> -----Original Message-----
> From: Bill, W6WRT [mailto:dezrat1242@yahoo.com]
> Sent: Sunday, August 09, 2009 6:40 PM
> To: Gary Schafer
> Cc: amps@contesting.com
> Subject: Re: [Amps] plate bypass capacitor
>
> 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
I believe that the recommended impedance of the RF choke should be 10 times
the plate load impedance of the amp not the reactance of the plate tune
capacitor.
That is the ideal value but the ideal can not always be obtained when multi
band operation with a single choke is desired. A larger choke will have many
series resonances that fall in the ham bands. When using a small value choke
on the lower frequencies the choke uses enough capacitance to become
parallel resonant.
What about the popular 1.8 to 30 MHz choke that is used in the ameritron
amps and many others. It is 225 uh. It has a reactance on 1.8 MHz of around
2.5 K ohms. That is a long ways from 10 times the load impedance of an amp
with a 2 k plate impedance.
It only takes around 30 pf to resonate that choke on 1.8 MHz. why do you
think that it would not be tuned to resonance no matter if you wanted it to
be or not?
If the choke operated at 2.5 k ohms and the plate tank circuit operated at 2
k ohms wouldn't you think that a significant amount of power would be
consumed by the choke?
The fact is that the choke uses enough of the plate tune capacitance to
parallel resonate itself in the circuit and present a much higher impedance
than it does in the non resonant mode.
The choke gets tuned to resonance no differently than the coils in a pi-L
network do. Each coil shares and uses an appropriate amount of the common
tune capacitor.
73
Gary K4FMX
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