An outline in the National Semiconductor data book provided the starting point for this 0-200V power supply. Current limiting occurs at 30mA,' set by the 150Ω resistor, and load changes up to this point cause output variations of at most 0.2V. Since the valve grid is connected to the output line, load compensation is very fast.
Most of the voltage is dropped by the EL84; only about 20V appears across the 317. With my prototype, a short circuit with output set to 200V causes no damage.
See also A Valve Power Supply
(The PL81 was designed for series regulator use and may be worth considering, especially as it will be cheaper than an EL84. Ed.)