[Amps] Bad tubes from Ebay

[Tom W8JI]

How can a gassy tube cause a flashover?>

You might read this:

http://www.allaboutcircuits.com/vol_3/chpt_13/9.html

You would expect the electrons to collide with gas molecules 
within the
tube, hence to decrease plate current.>

Current in a tube with a control grid is limited by negative 
electric fields that repel the electrons. Positive ions from 
gas have the opposite effect.
Gas can also neutralize negative space charge around the 
cathode increasing current, as well as ionize into a plasma 
that has very low resistance.

Look at how much better an 866 mercury vapor rectifier 
conducts than a high vacuum rectifier. Some gas rectifiers 
like the 0Z4 didn't even need a hot cathode.


73 Tom