You probably don’t need voltages that high. DIYers can stick with 200 V or something reasonable. The remaining people who deal with 450 V are probably servicing guitar amps.
Plate voltages can exceed much more than 450 V. The 811A is rated for 1500 V of plate voltage. But the same is true for transistors—you can find transistors with similar voltage ratings.
You probably don’t need voltages that high. DIYers can stick with 200 V or something reasonable. The remaining people who deal with 450 V are probably servicing guitar amps.
Plate voltages can exceed much more than 450 V. The 811A is rated for 1500 V of plate voltage. But the same is true for transistors—you can find transistors with similar voltage ratings.