[AMRadio] filament voltages

Bob Bruhns bbruhns at erols.com
Mon Aug 16 16:18:36 EDT 2004

The theory that I have seen holds that the cathode of a tube is
protected against ion bombardment by the electrons it emits.  When
the emission capability of the tube is not exceeded, the cathode
tries to emit too many electrons, but they have nowhere to go, so
they cluster around the cathode and bounce around.  Positive ions
from gas in the tube are repelled from the plate and screen, and
they accelerate toward the cathode - but they encounter this cloud
of electrons, and they tend to hit the cloud and dissipate, rather
than hitting the cathode and damaging it.

But, if the emission capability of the cathode is exceeded, due to
excessive current peaks or low emission due to age or low filament
voltage, then the protective cloud of electrons is pulled away (the
cathode can not replenish it), and the stray ions can then smash
into the cathode, and do damage to it.  And when the emission
capability is exceeded, it usually means that the plate voltage is
high during the period in the signal cycle when the emission limit
is reached and exceeded (the tube can not pull the plate voltage
down enough)... and this means that the ions have more voltage force
accelerating them, and they hit the cathode harder.  Bad news.

So, too low of a filament voltage reduces the amount of current that
can safely be handled by a tube, and if we do not observe the
reduced limits, we can damage the tube.

In my opinion, oxide cathodes are much more sensitive to this ion
damage than thoriated-tungsten filamentary cathodes.  I have read
that non-thoriated tungsten filaments are even more resistant to ion
damage than thoriated tungsten filaments.

  Bacon, WA3WDR

----- Original Message ----- 
From: "Donald Chester" <k4kyv at hotmail.com>
To: <amradio at mailman.qth.net>
Sent: Monday, August 16, 2004 3:16 PM
Subject: RE: [AMRadio] filament voltages

> >
> >A while back, you said something about running too little voltage
on the
> >older
> >big jugs, like 833's, 250THs, 4-1000's, etc...
> >Of course, we know that running voltages that are too high will
kill the
> >life
> >expectancy of the tube, by why is running too little voltage bad
for the
> >tube,
> >if it's developing full emmission at a lower voltage?
> I think low voltage would be ok as long as you are not exceeding
the current
> rating of the tube, starving the filament emission by running too
> filamrnt voltage, or exceeding the rated plate dissipation.
> Don k4kyv
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