Hi Michkhol,

The frequency response was measured in a similar way and with similar results.  The -3dB point on the current model is about 38Hz.  The stated response 20K-20K does not claim +/-3dB, although this was not intentional - the site simply needs to be updated to reflect the current model.  Over the years and revisions to this amp we have found better sound quality and achieved higher usable power with this mildly rolled response.  SET amps without feedback and under 5 or 10 watts are obviously going to perform better if they're not trying to reproduce 20 cycle notes during large classical movements.

As for explaining the phase shift you measured at low frequencies I think you've already answered your own question.



The differences are attributed to the caps used on the output tubes, ie., bypass cathode cap and suppressor grid to cathode cap.

If you're asking why there is a phase shift at low frequencies at all, it's a natural side effect of amplifier design.  Even if one could eliminate it completely, the amplifier would have to be re-voiced and frankly I am happy with the way it sounds.