Which tubes are suitable for organ and Leslie amplifiers as replacements or as a sonically useful exchange?

With organs and Leslie amplifiers, a technically clean and robust replacement is especially important. This applies, for example, to classic Hammond models such as B3, C3, A-100 or RT3, as well as popular Leslie cabinet systems such as 122, 145, 147 or 251. Especially in Leslie amplifiers, the 6550 plays a central role; Hammond Organ explicitly names it as the standard for most Leslie tube amplifiers and points out that it should be used in matched pairs. In many Leslie amps, 12AU7 and, depending on the model, also 0C3 are used; in classic Hammond organs, 12BH7, 12AU7 and, depending on the model, other types are also relevant.