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MMD > Archives > February 2000 > 2000.02.06 > 11Prev  Next


Welte-Mignon T-100 Recording System Technology
By Craig Brougher

Hi Robbie,  A few things about the Welte recording system we aren't
taking into account yet is this:  The best 90-ohm pipe magnet available
won't lift off the end of a 5/32" nipple with 90" of vacuum in it.  But
suppose it did.  A colloidal ink trace is probably going to measure a
maximum of 1000 ohms or even a little more.  This is what we don't yet
know.  All we know is that it is made from deposited graphite.  I don't
think a pencil mark even registers ohms (in the range practical for
this), and when it does, it's high ohms and varies a lot.  That's why
they used graphite in suspension.

The ordinary 90-ohm pipe magnet requires about 170 ma apiece.  If you
operate something that big directly in a DC circuit with an ink trace
and no diode, you would have a continually shortening ink trace because
the spark at the end would vaporize the paper in the necessary 110 volt
or higher DC circuit.  Even a 6 volt DC circuit sparks like crazy with
iron-stored inductance.

Anybody who's ever worked on a Mills Violano realizes that certain
magnets, like the piano expression and sustain, vibrato, etc., tend to
burn up the contact fingers quickly, and some have tried to put modern
relays in their circuits to protect them.  But they quickly end up with
molten plastic blobs where they had put relays before.

The reason is simple: It takes a special kind of relay to switch higher
voltage DC, because the inductive kick of even a small iron core magnet
creates a plasma in the relay that never stops, once struck.  You can
watch it with its fully open points still firing happily away until it
incinerates the relay.  (The way to do it with modern silver and nickel
bearing points and short travel is to series together two or three sets
of contacts in the relay, forcing the current to jump all three sets of
contacts at the same time -- which it can't do).

The point is this: Seemingly small magnets having iron cores have a
tremendous inductive kick.  Remember the little metallic covered book
that says "Shocking Pictures of Our Sex Life" or some provocative title
like that?  That coil is only about 3" long and 1/2" diam., wound with
tiny wire and powered by two penlight batteries.  But simply making and
breaking its circuit will give you a 10-20,000 volt jolt.  That's about
a 1/4" long arc.  There's no way around it.  That coil turns off fast
and the energy has to go somewhere.  The more work you make it do, the
harder and more destructive becomes the back emf.

Trying to operate working magnets directly with a resistive ink trace
is, in my mind, impractical -- that is, if you wish to keep the ink
trace.  Even a very tiny magnet, like the ones I have in the miniature
relays in SofI (350 ohm coils) have burned out more chips in the
"protected" Octet controller than I care to remember by knocking out
their diode protection.

I finally was forced to put a solid state diode (1N4004) across each
one.  Laurent didn't think that could possibly be the problem, but it
was.  And it went without saying that each pipe magnet had to have one.
Welte didn't use quench diodes because there was no such thing.  (They
would play just fine for a week or a month, and then suddenly pop the
driver.  It was goofy.)

If you were Welte and wished to keep your electrical master for more
than one performance, I think you will discover that you cannot operate
stuff safely and directly from the magnetism created through the ink
traces.  If the maximum ink trace resistance was only 200 ohms, or even
20 ohms, it still wouldn't work, to my mind.  The dc voltage reverse
current, even in a small magnet circuit doing work, could vaporize the
paper at the point the contacts came off the ink.  And the contacts in
this case are wider than the widest ink trace expected, so the burn is
going to be distributed like a surface gap spark plug.

That is why I suggested possibly a parallel circuit with constant ac
saturation voltage in the control winding, maximum when the contacts
are open, blocking all current in an ac transformer secondary.  As soon
as the trace partially shorts the reactor coil, energy flows from
primary ac supply to secondary in a totally separate circuit.  Remember
now, this "thing" I speak of could be as simple as a relay with an
extra coil on it.  At the end of the ink trace, there is no spark
because the control winding is going from less current to more current,
and the current required is 100 times less than that needed to operate
the control magnet.

One other thing we forgot to mention, however, is what used to be
called a "quench" coil.  It was a reverse-biased inductor that
canceled out the back emf.  However, it doubles the weight and
complexity of circuits and slows them down.

Monday, I am going to have the opportunity to read the jacket and hear
the Welte perform in the album mentioned by James Crank.  My friend
says there's a lot of information on the jacket, provided by Richard
Simonton.  Maybe it will give us a clue to something heretofore
ignored.  I'll check it out and if there's anything I can add, I'll
let you know.

Craig Brougher


(Message sent Sun 6 Feb 2000, 14:32:37 GMT, from time zone GMT-0600.)

Key Words in Subject:  Recording, System, T-100, Technology, Welte-Mignon

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