On Jan 29, 2005, at 12:22 AM, Tomm Aldridge wrote:
Thanks Will, that makes perfect and practical sense. Plate choke
valuse for 1.8 through 30MHz amplifiers need to be high enough to
present a large reactance at 1.8MHz with large defined as >> greater
than the plate impedance, correct? And they must not produce any
resonances withing the 1.8 to 30MHz band, correct?
Parking a choke resonance on WWV is okay.
But, what about the fact that the tubes have gain well above 30MHz and
well below it as well?
If I say that the plate Z is 2k ohms and therefore I want 20k ohms at
1.8MHz to satisfy the >> larger condition above, I get an inductance
of 1.77mH. Looking at some plate chokes for QRO amps out there, I see
values in the range of 200uH (a bit greater than the plate Z) to
500uH, much lower than I would consider to be an effective choke.
An effective HV-RFC is one that does not catch fire on any operating
freq., and does not incinerate the HV bypass caps on the lowest
operating freq.
A 300uH choke is approx 1" x 6" with 278 turns of 26AWG. Seems to be
a reasonable DCR to be putting in a plate circuit. I calculate about
2.9 DC ohms with a large surface to distribute the losses. But why
such a small inductance???
Tomm
Will Matney wrote:
Tom,
Actually, a ferrite core can be used if it's of the correct type of
material. The material is determined by the frequency that the coil
will operate at. There a couple of ferrite and iron powder types
that would work. The reason most are air coils I would think is they
are cheaper to make. An insulated form is all that's really needed.
The air coil formula is then used to determine the number of turns
for the amount of inductance wanted. The higher the frequency, the
lesser amount of inductance is needed to block the RF, so the choke
needs to be designed around the lesser frequency that will be
encountered. Then you need to make sure the choke is not
self-resonant at any frequency you wish to operate it on. This is
done by using a grid dip meter and shorting the coils leads
together. Any dip at any desired frequency means that the inductance
will have to be changed slightly to move the resonance point to
where it wont be encountered. Most of the time this is done by
simply adding or shortening a few turns of wire. Those staggered
windings on some chokes are done to stop self-resonance at a
particular operating frequency, and are really several inductors
being connected in series where Ltotal = L1 + L2 + L3, etc.. Hope
this helps as an explanation.
Will
On Fri, 28 Jan 2005 23:42:10 +0000, Tomm Aldridge <KD7QAE@ARRL.NET>
wrote:
Why are plate chokes seemingly black magic? Don't you just want a
good decoupling of the PS from the Plate; i.e. lots of impedance
from DC to Light and no resonances? How I get that should not be
an issue but all teh plate chokes I see are long skinny and
sometimes segmented single layer solenoids of questionable wire
size. Why wouldn't a really lossy powdered metal toroid with a few
fat turns on it work, assuming the inductance was high enough?
KD7QAE
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