Just out of curiosity, have you tried other Atma-sphere amps? How do they compare?
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Actually this is my first Atmasphere amp AND first OTL experience. I believe Ralph's amps are very similar if not identical, in design, except for their power output capabilities and the available mods. The bigger Atmasphere models might have more edge in the usual sonic parameters due to bigger and beefier output stages. The main sonic strengths are undoubtedly shared within all the range.
Pubul57, its true that we don't match the tubes. They have a mu of 2, which is to say that they have the lowest gain of any power tube made. So you have to drive them with some really high input voltages, compared to a pentode. The result is that for variations in the tubes, you're not going to see a lot of difference between the actual sections. We can only do this because the mu is so low, and because the transconductance is not outrageously high either. A different power tube would have required matching.