Partially the size of the transformers, but also the voltage/current being put through the tubes. If you put more power into the tube, you get more power out. As I understand it, you will also typically get more clean headroom, up to the point where you start getting over the tubes limits and all hell breaks loose. I haven't spent much time experimenting with that one, though.
Light
"Cowards can never be moral."
M.K. Gandhi
what it really comes down to though is different circuits.
But sorry, Ed Dixon, the "tubes are different" thing is not gonna carry the day- sure, any two 6V6 tubes will be different, but those differences would not account for a 40 percent difference in power.
\But sorry, Ed Dixon, the "tubes are different" thing is not gonna carry the day- sure, any two 6V6 tubes will be different, but those differences would not account for a 40 percent difference in power.