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![]() "N2EY" wrote in message ... 2) Close the key and adjust the "plate" capacitor (the one nearest the tube in the circuit diagram) for minimum plate current. Maximum output should occur at the same time. It is a characteristic of pi-network amplifiers that efficiency and power output are slightly better when the plate is tuned slightly off resonance. Tuning for a dip is good enough for most purposes. Why do I tune for minimum current?? Because that's when the impedance of the load circuit is closest to matching the tube's output. If plate voltage is 200 and plate current is 35 mA, plate impedance is 200/0.035, or 5714 ohms. The impedance of the load has nothing to do with this, it is due to the reactance at resonance of the plate tuning capacitor and the coil, which are equal but opposite in phase. You can have an open circuit on the other side of the coil and still tune for a dip in plate current. Assuming stable plate voltage it is possible to have the same plate current over a wide range of load impedances. Because the pi network looks like a lumped section of transmission line, and because the plate tuning capacitor serves the dual functions of resonating the plate circuit _and_ matching the plate circuit to the input of the pi network, variations at the load end require slight adjustments of the plate tuning capacitor. To get maximum power transfer to the load, the pi network must transform the plate impedance to the load impedance, while also resonating the plate circuit. There's more going on than simple impedance matching. 73, "PM" |
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