[Amps] toroid filament choke?

[TexasRF at aol.com]

The filament transformer will limit the current to a large extent. Also,  the 
resistance of the filament or heater will rise very quickly when the voltage  
is applied. For example, the TH347 filament reaches rated current in just a 
few  milliseconds. I haven't measured the current of an indirectly heated 
cathode  tube but you can see the color of the heater turn red almost immediately 
when  voltage is applied, suggesting that the resistance has increased  
substantially.
 
73/k5gw