As Rich says
? In any event, reducing RF resistance in L1 improves tank efficiency.
Get this in perspective. If you have a 4CX1000 with 2000 volts plate swing
(Peak, not peak to peak) and a 2000ohm plate load with a Q of 10, and Ip
peak of 1 amp, the circulating current in the tank is 10 amps. With a coil
Q of 100, on 20m the reactance is about 175 ohms and the series resistance
1.75 ohms. This gives 175 watts loss in the tank coil. No wonder they get
hot!
If coil Q is 200, the series resistance is .875, and the loss 87.5 watts.
Now take a tube with 4kV plate swing, and Ip peak 0.5 amp. The circulating
current for the same Q is 5 amps. The inductance is about 350 ohms of
reactance, so for Q = 100, R series is 3.5 ohms, and the losses are
87.5watts.
Now look at the losses in the padding capacitor on 160. With a Q of 500 and
a 100ohms of capacitor, R = 0.2 ohms and in the first case, losses are 20
watts.
In the second case (the 4000 ohm load) we may not need the capacitor.
So doubling the Q of the tank coil (NOT the working Q of the tank circuit)
halves the losses, and this is more important proportionally with low plate
load impedances.
All of which says in a lot of words (but hopefully amplifies with
explanation) what Rich said in a few.
And also explains why good amps have big plate coils with high Q.
73
Peter G3RZP
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