The HAMMOND ORGAN

North Suburban HAMMOND ORGAN Society

The following diagram, figure 22, represents five typical drawbars. At the top of each is the signal lead-in wire which is soldered to a spring metal tab with two small contacts placed close together on its tip. A one ohm resistor connects the two contacts of the tab. The idea is that at all times one or the other contact will be touching one of the horizontal busbars which are wired to the nine taps on the primary of the matching transformer. The one ohm resistor prevents short circuiting two adjacent busbars if the drawbar should be in a mid position setting.

If you click on a drawbar with your mouse, you can slide it out and see how the tab with two contacts slides under the group of busbars. For greater clarity, you can also click on the group of busbars and slide it out of the way to the right. [Note, this feature will not work on a cellphone, unfortunately. You'll get the picture but it will not be clickable] This diagram details the construction of a typical set of Hammond harmonic drawbars and shows you how, by sliding them in or out, you change the strength of each harmonic by changing the point at which each connects to the primary of the matching transformer. If a drawbar is all the way in, it connects to the first drawbar which is grounded, thus the harmonic it represents does not enter the primary of the matching transformer and it will not be present in the resulting mixture.

When the signals are combined and the resulting composite leaves the secondary of the matching transformer, the signal next goes through a double throw single pole switch that directs the signal either to the no vibrato or to the vibrato inputs on the preamplifier. If the signal is to go straight through with no vibrato effect, it is amplified somewhat and then passes through the volume control and then to the output stage of the console preamp. If the signal is to have vibrato applied, it must go through the vibrato system which I already described. However, the signal from the secondary of the matching transformer is a very weak, low powered signal.

Before it can go to the vibrato line box, it must be amplified. Now, on the next page, we'll see how a vacuum tube can amplify a low level signal to a much more powerful signal. Likewise, the signal that has the vibrato added and which appears on the vibrato scanner pickup arm must also be amplified in order to have sufficient strength for the subsequent sections of the Hammond system. The greatest amount of amplification takes place in the so-called power amplifiers of the speaker cabinets where the electrical output signal of the Hammond console finally becomes audible music.

The Hammond percussion feature is a somewhat specialized form of amplification which varies while you hold a note down. When you hit a key on the top keyboard with the percussion feature turned on, a tone representing either the second or third harmonic (musician's choice) begins loudly and then fades away while the key is held down. If the musician releases the key, the tone stops altogether, but while the key is held, the tone gradually fades out. This is accomplished by a special amplification circuit with a variable gain. That is, the amount of amplification provided by the circuit changes over time. In order to make the percussion behave realistically and fade out like a bell or a gong, the gain of the percussion amplifying circuit must be high when a key is pressed and then gradually decrease over time while the musician holds the key down.

In order to simplify things, there is only one percussion amplifying circuit in a particular Hammond instrument. It is triggered whenever a key on the top keyboard is played, if the musician has selected the percussion feature. In order to trigger again, all keys which the musician may be holding down on the top keyboard at any time must be released.

Although providing only one percussion amplifying circuit was an obvious economy move, it conferred an interesting advantage, and that was that the percussion effect was touch responsive. If the musician played in a legato or sustained fashion, there would be no percussion effect. But, if he even momentarily let go of all keys and then hit another one, the percussion effect would return on that key. With a little practice, a Hammond musician can quickly learn the technique and bring the percussion effect in and out at will, simply by slightly changing his playing technique. Thus, what could be considered a defect and an economy move on the part of Hammond could also be a definite advantage for a creative musician.

We have to realize that when these instruments were developed and manufactured, transistors did not exist. To put independent percussion on all 61 notes of a keyboard would require 61 percussion keyers, adding considerably to the complexity, weight and power consumption requirements of the instrument. After solid state circuits became the norm for keyboard instruments, having independent percussion triggering for each key became standard practice on all organ-like electronic musical instruments. The Hammond X66, which relies entirely on solid state circuitry, has a very realistic and excellent percussion system with independent percussion keyers for each generated frequency. However, in this article, we're looking at the traditional Hammonds which were developed prior to the introduction of the X66. When the Hammond B3 and C3 models appeared in 1954, the commercial development of the transistor was still three years in the future.

In the next part of this article, we will first see how a vacuum tube can amplify a signal and then look at specific parts of a typical Hammond B3 or C3 organ's amplification system to see what actually happens, including a look at how the percussion works. Then finally, we'll follow the musical signal through the power amplifiers and to the actual loudspeakers which convert the electrical output signals from the console to audible music. While in the speaker cabinets, we'll also look at the ingenious methods the Hammond company de-veloped to add artificial reverberation to the sound, for it very quickly became evident that organ music, or organ-like music sounded much better with some degree of reverberation as you would get in a large room or hall.

With pipe organs, lack of reverberation was rarely a problem because they were always installed in churches or theaters or other suitable halls. But after the development of the Hammond, this meant that now an organ-like musical instrument could be installed in a living room where reverberation was practically non existent. The folks at Hammond, however, were very clever and innovative. It was not long before they offered an artificial reverberation effect in many of their speaker cabinets, and over time they improved the effect until you could create the illusion of playing in a large room even in a tiny studio apartment.

Figure 21. Note, if you view this picture on a computer, you can click on and slide the drawbars in and out and you can also slide the entire group of busbars out of the way to see what the drawbar contacts look like. This entire figure may not show on many cellphones, and if it does, you will most likely not be able to click and drag the movable elements. For best results, view this page on a computer.

Figure 21. Note, if you view this picture on a computer, you can click on and slide the drawbars in and out and you can also slide the entire group of busbars out of the way to see what the drawbar contacts look like. This entire figure may not show on many cellphones, and if it does, you will most likely not be able to click and drag the movable elements. For best results, view this page on a computer.

Figure 21. Note, if you view this picture on a computer, you can click on and slide the drawbars in and out and you can also slide the entire group of busbars out of the way to see what the drawbar contacts look like. This entire figure may not show on many cellphones, and if it does, you will most likely not be able to click and drag the movable elements. For best results, view this page on a computer.

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