maybe a variac??? and another question to follow jim redman using 50 ohm pots--can you elaborate on the 22 ohm resistors and 10 ohm pot and the 50 ohm pot alone-
this from diytube.com
hum pots are only necessary for AC powered filaments, the idea is to reduce (and not totally eliminate) the amount of audible 60 Hz hum, with careful adjustment, generally close t the halfway point. Bottlehead goes ths one step further, they use a 10 ohm WW pot with a 22 ohm, 5W WW resostor connected to each end, this effectively behaves like a 54 ohm pot with a physically "wider", and less "sensitive" adjustment range.
The way the hum pot works is to create a very simple resistor "bridge" circuit, with the filament and the two halves of the hum pot, and by careful adjustment, the AC voltage on the filament can be balanced out in relation to the tube's grid, which will reduce the amount of hum heard on the output