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Originally Posted by adibrook
I didnt want to add anythgin on the cathode because as far as i know it would still amplify something with no grid bias.
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The problem is that without the cathode complex you won't get 'no grid bias'.
What will happen is that the input signal will be rectified when the grid goes positive, chaging up the grid blocking capacitor, thus creating a negative grid bias. But the bias will be much too low for an anode supply of 250V, being only about -0.5V or so, equal to the peak drive voltage.
Trouble is that this bias is depending on the oscillator being connected: It draws its power for biasing the EF80 from the RF power of the oscillator. So to the oscillator the grid represents a relatively low impedance and not the high input impedance one would expect.
In this case simpler isn't better I'm afraid.
Remember, valves are voltage controlled devices and do not respond well to no negative grid bias. Unless you really want a positive grid for special occasions, then you don't. Didn't we have this discussion before?
Best regards
Frank N.