|From Hammond T-595|
The Hammond T-500's were the last of the tonewheel organs that Hammond produced. Tonewheels generate sound via spinning metal discs and electromagnets. The drawbar controls allow a specific mix of harmonics in the output.
Amusingly, the T-500 Hammond incorporates circuitry to eliminate the uniqueness, classic, Hammond B3 sound. Forutunately, The modifications to reverse these "improvements" are well documented.