HAMMOND ORGAN
Figure
seven, below, shows a portion of the interior of a Hammond tone generator,
giving you a good look at the layout of its various components. As you
can see, there are many bearings in a Hammond tone generator and they
all need lubrication. Each bearing is, therefore, made of porous bronze.
Tied around each bearing is a cotton thread, and the other end of the
thread terminates in a narrow metal trough at the top of the tone generator.
Two oil tubes protrude through the top cover of the tone generator and
each has a small, flat funnel. Once a year, a conscientious Hammond owner
fills two oil cups in the console, from which oil runs down through tubes
and slowly drips into the two funnels which then convey it to the trough
in the generator. The oil soaks a long wick in the bottom of the trough
which wets the cotton threads. The oil travels through the threads by
capillary action to the porous bronze bearings, lubricating them for smooth
and amazingly silent operation.
The mechanical forces which are encountered in a Hammond tone generator are very small, and the machine was built to last because Laurens Hammond was a stickler for quality. Therefore, with periodic lubrication, a Hammond tone generator can work properly indefinitely. There are many original model A and B Hammonds from the late 1930s that still function perfectly. As you can also see now, there is no possibility of the instrument ever getting out of tune as long as the power line frequency is constant. Furthermore, the instrument can never get out of tune* with itself. In most other polyphonic [capable of sounding more than one note or pitch at a time] instruments, the various strings of a piano, or pipes of a true organ, or vacuum tube or transistorized oscillators of some electronic instruments, all operate independently and are subject to many minute mechanical and/or electrical variations, component degrade, etc. All of these effects conspire to make tuning drift away from its desired precision, necessitating periodic retuning.
The mechanical forces which are encountered in a Hammond tone generator are very small, and the machine was built to last because Laurens Hammond was a stickler for quality. Therefore, with periodic lubrication, a Hammond tone generator can work properly indefinitely. There are many original model A and B Hammonds from the late 1930s that still function perfectly. As you can also see now, there is no possibility of the instrument ever getting out of tune as long as the power line frequency is constant. Furthermore, the instrument can never get out of tune* with itself. In most other polyphonic [capable of sounding more than one note or pitch at a time] instruments, the various strings of a piano, or pipes of a true organ, or vacuum tube or transistorized oscillators of some electronic instruments, all operate independently and are subject to many minute mechanical and/or electrical variations, component degrade, etc. All of these effects conspire to make tuning drift away from its desired precision, necessitating periodic retuning.
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Figure
7. A detailed look at a portion
of a Hammond tone generator shows the various types of tone wheels and
associated drives, vibration filters, magnets, coils and mech-anical and
alignment parts. Note that in certain cases, where an actual tone wheel
is not needed, there is a blank wheel to maintain the necessary mechanical
balance and flywheel effect. Roll mouse cursor over picture to see individual
part names.
In
the Hammond tone generator, every tone generating element is mechanically
synchronized to all of the others through the gearing and the main shaft.
Therefore, they are all locked into a precise relationship to each other from
which they cannot deviate*.
Because of the method used in the Hammond to create various tone colors and instrumental effects, it is very important for most of the generated tones from the tone generator to be sine waves, that is, waveforms with absolutely no harmonics or extraneous frequencies present. The only exceptions to this are the first twelve frequencies for the pedals. The outputs from the tone wheels approximate a sine wave, but to make the midrange and higher frequencies really accurate sine waves, there is a resonant filtering circuit associated with each tone wheel. On the Hammond tone generator, these filters are made up of small transformers and capacitors as shown schematically in figure eight and physically in figure nine.
Because of the method used in the Hammond to create various tone colors and instrumental effects, it is very important for most of the generated tones from the tone generator to be sine waves, that is, waveforms with absolutely no harmonics or extraneous frequencies present. The only exceptions to this are the first twelve frequencies for the pedals. The outputs from the tone wheels approximate a sine wave, but to make the midrange and higher frequencies really accurate sine waves, there is a resonant filtering circuit associated with each tone wheel. On the Hammond tone generator, these filters are made up of small transformers and capacitors as shown schematically in figure eight and physically in figure nine.
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Figure 8, left.
This is a schematic repres-entation of a typical filtering circuit through
which the signal from the tone generator passes on its way to the key
contacts. The capacitor, C, and the grounded center-tap transformer are
different for every note. Together they form a resonant filter which favors
only one frequency, the pitch of a particular note for which the filter
is designed. All other frequencies are significantly at-tenuated which
leaves only the fundamental frequency for that particular pitch which
the associated tone wheel generates.
Figure 9, below. This close-up of the top of a Hammond tone generator shows the var-ious capacitors (cylindrical units) and the transformers that are the filters for each gen-erated frequency.
Figure 9, below. This close-up of the top of a Hammond tone generator shows the var-ious capacitors (cylindrical units) and the transformers that are the filters for each gen-erated frequency.
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* If a tone wheel bearing should develop
excessive friction, that particular pair of tonewheels may fail to run
at full rated speed in which case the two notes which that tone wheel
pair generate will be out of tune with the rest of the instrument. Likewise,
if a bearing should seize up completely, those two notes will be missing
entirely. As long as the correct lubrication schedule is maintained
so that the tone wheels run properly, tuning of the instrument will
not deviate from its design standard.
