[Amps] Plate choke magic?

[Tomm Aldridge]

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 at 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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