[Amps] Parasitcs

[R L Measures]

On Jul 2, 2006, at 1:26 AM, Jan Erik Holm wrote:

> No, it´s interesting.


Indeed, Jan.  To me, one of the more interesting facets of the  
phenomenon is why did we ignore feedback-C for so many years.  Sure,  
C-fb looks insignificantly low, but when Mu and f are high, C-fb's X  
can be in the k-ohms range.  For example, the 3-500Z is rated f is  
110MHz, and a pair of 3-500Zs have 0.3pF of C-fb.. Thus, at the rated  
f, there's 5000-ohms of X between the anode output and the cathode  
input that does not appear on schematics.  Compared to the anode's RL  
of 2000-ohms, as I see it, 5000-ohms feeding back to the input is  
nothing to sneeze at.
   In hindsight, since the Heath SB-220's anode resonance rings at c.  
110MHz** whenever anode-I changes, and it uses a pair of 3-500Zs, why  
should we be surprised that there's enough feedback to cause it to  
occasionally oscillate when anode current abruptly changes?
    Since newer tubes, such as the 8877 have only 0.1pF C-fb, it  
might seem that they would be less prone to oscillate, but the 8877  
has more Mu and over double the f capability.  As a result, at its f  
rating, the X of C-fb is 5500-ohms for an 8877.
** seen with a spectrum analyzer loosely coupled to the anodes.
end
>
> 73 SM2EKM
> ---------
>
> Tom W8JI wrote:
>>
>> and I am not just wasting
>> time.
>>
>> 73 Tom
>>
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org