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.

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.

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.