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.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Joe Subich,
W4TV
> Sent: Tuesday, April 22, 2014 9:34 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Filament Voltage Question
>
>
> Perhaps because the low voltage, high current filament is more
> physically rugged?
>
> 73,
>
> ... Joe, W4TV
>
>
> On 4/22/2014 11:19 AM, Jim Garland wrote:
> > I was reading the data sheet this morning on the 4CX3500A and noticed
the
> > filament requirements are 5V@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@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
> >
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> >
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