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
        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

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.