Cathode Bias resistor values

[pickupcentral]

Most of the cathode bias amps I have seen use a 470 ohm resistor for the power tube, whether it is a 6L6 or a 6V6. For two tubes, generally it is a 250 ohm.

What is the typical value for an EL34?

Is there a "cheat sheet" for these resistor values vs. plate voltage?

The Univalve is one of a few amps out there that can handle all three of these tubes and more without any adjustment. I assume it is cathode biased. Is that all?

If I have a two tube power section and each tube has its own 470 ohm resistor, can I swap between these three at will? (pin one and pin eight tied together of course)

Thanks!
-Phil