So I have been trying to get this circuit to work with no success. I started with R5 using 200 ohms and finally concluded that that was just too small of a resistance. So I am opening this project for general comment. Ideas? Suggestions?

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to start off I adjusted R5 to 600Ω/300Ω for a gain of two.

 

What are you using for R6?
What is the supply voltage?

U2b has to be able to supply enough voltage/current via R5 to heat the filament of the lamp.

 

At this point the lamp is irrelevant it measures less than 1Ω cold. I could remove it entirely if it would help. 12 volts DC

 

Your circuit will work with a power amplifier like an LM386, it is in its datasheet.
For a low power opamp, a low power incandescent light bulb might not be found.
The gain for a Wien Bridge oscillator must be 3.

Pin 5 of the opamp connects to nothing.
R1a and R1b do not connect together.
There is no supply voltage.

 

OK next brain fart:

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Now R5 does not connect to pin 7 because it is cut away and instead it wrongly connects to pin 5.

Then when fixed and the gain is set to 3 or a little more does the oscillator work with no AGC because the light bulb needs too much current?
Can you see and hear and see on a scope the crossover distortion and hiss from the lousy old LM358?

 

This circuit works very well, using diodes to stabilize the feedback. LM358A is used to give a virtual ground center tap to the single supply.

 

OK so I will get rid of the light bulb, try this instead:

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There are other approaches I can use to create an AGC function. Once I have oscillation I will tweak the design.

 

That should work but the output may vary between clipped and nothing with temperature changes. The non-linearity in the signal diodes in my circuit stabilize the amount of feedback and avoid that problem..

 

This I understand. I am hoping to come up with my own design. With the brain fog I no longer grok in fullness wiring that was once simple for me. Any Robert Heinlein fans here?

 

Keith, your diodes add distortion.
I corrected errors in your schematic:

 

Instead (or addition to) checking the output for AC, I'd check various points in the circut for DC voltage.

Since the circuit is single supply, I'd measure between a point and ground (not the virtual ground).

What is the voltage at Vcc? pin 3? pin 1? pin 7? Are these as "expected"?

 

 

Keith, your diodes add distortion.
I corrected errors in your schematic:

Thanks for the correction. That was a last minute addition.
If the pre-set pot is adjusted correctly there is very minimum distortion.

 

View attachment 251121

Sparky, what prevents the opamp from clipping since its gain is higher than 3 times?

 

On your breadboard R1a is connected to the +ve rail, not to ground.

 

Look again. While I did bad job of showing it the outermost rails are 12 volts DC.

 

I had been wondering about this idea for some time, so I thought I'd try it on SPICE to see how well it works.

This is the Linsley-Hood two-op-amp Wien Bridge, made with rail-to-rail op-amps. The final output (from U2) is a clipped sinewave, but that output goes into the Wien network then it appears band-pass filtered, out of phase and at half the amplitude at the output of U1.
The band-pass filtering removes quite a lot of the distortion, so that the U1 output has 3rd harmonic distortion at 50dB below the fundamental (0.3%).
[Edit] SPICE seems to require me to add a bit of noise to make it start up - hence the Gaussian noise source.

 

A Jfet used for AGC reduces the distortion much better.

 

Next brain fart:

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I was going to use a 2.7KΩ for R5 but all I had handy was a 2.0KΩ. Results will follow.