No, the output pentode delivers a *current* 'swing'. It becomes a voltage when flowing through a load impedance.
What 'limit'? What's the voltage created by 30mA through an 8k load?
240V. What's the voltage created by the same 30mA through a 1 Megohm load? 30,000 volts.B+ is not a 'limit'.
You're forgetting first principles. An OPT is coupled inductors. Pull current through an inductor, then try to stop it, and you can create any voltage you want.
If there's no load then there's no place for the current to go when the tube cuts off so the inductance is going to create whatever voltage it takes to make that current go SOMEwhere. If it can arc at
1,000 volts then that's what it'll do but if it takes 30,000 volts to get there it'll do that as well. There is no 'limit'.