Duo-Art Vacuum Numbers
By Craig Brougher
Pete Knobloch presented a set of vacuum readings that I would say I have never seen the likes of. I don't even know how to get a Duo-Art to do that if I wanted to, given that everything else is correct.
If in fact his pump speed is actually running 7% slower than optimum, all the while putting out these awesome slopes of expression gradient, then I would have to agree with Pete-- I wouldn't like it, either. I also would not allow the machine to beat my piano to death.
Without actually having the expression box in front of me, I cannot offer a positive solution. I cannot explain how a Duo-Art pump could be capable of doing what he claims for it at that speed with fixed leaks required to do this test. It has a fixed capacity, and if 100% efficient, cannot propel a piano into orbit like this one apparently is. The concert piano pump speeds were doubled, and perform about similarly, except their Theme curves were still straight. This one is just nuts! The Theme chart is really wild.
My first question is, Were these figures made with the suggested fixed bleeds, or dead-headed straight into a stack? The linearity of the accompaniment side of the box however is pretty good up until the 13th step, at which time it goes off "into the Boondocks". That suggests some problem like a warped knife valve that cannot close (or a leaky gasket under the knife slide), at the same time the stack valves have been sucked down cartridge-tight, so up goes the pressure.
If we are in the ballpark, here, and the gauge used to chart with was accurate, then I suggest that the only way these figures could be had is if we are playing into a (believe-it or not) leaky stack! What might happen is this: If we do not have fixed (linear) bleeds of at least 5/32" to represent valve playing losses, then these figures are irrelevant. As the pressure builds in the stack, the valves get tighter and tighter, and at the accompaniment pressure of 26", suddenly jumps to 34" (as it shoots past the 29.5") and tops out to whatever the spring allows. [Warped knife valves will still close tight, usually, but not exactly where they should. ]
However, we don't see the same effect during the theme portion of the curve. It actually rises dramatically between 6 and 9, then loses slope between 9 and 12, becoming less slope than the accompaniment (although higher in intensities). So that might tell us that the half of the stack tested under theme air simply did not have the same leaky valves which could have caused the effect we see when reading accompaniment.
Let me explain that. You do not have to even move your gauge. When you test accompaniment, you are actually testing the entire stack characteristic. There is no stack separation. When you test the theme air, only then is your stack divided. So if the box is linear for a ways and then jumps up, that suggests a combination problem of leaky knife valve plus tightening stack valves. In other words, perhaps not enough clearance above the pouches, or a bunch of cross valves whose time has (ahem) come. (Characteristic of rotated cross valves, too). And often, you'll see a combination of those problems.
The nonlinearity of the Theme chart is a puzzle, frankly. However, I would not gray-scale the box to suit your fancy. Every thing is dependent upon everything else, so it just can't be done. The piano will never play realistically that way. When you misadjust one thing, you cause something else dependent upon it to be really fouled up.
With the appropriate bleeds, My box tests begin at 5" and 5.5" usually, and end up at somewhere between 24-26" for the accompaniment and 34-38 " for the theme. It depends on the springs your box comes with, but the important thing is the linearity, not the top end. If you get good linearity, then you can hop up your pump and still maintain it. My bleeds are .161" and I clamp my pump at something like 40" so that I purposely cannot get above that. (It is a Duo-Art pump for a test pump, but has its own regulator.)
Before wondering what to do next, I would use the leakage tester device pictured in the Orchestrion Builder's Manual to test both sides of the stack, first. Disconnect the expression box, cover the trackerbar, and test the leakage with a steady vacuum source set at about 15". You can just bleed off the excess, as long as the load doesn't change the pump pressure. Test both ends of the stack at 7-1/2". Then raise the pump pressure to 40" or 50" and test each end by setting the tester to half of that. If it tests considerably different, you have leaky valves which are pressure sensitive. Probably cross valves, but sometimes valves set too close to the pouches, too. A tight (but not too tight) Duo-Art stack should read a leakage equivalent of about 2-3/32" dia. holes, or a little less. just one 3/32" hole may be sufficient leakage to use with a knife valve, but I prefer a leakage equivalent to about 1-1/2 3/32" holes. The Duo-Art must have valve leakage to work correctly, and that leakage should be fairly constant, although not perfect.
Any Duo-Art having patent leather valves for instance, will never adjust correctly. Neither can you add a fixed leak to it, because that leak is too constant. Don't get me wrong, now. The valve leakage will never be "linear," but it is necessary at all times except at the very last step intensity. So it's normal for valve leakage to decrease as pressure rises, but not very much or very fast. When you see an abrupt change as the pressure is slowly increased, that is a problem valve(s). Let me know, and I'll tell you what to do next. With luck you will be able to fix this problem right in the piano. And if your vacuum numbers were taken without bleeds, then before we can know anything else, you will have to provide them.
Craig B.
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(Message sent Fri 6 Sep 1996, 16:34:51 GMT, from time zone GMT.) |
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