Good point, Bill. I hadn't thought of that, but that's obviously a
consideration for VHF/UHF tubes with directly heated cathodes.
Jim
> -----Original Message-----
> From: Fuqua, Bill L [mailto:wlfuqu00@uky.edu]
> Sent: Tuesday, April 22, 2014 11:37 AM
> To: Radio WC6W; Jim Garland; amps@contesting.com
> Subject: RE: [Amps] Filament Voltage Question
>
> Thinner and longer filaments also have more inductance.
> 73
> Bill wa4lav
>
> ________________________________________
> From: Amps [amps-bounces@contesting.com] on behalf of Radio WC6W via Amps
> [amps@contesting.com]
> Sent: Tuesday, April 22, 2014 11:28 AM
> To: Jim Garland; amps@contesting.com
> Subject: Re: [Amps] Filament Voltage Question
>
> Hi Jim,
> That tube employs a DIRECTLY heated filament.
>
> Two good reasons for low voltage in this case:
>
> High voltage operation would require thin elements that would be
mechanically fragile.
>
> Current distribution would be way whack with the potential differences
in a high voltage
> setup and induce lots of hum.
>
> 73 & Good morning,
> Marv WC6W
>
> http://qsl.net/wc6w/
>
>
> --------------------------------------------
> On Tue, 4/22/14, Jim Garland <4cx250b@miamioh.edu> wrote:
>
> Subject: [Amps] Filament Voltage Question
> To: amps@contesting.com
> Date: Tuesday, April 22, 2014, 8:19 AM
>
> 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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