The magic is in the directly heated, oxide-coated filament. While this combination is certainly not unique to the 300B, the coating of the filament is. A strontium and barium mixture is combined with other proprietary components in the ribbon to form a coating originally developed for use in underwater marine cable applications.

Western Electric often calls it the secret sauce or secret formula. This special coating not only improves the life of each tube, but also supports the dynamics needed for superior music reproduction.

While a signal is propagated through a 300B, the filament remains "hot", never completely depleted of electrons and always on standby. When we experience the pleasant, water-clear sound of a 300B, we experience the thermal inertia of a directly heated cathode.