[Amps] Plate choke magic?

[Tomm Aldridge]

This guy is my god now, mainly because he agrees with me on Plate 
Chokes:  http://www.qsl.net/kk5dr/ampbuilders.htm check out his treatise 
on plate chokes.

Tomm Aldridge wrote:
> 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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