> 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
