Circuit simulations ... arise because the op amp isn’t really perfect. A real op amp would not hit the 15V rail exactly. It will get close depending on how much current you draw from the output.

In fact, if you need more current than your preferred amplifier can deliver, it might be a better idea to make an output boost circuit ... measure of speed though. Another spec that you find in ...

Part 5 provides spec summary of the GP/R K2-W vacuum-tube op amp and discusses measurement constraints ... modeled result shows 1.00 nA of input current. Early SPICE simulations based on that ...