[AMPS] Re: Ferrite Rod for 6M Amp

[Tom Rauch]

Hi Rich and all,

While I agree an air core alone is sufficient to make a reasonable 
filament choke on 50 MHz, Rich has a total misunderstanding of 
what creates "distortion" that we can hear in a linear PA.

Not only that, something must have been wrong in Rich's test 
setup. Operation at higher frequencies does not suddenly create 
waveform distortion in a ferrite material. 

If a certain material did create harmonic distortion as Rich wrongly 
claims, the manufacturer would NOT recommend it for EMI 
suppression!! That's because it would create harmonics and create 
EMI through generation of harmonics.

The claim operating 61 material at 14 MHz creates harmonics is  
nonsense. Clipping is a flux density problem, not a frequency 
problem. 
>
> €   In the test I performed with type-61 material operating at 14MHz, the
> slope of the waveform was noticably different from that of a sinewave --
> esp. between 60-degrees to  80-degrees and 100-degrees to 130-degrees.   .
>  My guess is that the distortion was >15%.  

Great "guess". What scope did you use? Maybe it needs repair.
 
> >Quoting from Fair-rite Corp, an actual manufacturer of soft iron 
> >materials, 61 material is suitable in broadband transformers and 
> >EMI/RFI suppression devices up to 200 MHz, and tuned 
> >applications up to 10 MHz. Q at ten MHz is 80.
> >
> € The cathode of a g-g amplifier is akin to a tuned circuit. 

Nonsense. The choke is intentionally designed so its impedance is 
several times the cathode impedance. That means the choke has 
little RF current, low magnetization, and is NOT a part of any 
resonant circuit.

Besides that, the eddy currents (the "thing" that limits upper 
frequency Q) simply add linear loss resistance to the system.

> >> I would consider a filament choke to be in the latter category and if
> >> its inductive reactance is the correct value, its impedance should be
> >> far greater than that of the cathode and therefore not affect the
> >> waveshape a significant amount.>
> >That effect has NOTHING to do with waveshape distortion.
> >
> €  Harmonic distortion has everything to do with sine waveshape 
> distortion.  

That's right. But harmonic distortion has no ill effects in a narrow 
band PA.

People who aren't totally familiar with PA operation sometimes  
confuse harmonic distortion with the type of distortion that causes 
splatter. Splatter is NOT caused by harmonic distortion, splatter is 
caused by amplitude non-linearity in the envelope shape.   

That's the reason a single-ended class B PA is just as linear and 
can produce just as low distortion in a SSB signal as a push-pull 
PA.

If 61 material did cause harmonic distortion (which it doesn't), it 
could STILL be successfully used in a resonant PA. It would be 
useless in broadband applications, and EMI suppression.

There is no other explanation for your results and conclusions.  The 
"test" you made must have been flawed, and your end-conclusions 
make absolutely no sense at all when compared to well published 
data.



 
73, Tom W8JI
w8ji@contesting.com

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