The circuit uses only a small number of components, plus a battery. Its sound is mostly dependent on the selection of transistors (type, hfe and leakage in case of germanium transistors). As germanium transistors are sensitive to temperature, the amount of leakage is variable. Thus the bias voltages will shift up and down, so the sound produced by a germanium Fuzz Face may change as the equipment heats up or cools down. Also, the battery is part of the circuit. Variances in voltage and internal resistance can make an audible difference.