> Let me ask a question here. Lets say we took away the
> current from a cathode/heater and let it cool off
> completely. Then we grounded both ends of the
> cathode/heater where no current would flow through it to
> heat it up. After this we applied plate voltage. Would any
> current flow from the cold cathode to the anode?
No, but that is not what we are talking about. In order to
have a fault the grid is under operation and subject to
excessive dissipation or to a direct arc (which means
plasma).
> Here's an experiment for you. Take a 1/4 watt resistor the
> size and type Rich mentiones, I believe he said 30 ohm 1/4
> watt, and apply say 2500-3000 Vdc across it. Let me know
> what happens and how long it took to obliterate it.
The real question is what happens to a grid in series with
that resistor, or a cathode spaced a few thousandths of an
inch from that grid.
Also you, like Rich, are ignoring the fact a good amount of
grid damage comes from grid currents high enough to cause an
accumulated failure. I can ruin a 3CX800A7 in a matter of
5-20 relatively brief operational cycles at 100-150 mA of
grid current. How good does the "resistor fuse" do in that
situation? If the grid is hot enough to go into secondary
emission and you open it, what happens to anode current? Do
you think the fault occurs with a cold tube that isn't being
used or abused?
73 Tom
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