Philip Jamison asked, "What is the difference between 'vibrato' and
'tremolo'?"  Robbie Rhodes responded, "The engineer's simple definition
is that vibrato results from frequency or phase modulation of the
waveform, and tremolo is amplitude modulation of the wave.  Amplitude
modulation of an audio signal is easily applied, but for many years the
only effective phase modulation system was the spinning loudspeakers
developed by Don Leslie.  I don't know how the terms are used on the
Hammond and other electronic organs."

Electronic organs do use the same terms.

The original Hammond Organ, introduced in April 1935, had neither
tremolo nor vibrato.  The Hammond Company struggled for years to
produce something like vibrato using an electrical or electronic
system.  Their first solution, introduced in 1936, produced a chorus
effect by having a second set of tonewheels, tuned slightly off from
the correct frequency and thus producing a beat with the primary note.
The second solution, introduced in 1942, provided vibrato from a
rheostat continuously rotated by the tonewheel motor.  The third
solution, from 1949, provided vibrato by phase-modulating the signal in
the preamplifier, by a band-pass filter with variable capacitors being
rotating by the motor.  Early Hammond Organ model numbers reflect the
tremolo/vibrato system they incorporate.  For example, the Model B has
nothing, the BC has the chorus system, the BV has the tremolo system,
and the B-2 has vibrato system.  The B-3 has the vibrato system plus
percussion.

Most pre-digital electronic organs produced vibrato by frequency
modulating the oscillators.  Purely electronic vibrato by phase
modulation in the preamplifier has been invented twice in the 1950s: by
Wurlitzer for use in their early reed organs, and by Richard H. Dorf of
the Schober Organ Corporation.

All of these systems are described in Dorf's book "Electronic Musical
Instruments," second edition (New York: Radiofile, 1958).

Paul West
Mountain View, CA