I’m publishing this chapter break out of order again.  One of these days I’ll remember to do one before starting the next chapter.  As with last time, I’ll rearrange the archive correctly after the next page is published so things make sense when browsing older pages.


Hammond B3 Organ

Hammond tonewheel organs – particularly the B3 (and C3) – are the stuff of legend.  They’ve been a staple in gospel, jazz, rock, pop and everything in between for well over half a century.  Not only do they look and sound amazing – they are also incredibly interesting on the inside.

First let’s talk about the basic controls on a Hammond.  It has two manuals (keyboards) with five octaves each, and an additional octave of opposite-colored keys at the bottom to select presets.  Two of these presets for each manual are controlled by four sets of nine drawbars at the top of the console.  These drawbars mimic the stops on a pipe organ, and determine how much of each of nine harmonics should sound when playing a note.  If you want low tones, you pull out the low drawbars.  If you want high tones, you pull out the high drawbars.  If you want total and absolute fatness, you dime all nine bars, floor the volume pedal to drive the tube amp into distortion, and wail on the keys in 4ths and 5ths.

The coolest part about these organs other than the sound they produce, is how they produce their sound.  Taking up most of the inside of a B3 is a large, heavy, metal contraption called the tone generator.  This tone generator consists of a single driveshaft with dozens of notched wheels (called “tonewheels”) all along it.  The shaft spins at a specific speed (timed by the 60Hz AC frequency of the wall) and moves these notched wheels past magnetic pickups similar to what you’d find on an electric guitar.  The notches (or “waves” might be a better descriptor) in the wheels are spaced in such a way as to produce a single sine wave at a specific frequency per wheel.

The mechanics of this sound production are very similar to a guitar.  The pickup is a coil of wire (an inductor) with a magnet in the center.  The magnet in the organ pickup magnetizes the metal tonewheel as the magnet in a guitar pickup magnetizes the string.  Then, as the wheel spins, the notches cause the distance from the wheel to the coil to vary sinusoidally over time.  This distance produces a corresponding change in magnetic flux through the inductor, which produces a corresponding electrical current (and hence voltage) in the coil.  Thus, mechanical motion of the tonewheel produces an electrical signal of a sine wave – one wave per wheel.

This is where things get really funky.  Remember the nine drawbars?  Like I said before, each drawbar selects the volume of the corresponding harmonic of the key(s) being pressed.  This means that each key actually selects the sound of nine different tonewheels, and the drawbars select the volume of each of these tonewheels.  Each key has nine switches under it over nine busses that run the length of the manual.  Each switch connects to one of the nine harmonics of the key.  If the player presses multiple keys (which they often do), all the first harmonic tones are connected to the first buss, the next harmonic to the second, and so on.  The busses connect to a transformer that adjusts the amplitude of the signal of each buss.  The tranformation ratio is set by the drawbars, which select between transformer taps.  All the sounds are mixed together and run into a tube preamp with a volume control (that big foot pedal on the bottom).

From here, the signal is sent to an external power amplifier (or a Leslie rotating speaker cabinet – a topic for another time 🙂 ) so that people can hear the wonderful funky sound that you’re playing.

So there you have it.  The Hammond B3 (and C3, B2, C2, A100, M2, M3, and countless others) is a beautiful instrument on the outside and the inside, and especially the audible side.


Sources:  Wikipedia and my memory (based on dozens of articles read on the subject).