[Amps] Re: 4-400 Question - It's Grounded Grid!

[Will Matney]

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