Of the suggestions offered by Robbie Rhodes to solve Mr. Code's
requirement for a 2-speed motor for his "red' Welte Vorsetzer,
I believe Option 4, use of a DC motor, is the most practical.
However, I disagree with his "basic field-current speed control";
more on that later.

First, let me say that I have the same problem.  I have a "red"
Welte Vorsetzer of which the original motor is missing.  I do not
yet have it working, but here's what I have done and will do, and
what I suggest Mr. Code do, or at least try.

I obtained a DC motor built for a mechanical music instrument.
Sound impossible?  Until the late 1930's, many cities in the USA,
and I believe other countries, were supplied with direct current.
Naturally, pianos and organs delivered there had DC motors.  When
the grid was converted to AC, these motors were removed and AC motors
installed.

Fortunately, not all the DC motors were junked.  I know this because,
several years ago, I advertised for a DC motor for the Vorsetzer.
I received 3 replies, and picked one that was near enough so I could
drive there.  I now have a 1/4-horsepower Emerson motor for 110 Volts
DC, compound wound.  It has sleeve bearings, an important point
because ball bearings are inherently noisy.  I cannot prove it is
from a mechanical musical instrument but this seems likely, since
I placed the ad in the AMICA Bulletin.

It will, of course, be necessary to build a DC supply.  The field
voltage should always be the rated voltage of the motor, and only
the armature voltage should be reduced to obtain lower speeds.[*]

To achieve this, the voltage at the motor terminals will be the rated
voltage.  One brush will be isolated from the motor's existing circuit,
and fed to the appropriate side of the line through adjustable
resistance.  In my motor, half of the series field winding is in
series with each brush, so when I do this only half of the series
winding will be in use.  I do not expect this to cause any problem.

Anyone who does not understand why reducing the field voltage will
increase the speed (the opposite of what many expect) should read an
old (circa 1920) book on motors.  This will also explain why a compound
motor maintains a more constant speed than a shunt motor.  Although
I do not know for sure what Welte did, I cannot believe they varied
the field current to control the speed.[**]

Unless Mr. Code is very lucky, he will find that the fancy two slider
rheostat furnished by Welte has the wrong resistance for the new motor.
Unfortunately (of fortunately) I was spared this problem, as the
rheostat on mine was missing when I bought it.

Musically yours,

Cecil Grace

 [*  This statement is generally true for industrial DC motors.
 [** Nonetheless, the faster speed of the original "shunt wound" motor
 [ supplied in the Welte-Mignon is achieved by reducing the field
 [ current, with the armature operating at constant voltage.  Since the
 [ torque demanded by the pump is constant at all speeds, the armature
 [ current doesn't increase too much as the field current is reduced
 [ and so the commutator is not overstressed; that's the main concern.
 [ It's possible that the Welte motor was designed especially for this
 [ type of service.  -- Robbie