We have a number of possible scenarios:
1. We get, for some reason, an arc from plate to grid. The resistor/fuse/what
have you opens. Because once the arc is struck, it's a low impedance with
around 50 volts across it, the grid starts to hare off towards the plate
potential. The arc doesn't immediately extinguish because it's got the grid
plate capacity to charge. This gives the chance for either the grid - cathode
to arc, or the grid to start pulling lots of current from the cathode because
it's trying to get to the B+ line, or both. The arc may well extinguish once
the cathode reaches saturation, since that will try to clamp the grid at some
lower voltage than the B+, and the grid plate spacing may then be enough that
the arc can't maintain itself. Either way, we may well have a bad tube as well
as a fuse or resistor to change.
2.We get, for some reason, an arc from plate to grid. The resistor/fuse/what
have you opens. The arc extinguished, teh grid floats. Internal plate grid
leakge starts pulling the grid positive. Depending on the extent, and the drive
conditions prior to the arc, we can have a variation of scenario 3, where the
grid is pulled positive by leakage as well as the secondary emission.
3. We overdrive the grid, and it gets hot, and the resistor/fuse/what have you
opens. The grid is now floating, is hot and emitting electrons, so it goes
positive atracting more electrons from the cathode, which hit it with kinetic
energy so it gets hotter so it emits more so it gets more positive so it
attracts more electrons...........and so on until we have a run away plate
current situation.
Depending on such factors as the actual voltages, the grid current and
temperature, it may not happen with some tubes and may with others.
What happens if the cathode is cold? Two capacitors in series across a direct
voltage distribute the volts across them in inverse relation to teh capacity -
the smaller the cap, the higher the volts. So depending on the internal
leakage, the grid under those conditions will float to some voltage, which
could lead to an exacerbations of scenarios 2 or 3 above.
With small receiving tubes, you rarely run much grid current or get arcs.
73
Peter G3RZP
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