More on the plate choke question. I just took a t200-3 core with pretty
high RF losses (good) and an Al of 42.5 nH/T^2 and wound 154 turns on it
to get about 1mH. Because it is a multi layer construction, am I to
assume that distributed capacitance in the winding will resonate with
the inductance in a manner that would come back to bite me? Maybe a
T200-26 core with a much higher Al and HF losses would be better???
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? 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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