Hexaborides show low work function, around 2.5 eV . They are also somewhat resistant to cathode poisoning.
4.
Some sources suggest that incomplete glow coverage of a glyph ( " cathode poisoning " ) or appearance of glow elsewhere in the tube would not be acceptable.
5.
Hexaborides, such as lanthanum hexaboride ( LaB 6 ) and cerium hexaboride ( CeB 6 ), have low work functions, around 2.5 eV . They are also somewhat resistant to cathode poisoning.
6.
The special quality could be anything, from rugged designs for military and industrial use, through devices with exceptionally low noise and microphony, to designs primarily optimised for long life without cathode poisoning when used for switching in a digital computer ( but not necessarily with exceptional qualities as an amplifier ).
7.
Use of special-quality versions outside of the purpose they were designed for may not be optimal; for example, a version for digital computers may be designed for long life without cathode poisoning when mostly switched to low-current mode in switching applications, but with little attention to parameters of interest only for linear applications such as linearity of transfer characteristic, matching between the two sections, microphony, etc.
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