Output stage schematic:
Been playing around with this in SPICE. This is sort of inspired by Brosie's single ended "power buffer" circuit, only it's push-pull. Maybe it's sort of like a McIntosh output stage, except that it doesn't use a separate feedback winding.
What I like about this idea is that it seems like a very easy way to apply a bit of damping right at the output stage, without worrying about low-frequency rolloffs and phase shifts during preceding stages. Might be an easy way to tame the low-frequency high-impedance resonance of a loudspeaker. While it might be difficult to get sufficient drive with low-gain triode or beam tetrode tubes, it doesn't look like a difficult problem to drive this circuit using EL84 or EL34 pentodes.
It's also an elegant looking circuit - to actually hook a speaker right to the tube cathodes, almost looks like an OTL! I could even pretend that some of the tube milliamps were directly driving the speaker, assisted by the transformer secondary.
I wonder why I've never seen this circuit before. Is there some reason that this hasn't been used?
I'm a little concerned about stability, about high resonance frequencies where the transformer induces a 180 degree shift. Any ideas how to be safe about this?
Thanks, I look forward to any suggestions or advice.