[Amps] Filament Voltage Question

[John Lyles]

When direct heated filaments of thoriated tungsten (usually 1-2% thoria) 
have a lot of hours of operation (thousands), they become brittle and 
easy to break. Older tubes are much more prone to shipping damage from 
broken filaments for this reason. Grain growth occurs in the 
microstructure of the metal. Thin higher voltage filaments would be even 
more fragile in this condition. Large filament wires or bars can be 
modified in thickness along the length to try and create a more uniform 
temperature and electron emission. Near the bottom, the leads help 
conduct heat away and reduce the temperature. By necking down the wire, 
the temperature at this point can be raised. Lots of tricks like this 
are possible with fat high current filament structures.

The filaments in the RCA 7835 are 96 vertical bars, each carrying ~70 
amperes DC. End result is a 5 VDC filament with 6800 amperes of current. 
The RCA 4616 has a similar complex of vertical bars, much smaller, and 
has a terminal voltage of 0.95 VAC!

73
John
K5PRO
-------------

There is another issue too: With a directly heated cathode (filament),
there is an unintentional bias across the filament. Whether using AC or
DC, one end of the filament will be more negative than the other and
that adds a grid-cathode bias to the equation. With DC the bias is
constant, with AC it alternates of course. More anode current will be
emitted from the negative end and less from the positive end.

I don't know if this causes any actual harm, but I can't imagine it
helping any.

73, Bill W6WRT



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Message: 7
Date: Tue, 22 Apr 2014 13:13:26 -0600
From: "Jim Garland" <4cx250b at miamioh.edu>
To: "'Fuqua, Bill L'" <wlfuqu00 at uky.edu>,	<amps at contesting.com>
Subject: Re: [Amps] Filament Voltage Question
Message-ID: <00ad01cf5e5e$f1790250$d46b06f0$@miamioh.edu>
Content-Type: text/plain;	charset="us-ascii"

Good point, Bill. I hadn't thought of that, but that's obviously a
consideration for VHF/UHF tubes with directly heated cathodes.
Jim

 >
 > --------------------------------------------
 > 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