The Hickman ball bleed valve is a remarkable answer to the problem that
faces all designers of a single valve action: if you make the bleed too
small, the repetition suffers, and if you make the bleed too big you
lose the ability to play softly because the large bleed "eats up" the
signal coming down the tube from the tracker bar.

Hickman's design allows the note to play with a very tiny fixed bleed.
The vacuum that exhausts the note bellows is then used to raise a small
ball bearing inside a brass cylinder that has a crimped top.  The ball
cannot seal against the top, so air moves past and quickly exhausts the
tube to the tracker bar.  Therefore, the note has a small bleed when
the note plays, yet it has a large bleed when it comes time to reset
the system.

One problem that many have mentioned is when the ball starts to stick
inside of its cylinder.  I can not offer any advice here, but I want to
ask a question: The ball seals when there is vacuum below it, and it
rises up when vacuum is above it.  This certainly sounds like a flap
valve to me.  Wouldn't a small flap of leather do the same thing?  I
don't know if it is possible to modify an existing valve block, and I
am thoroughly opposed to any sort of permanent change, but even if it
isn't possible on original blocks, it might be possible to work it into
new valves that are yet to be made.  A tiny flap of leather, less than
a quarter inch square is certainly cheaper and less prone to
malfunction than a precision ball and sleeve mechanism.

Clarence Hickman was a brilliant man, and he certainly could have
thought of using a flap valve.  Did he use the ball because flaps won't
work, or because balls are better or because he didn't think of the
flap?  I invite readers to speculate on this if they are interested.

Randolph Herr

 [ A drawback of the tiny #70 bleed, which Dr. Hickman didn't foresee,
 [ is that the unit valve is very susceptible to stray leakage when the
 [ gasket and wood become dry after many years.  This is one of the
 [ reasons that the double-valve Ampico A has a reputation for
 [ endurance, in comparison with the Ampico B.  On the other hand,
 [ a model B Ampico restored to new condition is a superb performer.
 [ See the articles by Craig Brougher and others in the MMD Archives.
 [
 [ Selected pages from "The Ampico Service Manual 1929" are reproduced
 [ at the MMD Ampico site, http://mmd.foxtail.com/Tech/Ampico/index.html
 [
 [ -- Robbie