to the group:
any comments on choke such as the RFC-3 offered by RF Parts
with 225 uH 1.5A & 4 kV rating ??
any bands to avoid with this one or close by ??
73 W5AJ
----- Original Message -----
From: "Tomm Aldridge" <KD7QAE@ARRL.NET>
To: "Tomm Aldridge" <KD7QAE@arrl.net>
Cc: <Amps@contesting.com>; "Will Matney" <craxd1@ezwv.com>
Sent: Saturday, January 29, 2005 2:49 AM
Subject: Re: [Amps] Plate choke magic?
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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>>
>>
>>
>
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