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MMD > Archives > March 1999 > 1999.03.15 > 12Prev  Next

New Valve Tester and Ampico B Valves
By Craig Brougher

Robbie was asking about the possibility of the valve tester simulating
a slow roll.

>[ It sounds like pulse duty-cycle modulation or similar will do
>[ the trick, Craig.  The reason I want to simulate the "lethargic
>[ hole" is to demonstrate the big before-vs.-after difference
>[ when a bad unit valve is properly rebuilt.  With your test setup
>[ I should be able to measure, and record, the dynamic pressures
>[ and motions in the valve and a pneumatic, and see the improvement
>[ expressed in numbers.

Yes, I would have to add a circuit to it, however.  Something that
would slope my leading edge of the pulse, like a very slow rectifying
RC circuit in the output stage.  The power Darlington I'm using would
no longer be just a switch, but biased as an amplifier that way, so I
might have to change to a different kind of output amp.

Still, you will be able to easily see the great differences between
unrestored and restored valves without having to resort to the "slow
roll" simulator.  Just put an "instantaneous pulse" input into an old
pouch, and the difference should become clear, once that is compared to
a new one.  It should especially become obvious as the vacuum is
adjusted down below 4" or so.  So if all you really need is just a
clear indication of weakness that everybody can appreciate, that should
do it.

Even at a tempo of 50, with a properly punched roll and full-sized
holes (anything less isn't "Ampico" so we're not testing well anyway)
The time it takes for an .080" diam. hole to activate a B valve should
not be longer than 1/50th of a second.  Old Ampico valves cannot operate
that fast generally because their pouches are stiff, due to the rubber
cement used to seal them.  What's worse, though, is the variation
between valves in sensitive timing situations.  Here is a list of
factors which slow them down.

1.  stiff pouches
2.  leaky pouches
3.  leaky ball bleeds (old)
4.  partially clogged tracker bar tubing
5.  loose pouch nipples
6.  porous shellac sealant
7.  leaky pneumatics
8.  rough jack let-off (due to jacks too far under the knuckles)

Those are about in order of their effect, too.  But old pianos or
repaired pianos have 3 or 4 of those factors (at least) extant at the
same time, and some -- to a degree -- have all of them!

I just momentarily tried a very quick test with two block valves --
a Wurlitzer valve and an Ampico B valve, just to see how long a pulse
I'd have to have to minimally actuate either one.  Once I found that
the Ampico B valve operated with an actuation time of between 7-8
milliseconds at 20" of vacuum, I put the Wurlitzer valve (also rebuilt)
on the tester, and saw that it would just lightly throb at the same

Can't as yet do real testing with it because I have to calculate my
timing, without a 'scope or any good way as yet of reading the pulse
width.  But, that's coming, gradually, as I have time.  Presently, it's
just a rough breadboarded circuit to check out my speaker valve, but
the actual values of the components can be first calculated by actually
counting and timing several hundred slow pulses and working backward
from the frequency to the actual values involved.

I was just yesterday talking to a rebuilder who received a call from a
shop which had taken in a Mason & Hamlin Ampico model B to restore.
They were delighted that they "wouldn't have to do much to it, since
the valves all "looked' just fine." So they told him, "We just rebuilt
the pump, covered the pneumatics, and retubed it, and it plays good."

I still can't get over this attitude.  The top-of-the-line Ampico
piano "restoration" is being temporarily patched up and charged for as
a full restoration job.  That piano will play mediocre -- not 'good' --
for about five more years if the customer is lucky, then it's going to
stop playing "good."  But when I think about it more, that owner is
getting what he asked for, and probably would not appreciate a fully
restored, great model B Ampico, anyway.

Craig Brougher

(Message sent Mon 15 Mar 1999, 13:48:37 GMT, from time zone GMT-0600.)

Key Words in Subject:  Ampico, B, New, Tester, Valve, Valves

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