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In Reply to: RE: "Why is the grid resistor 1 meg?" ....................... posted by Cut-Throat on March 27, 2021 at 13:39:48
You can make that grid resistor just about any value you like.
I use the 330k on my first stage because it fits well and gives me the frequency I want with the sliver mica caps I had. And it's plenty high as to not load down my preamp.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Can't you get a capacitor close to crossover FQ and fine tune with the grid resistor value?
Wouldn't it be cheaper to swap out resistors than caps?
What formula do you use to calculate crossover FQ?
What can a 500 ohm output impedance preamp be comfortable with?
"Can't you get a capacitor close to crossover FQ and fine tune with the grid resistor value?"
That's what I do.
"What formula do you use to calculate crossover FQ?"
-3db point of the filter = 159155/(cap value in uf times resistance value)
"What can a 500 ohm output impedance preamp be comfortable with?"
Almost anything. The rule of thumb is 1:10 so it will drive 5k without trouble. Just don't forget about capacitance reactance at the high frequencies which is in parallel with the grid resistor value. That would include cable capacitance and the Miller capacitance of the tube being driven. Also the source needs to be able to supply the current that capacitance reactance will require.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
T
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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