tube amplifier build power supply design
Your main sector is either 110V or 220V (or approx) RMS. The transformer multiplies that by a constant factor to turn that into 290V RMS. As your main sector may not be exactly 110 nor 220V but a bit (10% is ordinary) more, this difference is multiplied by the transformer and it outputs 330V RMS = 330 * sqrt(2) = 466V peak. Without load, no current is drawn from the transformer so diodes will redress the peak voltage and charge the first capacitor with Vpeak (466V). 

With a charge, the transformer impedance will start to dissipate hence the voltage will be lower. As a rule of thumb, a loaded high voltage power supply delivers approx. RMS × 1.2 = 396V (400V) but the first capacitors must be able to withstand the full unloaded HT (466V). Choose at least a 500V rated capacitor.

PS: Yes tubes amps do love regulated power supplies.