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MMD > Archives > February 1999 > 1999.02.26 > 06Prev  Next

Soft Pedal Problem
By Craig Brougher

Larry Toto had a question about soft pedal response in a Pratt Reed
player being too slow to return.  Before redesigning the soft pedal
circuit for overly slow return operation, do these two things in

1.  Increase the bleed in the pedal valve.
2.  Increase the valve travel about double.

The reason most 88 note players have a slow soft pedal is because
their bellows are too large for the valve operating them.  When the
valve starts down, you have vacuum from two directions: The second
direction being a heavy bellows "supply" trying to exhaust and
creating, in effect a supply subtraction effect which would normally
return the poppet.

So instead of the poppet being vacuumed down by supply directly with
air on top to help push it closed, instead, you have a vacuum in the
chamber you would hope to have air pressure, subtracting from the
vacuum of the supply on the poppet.  It stands the poppet in limbo
momentarily.  (It's the old story -- you don't get something for
nothing, and "for every action...").

Most rebuilds I have examined have pedal operators with the same gaps
in their valves that are used for the note pneumatics.  This is wrong,
as Larry will find out by adjusting his valve accordingly.

There is one other thing he may have to do, too.  Those valves should
have a bit more pouch travel too, and concurrently, more distance
between the stem and the pouch -- say, about 1/16" or a bit more.

The Pratt Reed has those little bitty valves in it and valve cover
boards, making them difficult to set, but it can be done.  If I recall,
it uses two valves (maybe 3, I forgot).  They are still too small for
a large pedal operator.  But remove the pouch board and dip the pouches
more deeply, then gasket out the valve cover board for more travel.

One thing that most rebuilders of Pratt Reeds don't know is, the valves
were gapped with pressure after being placed in their respective holes
at the factory.  Then the shellac dried, holding the gap constant for
that leather, in that hole.  That's why they dump out all the valves
and when its time to replace them, they are all out of order.  Some
will have very wide gaps, some very narrow if any gap at all!  That's
what probably happened.

These three things will usually take care of the problems with all
but the most outrageously designed pedal operators.  If the system
needs to be redesigned for the kind of response he wants, I would make
the bellows smaller, increase the channel or hose diameter between the
valve and the bellows, and double or triple-up the valves used.  I
don't think he will need to, though.

Craig Brougher

(Message sent Fri 26 Feb 1999, 18:11:38 GMT, from time zone GMT-0600.)

Key Words in Subject:  Pedal, Problem, Soft

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