[Amps] Filament Voltage Question

[Jim Garland]

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 at uky.edu]
> Sent: Tuesday, April 22, 2014 11:37 AM
> To: Radio WC6W; Jim Garland; amps at contesting.com
> Subject: RE: [Amps] Filament Voltage Question
> 
>  Thinner and longer filaments also have more inductance.
> 73
> Bill wa4lav
> 
> ________________________________________
> From: Amps [amps-bounces at contesting.com] on behalf of Radio WC6W via Amps
> [amps at contesting.com]
> Sent: Tuesday, April 22, 2014 11:28 AM
> To: Jim Garland; amps at 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 at miamioh.edu> wrote:
> 
>  Subject: [Amps] Filament Voltage Question
>  To: amps at 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 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
> 
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