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EF806 pentode + CCS

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last month (i believe), somebody mentioned pentode characteristics glueing together with ccs.
i first started with ccs+ led bias; first thing i noticed was enorm gain, but shape was heavy deformed. (screen resistor was connected to B+ originally)

screen resistor moving to source of fet helped, it has kind of "selfcentering". (connecting to anode dropped gain a lot...or touching it)
second weird thing - gain roll off from >5khz (i thought mosfets has to be wrong, because you guys use dn2540 which has way low Ciss)
my mosfets i have almost for free :), but has 1nF Ciss.... so i added 50k "gate stopper" , and shape was even worse.

then i changed components, removing led -> 1k; screen resistor 100k; screen cap 1,5u foil; gate resistor ->0R.
this yields-undistorted waveforms (Vpp 200V scope load, or ~130-150Vpp 100k+u47 series// input 1,8Vpp)
-screen resistor helps a lot to correct the shape, 50k too low, 330k too much. (distorted bottom or upper halfwave) you might be best with 150k.
-and most important, no such huge roll off.
B+ ~260V; G2 ~105V; A ~135V; Rk ~1,8V

question is, why i see so many led bias schematic? what is wrong?


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Nothing wrong with cathode LED bias. Nothing wrong with anode CCS. Put the two together, especially with a pentode, and you get problems. This is because you cannot independently choose grid and anode bias voltages and anode current for a triode. You choose two of these and get whatever you get for the other one. Or you can use a resistor and set a linear combination of two, plus one other. Basically you only have two degrees of freedom, not three.

For a pentode you effectively have two different CCS both trying to set the current - the result is that the voltage 'bottoms' on one of them so you get distortion.
It had a big HF rolloff because the anode is a very high impedance point so it doesn't take much capacitance to affect it. Also high gain means high Miller effect from stray anode-grid capacitance (mainly in the wiring, for a pentode). As you have built a stage with a very high output impedance, the next stage needs an even higher input impedance. Using a plate/anode resistor will reduce it a bit, but if you want low output impedance don't use a pentode.
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