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The Hammond organ was originally developed to compete with the pipe organ. Much of the discussion that follows is easier to understand if you have a little knowledge of pipe organ terminology. Here's a link to A Crash Course in Concepts and Terminology Concerning Organs

Drawbars were a unique Hammond innovation to keyboard musical instruments. Prior to the Hammond organ, pipe organs most commonly used stop buttons or tabs to control the flow of air into a specific rank of pipes. Pipes can sound flutey with few harmonics or reedy with many harmonics, and there are many different tonal qualities in between. The stops were two position controls; on or off. The organist blended the sound produced by the pipe ranks by opening or closing the stops. The Hammond organ blends the relatively pure sine wave tones generated by the ToneGenerator to make sounds that are imitative of the pipe organ. The Hammond organist blends these overtones - special ones called harmonics, from the HarmonicSeries - by setting the position of the drawbars which increase or decrease the volume of the harmonic in the mix.

As obviously, Jazz, Blues and Rock organists aren't always interested in imitating a pipe organ, the Hammond can also be used to produce NonharmonicOvertones as well.

Most Hammonds have nine drawbars typically organized (left to right):

 No.   Harmonic   Interval   Footage   Color 
 1   Sub-Fundamental   Sub-Octave   16'   Brown 
 2   Sub-Third   Sub-Third   5 1/3'   Brown 
 3   Fundamental   Unison   8'   White 
 4   2nd Harmonic   Octave   4'   White 
 5   3rd Harmonic   Twelfth   2 2/3'   Black 
 6   4th Harmonic   Fifteenth   2'   White 
 7   5th Harmonic   Seventeenth   1 3/5'   Black 
 8   6th Harmonic   Nineteenth   1 1/3'   Black 
 9   8th Harmonic   Twenty-Second   1'   White 

Looking from the top, a set of drawbars may look a little like this (.3 = 1/3' .6 = 3/5'):

  |8|  |4|  |8| (4') (2.3) ( 2') (1.3) (1.6) |3|
  |7|  |3|  |7|                              |2|
  |6|  |2|  |6|                              |1|
  |5|  |1|  |5|                             ( 1')
  |4| (5.3) |4|
  |3|       |3|
  |2|       |2|
  |1|       |1|
 (16')     ( 8')

(The above drawbar setting is often notated: 84 8000 003, an example of DrawbarRegistrations.)

The footages stamped into late model drawbars refers to the relative lengths of pipes used in a pipe organ. The number of feet refers to the approximate length of the pipe that sounds when the lowest key is pressed on the keyboard. The low C key on an organ normally sounds a note of approximately 65Hz. A pipe approximately 8 feet long is required to sound at this frequency. A longer pipe sounds lower and a shorter pipe sounds higher. To halve the frequency and sound a note one octave lower, a pipe needs to be twice as long. Thus, a 16' pipe sounds one octave lower than an 8' pipe and a 4' pipe sounds one octave higher. Fractions are used when the harmonics are not on octaves of the fundamental. 2 2/3', for example, is the third harmonic of 8' and it is an octave and a fifth above.

It might seem odd that the 5 1/3' drawbar was included, especially since it is positioned to the left of the 8' drawbar instead of the right. This is the third harmonic of the 16' drawbar and with the 8' drawbar (the second harmonic) provides a little tonal color to the 16' drawbar. It might seem odd, too, that the 7th harmonic is missing (which see below), but that overtone would sound distinctly out of tune from a Hammond ToneGenerator.

On H-series and X-series organs, there are two additional black drawbars appearing to the right of the 1', extending the HarmonicSeries. The first controls 1 1/7' and 8/9', the 7th and 9th harmonics; the second controls 4/5' and 2/3', the 10th and 12th harmonics. Again note that the first of these includes a pitch which is also out of strict sequence order on the drawbar layout, and that the 11th harmonic (likewise 13th) is missing due to tuning issues.

