[Amps] "Tubes 201" - How Vacuum Tubes Really Work

[Peter Chadwick]

Tom said:
>Not quite a peer reviewed engineering text is it Will?<
He does tell you not to leave grids open, though.
When you have grid current, you have grid dissipation. Which is why tubes designed for Class C had carbon coatings or whatever to reduce emission. See the RCA TT4 Tube Manual. It's fair to say that flash over dumping a lot of current into the grid could well heat it enough to get emission.
Whether 'arc' is the right term is arguable, in that 'arc' implies a plasma effect, which you don't get in a vacuum. However, 'flashovers' in high power tubes with a good vacuum have been written up many times over the years, with a number of explanations. In tube manufacturing circles, they were said to be 'barnacles', and could be removed by controlled energy flashovers - there was an article in about 1934 in the IEE journal. As I recall, flashovers are more prevalent with new tubes.
Sorry, I just don't buy grid fuses ( and even more so, resistors as fuses) as a good idea. I believe I'm not the only one, though.
73
Peter G3RZP