[AMRadio] filament voltages

Geoff/W5OMR w5omr at w5omr.shacknet.nu
Mon Aug 16 16:54:51 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.

This is great, Bacon.  What I was looking for.

So, with my tubes not showing a reduction of emission until 3.5v is
realized, with 120mA of grid drive (for a pair of 250TH's) a plate
voltage of not more than 1600v, and around 200mA of plate current,
the limits of the tube are in no danger of being reached so a reduction
of 1v (from 5.0v to 4.0v) would probably not do any damage to the

Would you say this is a correct statement?

Max Ratings on a 250TH (and others) can be found at:

73 = Best Regards,

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