[Amps] Plate choke magic?

Sat Jan 29 05:48:15 EST 2005

"A full-wave bridge rectifier configuration is the only way to go on a 
quality amp"

- - For RTTY-broadcasting, yes.  However, since the duty-cycle for SSB 
is only c. 30%, a high-quality SSB linear amplifier can utilize a 
full-wave voltage doubler / C-filter.

Also:
1.  The diagram showing "equalizer" resistors and capacitors across 
seriesed rectifiers is now widely recognized as not good engineering 
practice.
2.  The use of Delrin® in HV-RFCs is questionable since it has a medium 
D-factor.  ABS, G-10, or G-11 would be better.

On Jan 29, 2005, at 12:56 AM, Tomm Aldridge wrote:

> 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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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org