Hammond Tonewheel Organs
From detritu–(at)–x.netcom.com Sun Jun 6 12:02:53 CDT 1999
From: detritu–(at)–x.netcom.com(Lord Valve)
Newsgroups: alt.guitar.amps
Subject: Re: LV…What’s a Hammond B4 worth…
Date: 6 Jun 1999 08:18:02 GMT
X-NETCOM-Date: Sun Jun 06 3:18:01 AM CDT 1999
Xref: geraldo.cc.utexas.edu alt.guitar.amps:183206
In <3759477E.B599F0F--(at)--eleport.com> Simply Steve
>
>
>
>Lord Valve wrote:
>
>> Lord Valve Speaketh:
>> Hmmm..B4 doesn’t mean anything to me, but I believe an S6 is a
>> Hammond Extravoice chord organ. Did it have a single keyboard,
>> with a bunch of accordian-type buttons to the left of it? If
>> so, it’s an interesting piece, but not worth much. It isn’t a
>> tone-wheel organ, it uses tube-type oscillators. Accordian
>> players love ’em, though..they can kick ass on ’em.
>
>Curiosity about this tone-wheel technology…
>Exactly how does it work?
>
>SS
>
Lord Valve Speaketh:
A Hammond tone-wheel organ uses actual (miniature) mechanical
alternating current generators to produce the frequencies
used by the organ. These consist of a pickup (exactly the
same, in principal, as a guitar pickup) and a “tone wheel,”
which looks more like a gear than anything else. All the
tonewheels are driven by planetary gears from a central
shaft, which is powered by a hysteresis-synchronous motor.
(BTW, Laurens Hammond invented the synchronous motor, which
made it possible to have *accurate* electric clocks. The
earliest Hammond organs say “Hammond Clock Company” on ’em!
Ol’ Laurens invented the spring reverb, too, and a whole
shitload of other stuff.) Since all the tonewheels are
driven from the same shaft, the organ *always* stays in
relative tune…a Hammond *cannot* go out of tune, or be
tuned. The synchronous motor will turn at the correct
speed as long as it receives 60 Hz, and the organ will
be in tune. The frequency of the “A” produced by a
Hammond is specified on the original patent sheets as
440.000, which ain’t too shabby for a device invented in
1934. I calibrate my strobe tuner with my B3, in fact.
Anyway, each metal tonewheel has “teeth” (or bumps) around
the edge, and as the wheels spin, the teeth interrupt
the magnetic field produced by the permanent magnets
in the pickups; each pickup has a coil of wire around
a cylindrical magnet with a blunt conical point, and as
each tooth on the tonewheel passes the tip of the magnet,
one cycle of alternating current is induced in the coil.
If 440 teeth pass the magnet tip every second, you have
an “A.” There is a toneheel and a pickup for every
frequency generated inside the organ; for a B3, I seem
to remember there are 92. The output of each pickup
goes through a dedicated filter which removes spurious
harmonics, and then all the tones are mixed together by
the organist, using the “drawbars.” A B3 is really an
additive synthesizer, which allows the player to combine
a variety of pure sine waves to build any tone he seeks.
The drawbars are arranged in the standard harmonic overtone
series, ie., fundamental, octave, twelfth, fifteenth,
seventeenth, nineteenth, and tewnty-second. (The flat
twenty-first was omitted on the B3, but showed up in some
of the later Hammonds which don’t sound as good.) In
addition, a sub-fundamental and sub-fifth were added for
body, and it is these two “undertones” which give the B3
more balls than a Tyrannosaurus Rex. “Modern” electronic
organs either use samples, or a “top octave generator,”
which is a chip that produces all of the notes from C to B
at a very high frequency; all the rest of the frequencies
the organ needs are divided from the top octave. Almost
all of the other organs use “subtractive” synthesis,
where you start with a harmonically rich tone (a square
or triangle or other complex waveshape) and remove harmonics
with various filters. Only the Hammond tone-wheel organ
builds its sounds from combinations of pure sinewaves,
and puts the level of each harmonic under realtime control
by the organist. The Hammond “percussion” effect you
may have heard organists talk about is produced by keying
in a tone which decays (like a struck xylophone) over the
top of the continuous tones from the drawbars; if you have
trouble imagining what this sounds like, take a listen to
“Green Eyed Lady” by Sugarloaf…this is a good example
of percussion “crunch” (in the pop/rock idiom, anyway.)
Lord Valve
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