[AMRadio] filament voltages

Bob Bruhns bob.bruhns at verizon.net
Tue Aug 17 11:12:58 EDT 2004

Hi Geoff,

I don't know, exactly, and I am not familiar with the 250TH at all.  I'll look at the tube specs later when I have some time, but I will only be able to guess where to set the filament voltage.

But wow, that is a lot of reduction, you are talking a 20% reduction there...  the resistance of a cooler filament will be lower than a hot one, so the current may only be reduced by 10%, and therefore the heat reduced by like 28%... still, the emission temperature will be down quite a bit...  but if you do not see anything drop until you get to 3.5V, then 4V *may* be ok.  Is this a modulated stage?  If so, watch the positive peaks closely.  And I would really worry about tuning up, where you might get serious current surges if you slip up.  Maybe tune up with full filament voltage, and drop back afterwards?

There's a point... positive peaks, higher plate voltage and simultaneous higher current.. that's a place to really watch out.  But if you see no flattening even with a slight further reduction in filament voltage, with all other conditions the same, then that argues that you have an emission reserve, which protects the cathode.  Does it protect the cathode as much?  I don't know.  Probably, if there is enough reserve.

The whole point is to extend the emission life of the tube.  I think advanced practice used to be to start out with a new tube using reduced filament voltage, maybe 5-10% low, and raise the filament voltage over the months and years as needed, as emission gradually fell.  If any negative impact exists for the reduced filament voltage, it should be offset by the reduced overheat-breakdown effects, so that the net result will be an improvement in lifespan for the tube.

It is difficult to determine where the exact point of emission saturation occurs.  Also, transient conditions exist.  The lower plate voltage should reduce the damage from emission-limited flattening that happens, but some damage will still occur.  In an AM transmitter, you will see emission-limited flattening first on positive peaks.  In a modulator.... peak flattening.  In the modulator, look at the peaks of low frequency waves, below 200Hz.  The finite transformer inductance causes this to be a higher current test.  Flattening may start out soft... but cathode damage is cumulative... so don't
tolerate *any* flattening unless you are positive that it is from voltage limiting, and not emission current limiting.  A DC coupled scope may be able to show you this.. but in the RF amp, it will not be easy.  You would have to build in a fast, high voltage sensing probe of some sort, and watch the instantaneous rf voltage on the plate.

Line voltage fluctuations may surprise you.  You may want to use a constant-voltage transformer or some regulated arrangement for the filament suppply.  Maybe you should try 4.5V instead of 4V.  I can only guess.  Operating with lower plate voltage and lower cathode current will help.  Be extra careful of the tube, because its cathode will have less reserve emission at the reduced filament voltage.

  Bacon, WA3WDR

Geoff/W5OMR wrote:
> > 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
> tubes.
> Would you say this is a correct statement?
> Max Ratings on a 250TH (and others) can be found at:
> http://w5omr.shacknet.nu/~w5omr/hamstuff/AM-Stuff/XMIT-Tube-Data/
> 73 = Best Regards,
> -Geoff/W5OMR
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