WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
For ordinary rectifiers (capacitor input) the RMS ripple current will be about 1.5 times the DC load current. You shoudl be safe up to 230mA continuous DC.
If its a class AB amp then you can design for higher maximum currents if you think it won't be continuous.
Do you think occasional peaks of 500ma DC could be had? I realize that running perhaps 250-300ma steady state current creates heat, but I'm guessing the worse that will happen with peaks of 500ma will be reduced voltage. The transformer will be feeding a bridge and then into a 500v voltage regulator.
The numbers above are for choke input. Cap input would be good for a little over 200 mA continuous. Peaks above that will be OK - use "stepped current source" in PSU designer to see how much drop will occur.