By Craig Brougher
|When answering Frank Himpsl about his model A, Robbie mentioned this:|
>[ My Ampico B had similar anomolies before it was rebuilt. I think
>[ that when the vacuum rises slowly, as when the motor is turned on,
>[ the valve logic doesn't "follow the normal rules." -- Robbie
What is happening with the B is that you start the roll with an automatic play setting of the transmission switch which will not reverse the direction of play until the take-up spool switch in the drawer has allowed the repeat pallet to close. By then, some of the expressions have been turned on and left on, since the roll leader covers both cancel holes first, then the expression holes last. So unless cancel holes are operated by note holes later on, the B will play 88 note rolls at about a mezzo-forte level.
The speed at which the vacuum turns on shouldn't have any effect on the logic, as far as I have ever discovered. In the B, it's all proportional to the pump pressure at every level. That's the basis of how it repro- duces! The curtain is a proportional expression regulator. It selects a specific percentage of the vacuum supply, rather than a specific pressure irrespective of the supply, and the supply is constantly changing during play, as you know. Now granted, if you exceed the sensitivity of the expression primaries, they will stay inoperative or half-mast until the pressure comes up, and then they will start to work. But nothing is lost in the mix. There is no "logic" to get tangled up. And you can't imagine how grateful I am about that!
[ Authors response:
[ I believe that the explanation above is true only under the
[ manufacturer's "Recommended Operating Conditions", Craig. Nobody
[ expects TTL logic to operate to specs at lower-than-normal voltages,
[ and the same should be true for binary pneumatic logic.
[ Unless special provisions for "Power-On Preset" are included in the
[ design, the logic state following Power-On is indeterminate. That's
[ why, for example, all Ampico and Welte rolls have "Cancel Intensities"
[ commands before the music begins. I have no faith in a control
[ system until after the power supply has stabilized, and the proper
[ initial conditions established.
[ -- Robbie
(Message sent Wed 1 Jan 1997, 19:02:10 GMT, from time zone GMT.)