This is from a FB comment by me.
Speaking of audio signal generators, I made an audio signal generator, the classic Wien Bridge oscillator with the lamp, in my case it was a 28 volt low current lamp, I believe an 1869. Well I got the lamps from Radio Shack, and the circuit had an intermittent problem. I tried more than one lamp before I found a good one. The end of the tungsten filament is held between the lead which is folded over on itself and then squeezed closed. There is no weld, probably because tungsten is very difficult to work with. Apparently the leads were not closed tightly on the filament end, which doesn’t show when the lamp is tested at full brightness. But at lower temperatures, the leads are loose and make intermittent contact.
Once I got that problem fixed, the oscillator put out a clean sine wave. My circuit used three transistors.
