I am perhaps confused. In my experience, grain growth generally produces
increased ductility and improved (or restored depending on your point of
view) yield strength more-or-less according to the Hall-Patch relationship
(yield strength is proportional to the square root of grain size). It would
seem that grain refinement, not growth, would contribute to increased
brittleness.
Matt
KM5VI
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of John Lyles
Sent: Saturday, April 26, 2014 12:39 AM
To: amps@contesting.com
Subject: [Amps] Filament Voltage Question
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
------------------------------
Message: 7
Date: Tue, 22 Apr 2014 13:13:26 -0600
From: "Jim Garland" <4cx250b@miamioh.edu>
To: "'Fuqua, Bill L'" <wlfuqu00@uky.edu>, <amps@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@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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