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RE: Phase Shift

Yes, that's the problem. The grid must be at AC ground. I suppose the reason this doesn't immediately grab everyone's attention is that the same RC network creates significant attenuation of the signal at the grid. After all, that's its purpose (and to bias the grid to the same DC level as the upper section). The problem is, the phase shift is nearly constant at 90 degrees across the audio band, whereas attenuation varies as a function of frequency. Use of this circuit demands that we assume the shift to be inaudible across the entire range of attenuation, whatever it might be, and I don't think it's reasonable to do that. I also think this is the cause of the phenomenon the OP is describing.

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