"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"