Chorus Generator

Prior to the Hammond, an "organ" was in a church or theater and had dozens of pipes. It is impossible for a pipe organ to have every pipe tuned precisely. Heat, humidity and position all affect the tuning of a pipe. That means the sound of a pipe organ has a very complex character with slightly out of tune pipes beating slowly against each other. One of the earliest criticisms of the Hammond was its "pure" tone. With the tone generators locked precisely on pitch, the Hammond sounded more sterile and less interesting than a pipe organ.

In an effort to refute this complaint Hammond tried to find a cost effective means of providing this effect. This led to the development of the Chorus Generator (Patent No. 2,159,505. See HammondPatents.) which was added to the "Solo" generator. This also forced the change in console designs from the A series to the B series cabinet that increased its depth to provide enough room to add the Chorus Generator.

The Chorus Generator was basically half of a standard ToneGenerator with different GearRatios and ToneWheels to provide slightly out of tune tones to blend with those from the solo generator. The Chorus Generator covered only the range from C4 (56) through C6 (73). In the patent, for each note in this range, there were two generators; one slightly sharper and the other slightly flat:

Solo   Driving Driven   Tone     Chorus     Solo     Hz +/-
Note    Gear    Gear    Wheel     Freq.     Freq.     Solo

G4      57       92      63      780.652   784.000  -3.348
<pre>
        91       74      32      787.026            +3.026

G#4 65 99 63 827.273 830.270 -2.997

        99       76      32      833.684            +3.414

A4 48 69 63 876.522 880.000 -3.478

        80       58      32      882.758            +2.758

A#4 70 95 63 928.421 932.173 -3.752

        79       54      32      936.296            +4.123

B4 57 73 63 983.835 987.428 -3.593

       110       71      32      991.548            +4.120

C5 67 81 63 1042.222 1046.153 -3.931

       110       67      32     1050.746            +4.593

C#5 78 89 63 1104.269 1108.292 -4.023

       120       69      32     1113.043            +4.751

D5 65 70 63 1170.000 1174.794 -4.794

       105       57      32     1178.947            +4.153

D#5 60 61 63 1239.344 1244.444 -5.100

       121       62      32     1249.032            +4.588

E5 74 71 63 1313.239 1318.400 -5.161

       120       58      32     1324.138            +5.738

F5 85 77 63 1390.909 1396.363 -5.454

       103       47      32     1402.553            +6.190

F#5 62 53 63 1473.952 1480.000 -6.038

       130       56      32     1485.714            +5.714

G5 57 92 126 1561.304 1568.000 -6.696

        57       92     127     1573.695            +5.695

G#5 65 99 126 1654.545 1660.540 -5.995

        65       99     127     1667.676            +7.136

A5 48 69 126 1753.043 1760.000 -5.957

        48       69     127     1766.956            +6.956

A#5 70 95 126 1856.842 1864.346 -7.504

        70       95     127     1871.579            +7.233

B5 57 73 126 1967.671 1974.856 -7.185

        57       73     127     1983.288            +8.432

C6 67 81 126 2084.444 2092.306 -7.862

        67       81     127     2100.987            +8.681

</pre>

In the actual production Chorus Generator, there are 48 tonewheel assemblies. 24 were double tonewheels; two tonewheels stuck together revolving in front of a single MagnetAndCoil. 24 were single tonewheels with separate magnets and coils. These had their outputs wired together in 12 pairs to produce the same effect as a single double tonewheel.

Thus, the production generator produced 36 complex tones, a mixture of sharp and flat tones for the solo generator tones 56 thru 91. The difference between the solo generator and the sharp-and-flat chorus generator was .8% for frequencies 56 to 67, and .4% for frequencies 68 to 91.

The only chorus generator produced was considered the "Church" chorus generator. Hammond engineers prototyped a "Theater" chorus generator that was twice as out of pitch but the sound was so "wild" that no organ was ever sold with the theater chorus. The scanner Vibrato developed after World War II generally eliminated the chorus generator from ConsoleOrgans.