[Amps] Filament Voltage Question

[KA4INM]

   Hi Jim W8ZR Garland:

> A note from WC6W pointed out that the particular  tube in question, a
> 4CX3500A, actually uses a directly heated cathode, although I'm not sure
> that makes a lot of difference (except that the filament presumably runs
> hotter than it would otherwise). And the ruggedness issue could certainly be
> a consideration. Still, 90 Amp filaments strike me as excessive unless
> there's some other really compelling reason. Hum rejection (also pointed out
> by a commenter) would seem to me to be worse in low V/high I filaments,
> since the induced hum field would be greater. In fact, with 90A of current,
> I'd think there could be some significant distortion of the electron beam
> caused by the filament's magnetic field. I wonder if heat conduction from
> the filament to the socket is a consideration? It might be hard to get the
> heat out if running a filament at high V/low I.

   These tubes have two rings with wire strung between them like a round 
cage, many filaments in parallel.  A slightly larger cage surrounds the 
filament cage but the wire spacing is between the filament wires, it is 
the control grid.  A slightly larger cage surrounds the control grid 
cage and wire spacing matches the control grid wires, this is the screen 
grid.  (when present)  The final grid structure is surrounded by a 
cylinder this is the plate element.
   I'm sure they use the thinnest highest Voltage physically strongest 
filament wires possible but there are 12 to 30 of them in parallel.  If 
they used much higher Voltage strands for filament wires the AC filament 
supply Voltage could modulate the cathode current much more than it does 
already.  And if you used a DC supply the cathode current would be much 
higher from one ends of the filament wires than the other.
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.