Hammond Model A, Manual, Page: 73 / 298

Some of the coils have copper rings mounted on them for the purpose of
reducing harmonics. As these are used only on fairly low frequency coils,
the eddy current loss in such a ring is small for the fundamental frequency
of that coil, but high for its harmonics. This has the effect of reducing
the relative intensities of any harmonics which may be produced by irregularities
in the tone wheels. The wheels are cut so to give as nearly
a sine wave as possible, but the generated voltage seldom reaches that ideal
condition, since even a change in the air gap will change the wave form. The
tip of each magnet, as well as the edge of each tone wheel, is coated with
lacquer to prevent corrosion. For, should oxidation set in, the change in
tooth shape would introduce irregular frequencies.
Locations of the various magnet and coil assemblies are shown in figure 4.
They are identified by their frequency numbers, and the broken line between
any two numbers indicates that these two frequencies are supplied by one
tone wheel assembly.
Each magnet is set at the factory with the set screw partially loosened,
while observing an output meter. Experience has shown that the magnets
seldom need adjustment and that setting them without proper equipment involves
danger of damaging both magnet and wheel. Therefore it is not recommended that
the service man attempt this adjustment.
As a means of eliminating any vagrant harmonics that may be present, there
are filters consisting of small transformers and condensers associated with
certain frequencies. The transformers have a single tapped winding, and this
tap is grounded, so one side, which is connected to the corresponding magnet
coil through a condenser, forms a resonant circuit for the fundamental frequency of
that coil. This tends to emphasize the fundamental and suppress
harmonics.
Locations of these transformers are shown in figures 5 and 6.
These transformers and condensers are mounted on the top of the
generator assembly. The transformers are mounted at an angle. thus
minimizing interference between them. The cores of the transformers
are made of a special iron. and the number of laminations used is adjusted
to secure the proper inductance. Wires from the magnet coils connect to
the transformers. and wires from the transformers lead to the terminal
strip on the generator.
This terminal strip carries the output frequencies of the generator, which
are arbitrarily numbered from 1 to 91 in order of increasing frequency.
This frequency numbering is continued throughout the instrument. In some
models the frequencies are not in order on the terminal strip. and figures
5 and 6 indicate the arrangement for different models. Several terminals
at the right end are grounded to the generator frame and serve to ground
the manuals and pedals.
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