller
bellows, as the roll calls for varying numbers of notes to play at any
particular time.

If you ever get the chance to get into a pipe organ chamber, check out
the reservoir system.  It will be a quite large bellows, utilizing
weights and/or springs, and may have a tremulant feature, which makes
the whole works vibrate.  It's quite interesting to see in action..

Good luck with your rebuild.

Tom Lear
San Francisco, Ca.