Wein bridge oscillators

An interesting low distortion audio oscillator

I got interested in building a good low distortion sine wave audio oscillator some time back.  I wound up building two versions; one with discrete transistors and the traditional incandesent lamp AGC element and one using an op-amp and a FET as the AGC element.  I then combined them with an op-amp mixer to give a two-tone test oscillator. There were some interesting discussions on QRP-L and I want to thank Jeff Furman AD6MX for his circuit ideas and explanations.

Below is the schematic of my first oscillator.  It uses a two transistor amplifier circuit with the Wein bridge elements added to make the oscillator.  Jeff Furman posted a SPICE list for this circuit on QRP-L, but with back to back diodes as the AGC elements.  I wanted to go with the traditional incandescent lamp and did some experimenting to find a suitable lamp and associated resistance values.