[Amps] Plate choke magic?

[Will Matney]

On Sat, 29 Jan 2005 11:55:13 -0500, Will Matney <craxd1 at ezwv.com> wrote:

> On Sat, 29 Jan 2005 03:01:02 -0800, R.Measures <r at somis.org> wrote:
>
>>
>> On Jan 29, 2005, at 12:22 AM, 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?
>>
>> Parking a choke resonance on WWV is okay.
>
Done all the time!
>
>>
>>> 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.
>>
>> An effective HV-RFC is one that does not catch fire on any operating  
>> freq., and does not incinerate the HV bypass caps on the lowest  
>> operating freq.
>
Perzactly. But you forgot one other function, you see the power going
out to the load and not back to the transformer :) I know, I couldn't
resist it. Where my back is aching like he## today (where I once broke
it), I had to say something to get some fun out of it..LOL!
>
>>
>>> 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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>>>
>>
>> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>>
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>
>
>

Will

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