Now decided to try playing with the plate current. Tried putting a 220ohm resistor in parallel with the existing 220ohm cathode resistor to see what happens. Result. No appreciable change. Plate current measured as 29.8mA. Removed the resistor.
Now back to the original circuit and now trying a new tank coil with looser coupling. ie winding the link over the cold end. Seems to be better than the previous coupling.
9 turns. Coupling still loose. Sharp resonance and sharp dip. Power 2 watts. Stable freq.
Now tried tighter coupling. Got it very stable and looking good! Output power = 4watts with Anode curent at max dip showing a nice dip to 30.2mA. Keydown voltage =325volts. Therefore input power to plate = 325*30.2mA = 9.8 watts. Output power = 4 watts. Efficiency now 40%. Can I do better? Output freq = 7042.18Khz.
Further adjustments now very interesting since I am getting very close to the point where max dip on the input current is corresponding to max power output indicating a point of maximum efficiency and therefore stability.
29.7mA plate. Keydown plate voltage = 326volts. Input power = 9.68 watts. Output power now 4.75 watts. Efficiency = 49%. Excellent.
Now try raising the screen voltage and see if can get more power? Raised it back to original spec. ie 191 volts on key down. Now raised the plate current to 35mA with no difference in power output and freq not as stable. ie less effieicnt now.
Now feel like I have arrived at a good result for the tank coil. Next step is to wind a solid coil using the same dimensions.
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