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" <[EMAIL PROTECTED]> To: <[email protected]> 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 low > filamrnt voltage, or exceeding the rated plate dissipation. > > Don k4kyv > > _________________________________________________________________ > Is your PC infected? Get a FREE online computer virus scan from McAfee® > Security. http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] >