 No.   Harmonic   Interval   Footage   Color 
 10   7th Harmonic
9th Harmonic 
 Flat Twenty-First
 1 1/7'
 11   10th Harmonic
12th Harmonic 

On the bar which connects the tab to the switch inside the organ are marked numbers 1 through 8 (when the drawbar is pushed fully into the console, the drawbar setting is considered 0 and no harmonic will be heard). As the drawbar is pulled towards the player, each number represents roughly a 3dB increase in the volume of the harmonic that drawbar controls. Pulling it out by two increments gives a doubling of the volume of that harmonic. Conversely, as the drawbar is pushed in, each number represents a -3dB change in the volume of that harmonic. Pushing it in by two increments gives a halving of the volume of that harmonic.

In most Hammonds, each manual key has nine contacts which which close more or less simultaneously when the key is depressed. Each contact is wired to one of the 91 ToneWheels in the ToneGenerator. The ToneWheels selected for the contacts in each key represents a harmonic series of sine waves for that key. Each contact connects the nine ToneWheels for that key's assigned harmonics to nine BusBars which run the length of the manuals. Each busbar is connected to the sliding contact on one of nine drawbars. Each drawbar can be moved to connect the tones on that busbar to one of nine fixed contacts. Each fixed contact is connected to one of nine primary windings on the MatchingTransformer. The matching transformer is wound so that as each drawbar is pulled out, the busbar is connected to a winding that is half the impedance of the drawbar position beneath it.

Note that the most desirable Hammond organ drawbars are discrete switches and not potentiometers. In early Hammonds, drawbars were originally Ratchet type with distinct detents at each setting. Adjusting the drawbars on these models causes an audible pop as the contacts open and close. Newer organs have the more desirable Smooth type which do not have distinct detents. To eliminate this noise, these drawbars were constructed as make before break switches. This also made it possible to set the drawbar between whole number settings. It is possible to modify ratchet drawbars to be smooth --- see AddingSmoothDrawbars. In late model SpinetOrgans (e.g. L100) costs were greatly reduced by eliminating the 8 separate taps on the primary matching transformer and using variable resistors on the drawbars to set the harmonic levels.

The most popular Hammond organs have reverse colored Preset keys for the leftmost octave on each manual that work much like stop tabs. The preset keys are pressed and mechanically latch down until either another preset key is pressed or the very last key in the manual is pressed (the Cancel key) which releases the latched preset key and completely disconnects the busbars to the matching transformer. The B and Bb presets allow the drawbars to control the tone (see below). The other preset keys are wired to a patch panel inside the organ that disconnect the busbars from the drawbars and connect them to a set of screw terminals (one set for each preset) that can be used to custom tailor each preset's harmonics. Other Hammond models used key cheek studs or traditional pipe organ style stop tabs.

Each manual on ConsoleOrgans have two sets of drawbars. When a manual's B preset key is pressed the drawbars farthest to the right in the set for that manual control the harmonic mix. When a manual's Bb preset key is pressed, the drawbars farthest to the left in the set for that manual controls the mix. Note that the Swell or Upper manual drawbars are grouped on the left side and the Great or Lower are grouped on the right. The reasoning is that the performer is playing the Swell with the right hand and that means the left hand is free to adjust the drawbars. Obviously, the expectation is that the left hand is playing the Great and that leaves the right hand free to work the 'bars. On the Swell of the 3 series organs, the B preset must be pressed for the Percussion to work.

There is a single set of one, two, or four drawbars in the center of the organ for the ?PedalTones. These drawbars control the volume of preset mixtures of harmonics.

Pipe organs frequently have 32' pipes sounding two octaves lower than 8' pipes. But a single toothed ToneWheel necessary to reach these low frequencies in the Hammond ToneGenerator would be unbalanced and impractical. But Hammond was competing with pipe organs so he needed some way of generating lower pitches in ConcertOrgans. 32' tones were eventually added to some ConcertOrgans by using tube oscillators but ConsoleOrgans and SpinetOrgans never had these extra generators.

SpinetOrgans usually have:

In at least some of the M-series spinets, the missing 1' drawbar from the Great is combined with the 1 1/3' (and usually marked with a red dot).

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