[Arran]

It has the power output #42s tubes wired in as tetrodes, not triodes, the screen and plate maybe connected together but they still have a suppressor grid. In fact they can't be wired in as triodes because the suppressor grid is internally connected to the cathode. The power output stage is different in the 660 compared to the 118 in at least one way, in the 118 the type #42 driver is wired up as a pentode with the screen connected through a .006 uf capacitor to ground, in the 660 it's connected as a tetrode, with the plate and screen connected together, just like the output tubes.
Regards
Arran