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This is a very peculiar Oscillator I reverse engineered from an old tube oscillator. I'm not a 100% sure how to explain it, but I'll give it a go. Basically you've got one voltage gain stage as usual and a buffer stage. The interesting part comes from the feedbacks. The positive feedback is actually a negative feedback and vice versa. Let me elaborate. The complex RC circuit at the left is actually a notch filter. Since it's in the negative feedback path, all frequency except the fundamental will be attenuated, and the fundamental will be allowed to oscillate. This is due to the inversion of the transistor. If the negative feedback attenuates one frequency, after inversion, the transistor will only amplify that frequency. The second feedback, which is typically a positive feedback, does exactly the opposite, as everything is reversed. The regeneration feedback causes the amp to bootstrap at the frequency at which the notch filter is set for, thus reinforcing the tendency for oscillations of the amp. So if we add an amplitude compensation fritter diode (just two diodes in antiparallel) to nonlinearly and more sharply increase the gain after the output reaches a given amplitude level, the increase in gain, will actually cause the notch filter to further attenuate the fundamental frequency and decrease the gain. A very smart little device this is. It took me a while to trace how it works, but it's deceptively simple.
You may reference the circuit to a vintage heathkit ag-9u audio generator for the original Schematic.
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