[Amps] Plate choke magic?

[R.Measures]

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.

> 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.

> 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