A series RC and a parallel RC make a network that passes exactly one frequency with zero phase shift, at a third of the amplitude. Give the amplifier a gain of exactly 3 and it sings at that frequency forever. A little less and it dies out, a little more and it clips. The classic fix was a small lamp in the feedback path, its filament warming up to hold the gain steady.

R = 10 kΩ, C = 10 nF: f = 1591.5 Hz.