[AMRadio] filament voltages (oops, Harris link fixed)

Bob Bruhns bbruhns at erols.com
Fri Aug 20 10:53:33 EDT 2004


I looked at the 250TH specs, but I really can't make a solid judgement on
acceptable filament voltage at reduced specs for long life.  Generally I
would be conservative; where you might reduce filament voltage a great deal,
I recommed that you only reduce it slightly.

Emission falls off very rapidly with reduction in filament voltage, and a
small reduction probably has a large effect on tube life.  I really can't
specify how much you can safely reduce filament voltage, but with
thoriated-tungsten filamentary cathode tubes, I would recommend that you
don't reduce operating filament voltage more than 10% from spec.  And make
sure your socket connections are clean, etc.
Ref: http://www.bext.com/filament.htm

Some recommend not reducing filament voltage on large oxide-cathode tubes
such as the 8877, etc.  Ref:
Others claim remarkable improvements from filament voltage reduction. Ref:
(Note: I believe the 4CX20000 is an oxide-cathode tube.)

Other factors probably have as much effect on tube life.  Excess current, of
course, or operating before full emission temperature is reached, and
thermal shock at turn-on. ref:

Burn-in: cook long-unused tubes at rated filament power for 100-200 hours.
The idea is to allow the "getter" elements to absorb gas that has leaked
into the tube or out of the materials in the tube.  In tube operation, gas
becomes ions which attack the cathode.  Ref:

Filament and cathode structures do not like thermal shock.  The sudden
application of filament voltage to a cold filament is a significant shock,
because the resistance of a cold filament is much lower than its resistance
when it is hot.  A gradual application of filament power is best.  This can
be done with a resistor in series with the filament transformer primary, a
small filament transformer that overloads at turn-on, etc.

Filament voltage regulation is a good idea. Ref:
But, be careful not to make the turn-on shock situation worse.  Basically, I
suggest one or two big constant-voltage transformers and a few small
filament transformers, possibly with a series resistance of a few ohms in
their primary circuits.  The little transformers will overload on turn-on,
limiting thermal shock to the tubes, yet the system will maintain operating
filament voltage nicely, over a wide range of mains supply voltage.  Then
you can tweak the filament voltage to extend emission life, and the voltages
will stay put, and turn on will be gentle.  The gentler, the better.

  Bacon, WA3WDR

----- Original Message ----- 
From: "Geoff/W5OMR" <w5omr at w5omr.shacknet.nu>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Monday, August 16, 2004 4:54 PM
Subject: Re: [AMRadio] filament voltages

> > 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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