Bear,
Sorry, I didn't know which amp was using this so I assumed it having
screen voltage by it being a tetrode. Actually those resistors act as
fuses which should open up in case or an arc or short (whenever a large
amount of grid current occurs). The capacitors are bypass caps to kill
off any stray rf floating around. If both grids are connected to ground
then, and are running as grounded grid, about the only thing that could
damage this would be from oscillation. Parasitic oscillation of the tube
running without any signal applied. I'm not sure if that has a
neutralization circuit or not without seeing the schematic. I have seen
3-500-Z's melted down because of this. I don't know why they would have
used 4-400's in place of 3-500's unless they had some better gain
someway? This I wouldn't think as the 400 is a 400 watt tube and the
3-500 a 500 watt tube. You might read up more on parasitic oscillations
over on Richard Measures website. He posts on here all the time and has
seen this more than me.
Will Matney
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