[Amps] Grid fuses (was: Life and gain of 3-500Z)

[Peter Chadwick]

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