Can a purely thermonic cathod work?

Bob Bruhns bbruhns at
Mon Sep 12 01:01:25 CDT 2005

An electric heater is convenient, and permits some
precision of temperature and heat distribution.  But,
there are problems such as back bombardment, which can
add addtional heat to the cathode, resulting in
excessive cathode temperature in UHF and microwave
applications.  Some transmitters reduce filament
voltage during transmit to compensate for this effect.

A cathode could be heated with light through the glass,
if some means of cathode temperature regulation is
arranged.  Or, hot gas or liquid could be passed
through from the outside.  But the heating system will
have to produce temperatures of thousands of degrees
Kelvin in some cases.  Normal oxide-cathode operating
temperatures are about 1150K, and temperatures of 1500K
are needed in the manufacturing stage to process or
"form" the emissive layer.  Thoriated tungsten requires
about 2150K in normal operation, and temperatures of
about 2700K are needed in manufacturing to "activate"
the emissive layer, and sometimes up to 2850K is needed
to rejuvenate an old tube.  Pure tungsten does not
require activation, and it is the most resistant to
deterioration of its emissive layer due to aging and
ion bombardment, but its electron emission per unit
area is not as great as when thorium is added, and it
requires about 2700K in normal operation.  This causes
it to glow yellow hot.  Tungsten and thoriated tungsten
are usually used in directly-heated filamentary
cathodes of certain power tubes.  Oxide coating is more
emissive per unit area than thoriated tungsten, but it
is very sensitive to ion bombardment.  It is usually
used in indirectly-heated cathodes, but it was
sometimes used in filamentary cathodes for speed of
warmup, or for high efficiency in battery-powered
applications. The possibility of a small cathode area
makes oxide coating useful in the UHF and microwave

If heat would be pumped into the tube by a pipe, some
kind of tricks would be needed, or the glass seals
around the pipe would probably crack.  Maybe insulated
piping that can seal well, and a more
temperature-tolerant envelope.

Light-heating could probably do the trick, though.  You
could shoot the light through the glass envelope of the
tube.  Heating the cathode efficiently would be tricky.
The back of the cathode would need to be really black,
the heating beam would have to be very well converged,
and aimed at the right spot, the glass would have to be
very transparent, probably with anti-reflective
coating, and some means of cathode temperature
regulation would have to be used.

Or, if some means exists of releasing lots of electrons
into space without using a hot cathode, that could be
used instead.


----- Original Message ----- 
From: "hal" <hfeinstein at>
To: <tacos at>
Sent: Sunday, September 11, 2005 11:40 PM
Subject: Can a purely thermonic cathod work?

> In each and every vacumn tube I have ever seen the
> is connected to a source of current.  Therefore, I
reason, the
> current must enter the material that makes up the
> and launch the electrons that make it up toward to
anode or
> grid(s). The heater must, likewise, raise the
temperature of
> the cathode to make this possible, since a cold
> would never emit anything.
> But wait!   It this really the case, I wonder?   We
were all
> taught to think of electrical circuits through their
> analogy -- wires as pipes, a battery for a pump, a
> for a valve.  Gravity acted as resistace.  While
> for some circuits, it might have sealed in the wrong
ideas for
> an electron tube.
> What about a solid state theory of the cathode?
Here a
> cathode might be thought of as a material that when
> boiled electrons out of its metal.  Once free, the
> are attracted to the grid(s) and plate and begin the
trip to
> those elements.  The cathode is not a sprinkler
supplied by
> the batteries, but a metal that showers off
electrons when
> heated.
> If this makes sense then you should be able take any
> electron tube and re-engineer it so the cathode uses
> purely thermal energy source w/o ever connecting it
to an
> external current source. The tube should work just
> --h.
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