Some early organs had unclean power supplies, resulting in a particular rattiness in their tone, especially as the frequencies clashed with 60Hz. This was especially evident in 2005 when I edited some up for the Alesis Fusion project. When a single-cycle loop was used, which of course is the most memory-saving method, the character of the note(s) disappeared. Depending upon the note, loops of several wavecycles were required in order to preserve the original character.
A lack of rattiness and various tube characteristics might be why some of the earliest Hammond-alikes sounded too pure and clean.