On Sun, 9 Aug 2009 16:06:53 -0500, "Gary Schafer" <garyschafer@comcast.net>
wrote:
>
>"Normal way opposite reactances in parallel combine in a non-resonant
>manner"? what does that mean?
It means using the standard formula for opposite reactances in parallel. Perhaps
Icon fused you by saying "in a non-resonant manner". I was just emphasizing that
the values under discussion were not resonant. The formula is the same of
course, resonant or not.
>
>Calculate 100 microhenries in parallel with a 75 pf capacitor. I think that
>you will find it to be resonant at around 1.8 MHz.
Correct, but so what? Those values would not be used in a 1.8 MHz amplifier for
the plate RF choke and the tune cap. For a typical high power amp with the tube
load resistance in the 2000 ohm range and a Q of 12, the tune e cap will be
around 470 pF. If you use a "too small" RF choke, the necessary value of the
tune cap goes significantly higher.
The general rule of thumb for RF chokes is they should have a reactance much
higher than the reactance of the rune cap. Certainly never *equal* to the tune
cap. Different designers will use different factors, but a factor of ten is
typical. It can be somewhat more or less.
It should be noted that *any* RF choke used in the conventional pi-net circuit
will necessitate at least a small increase in the value of the tune cap. The
smaller the value of the choke, the more the increase of the tune cap. Since
large HV variable capacitors are expensive, it is much more economical to use
additional wire and make the choke larger in value.
>
>Why would you think that the choke can not be operated in a parallel
>resonant mode? It is no different than placing any other inductor of proper
>value in its place and tuning it to resonance.
If the RF choke is parallel resonant at the operating frequency, the circulating
current in the choke will be very high, an undesirable condition to say the
least. With typical high power plate RF chokes wound with wire in the range of
22-24 gage or so, smoke will ensue.
If for some reason you want the RF choke to be parallel resonant, use wire at
least equal to the size used in the tank circuit itself. Better yet, don't have
it resonant since there is no reason to do so.
73, Bill W6WRT
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