I think I see a grounded grid phase splitter...or is that cathode coupled phase splitter?
Anyway, There is a 70 volt bias on the cathodes (Limit = 180V), so that subtracts from the plate voltage.
Then there is the rating: 330 V @ Wikipedia, 300 volts @ GE, 300V @ Sylvania, 300 @ Tung-Sol 250V @ Phillips
I suspect the right answer is 300 volts and I know that tube amps are designed with the tubes at max voltage during idle.
Then there is the voltage drop across R4 or R5, not R4 AND R5.
480 - 70 = 410 so you need to lose at least 110 volts in R4.
That's not unusual. I expect the design, as intended, will use R4 to drop 170 volts, but I also expect Vb will have some resistance to limit the plate current of each tube.
Basically a valid design, but a major PIB when it has no parts values written on it :-(
Now...what was your question?
Oh yeah...bias supply. Build a voltage divider from the 480 volts to get to 70. Add a decoupling capacitor and put a resistance between the 70 volts and the cathodes.
I'm afraid I'm running out of Login time so, quick post.