[Amps] Filament Voltage Question

[Jim Garland]

I was reading the data sheet this morning on the 4CX3500A and noticed the
filament requirements are 5V at 90Amps.  It occurred to me that I've never
understood why so many tubes with indirectly heated cathodes have such
low-voltage - high current filaments. Since the only thing the filament is
used for is to heat the cathode, then why not design it to run at, e.g.,
115V at 4A? That sure would be a lot easier to implement. I'm sure there's a
reason, howevrr, and would appreciate somebody informing of it!

73,

Jim W8ZR