Ampico A/B Question.
By Pat Mullarky
Hello from the unusually warm Pacific Northwest:
You will recall the recent discussion over using one or two crescendos in an Ampico A/B system.
A follow-up question: with only one crescendo, how does one balance bass and treble expression pressures? (vacuums?): I rebuilt the "sort-of-A" expressions this weekend (due to rotten Perflex pouches...), and after setting the expression boards parallel and spring pneumatics 7/8" open with the regulator valve on its seat, if I set the bass expression to 5" I get 7" out of the treble expression. Of course the treble is *much* too loud.... If I set the treble to 5", the bass expression outputs 4" and of course won't play.....
These "sort-of-A" expressions do not have a re-regulator pneumatic or re-regulator valve. They have only the spring pneumatic and valve.
How does one balance the bass and treble sides with only one crescendo? It's clear that small differences in the expressions themselves and supply pressures are adding up in one direction here... because the spring pneumatics both have the *exact* same pressure in them....so if the expression pneumatics have the same force (which they won't due to tiny area and position differences) the output pressure should be the same. It isn't, of course.
I can adjust the spring pneumatic's closure distance up or down from 7/8", though my math tells me that this will make only tiny fractions of a percent change in the spring pneumatic's force on the regulator valve. The same for the expression pneumatic's control board. Setting it very slightly non-parallel one direction or the other will add or subtract very tiny amounts of regulator valve force.
Of course I'll be fiddling with it till it is balanced one way or another... .. but is there a "factory" or "standard" method of balancing bass and treble 1st level with only one crescendo that anyone is aware of?
I have a vague distant memory that there might have been a procedure I read about twenty years ago or so.... Anyone remember such a thing?
-Pat-
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(Message sent Tue 30 Jul 1996, 16:23:27 GMT, from time zone GMT-0800.) |
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