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? 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. 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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