[Amps] Parasitics

[rlm]

>There's a header that should wake people up!
>
>In the latest QST, the article  on maintenance of linears suggests that
>poor neutralization can lead to parasitics. Not something I've come across
>before, although obviously, it can depend on exactly what you mean by
>parasitics - personally, I tend to think of 'parasitics' being 'VHF
>parasitics' but of course, that's not necessarily the case, although
>arguably the most common.

***   I agree, Peter.  Oscillation at the operating frequency is the 
result of a grid-driven amplifier being out of neutralization.  This is 
clearly not a parasitic because a parasite is defined as regeneration 
beyond the operating frequency range. 
--   G-G amplifiers usually do not oscillate in the operating-frequency 
range because they are self-neutralized there.  The parasite problem 
comes at frequencies above the grounded-grid's self resonamce.  (For a 
3-500Z or 3-400Z, this is c. 90MHz).  One attempted solution in the 1950s 
Was to neutralize the G-G amplifier.  Gonset, and Heath tried 
neutralization at the operating frequency in G-G 811A amplifiers, but 
they discovered that it made the amplifiers more unstable at the 
VHF-parasitic resonance in the anode circuitry.  As a result, they 
stopped producing neutralized G-G amplifiers.  Heath's subsequent models 
were the SB-200 and the SB220, neither of which had neutralization.  //  
The intrinsic problem is that there is no known way to neutralize a G-G 
amplifier, or a grid-driven amplifier, on the frequency of the parasitic 
VHF anode resonance.  The solution that seems to work is to reduce the 
amplifiers VHF gain by using lossier (lower-Q) VHF-suppressors.  The 
tradeoff is c. a 2% power loss at 28 - 29MHz.  
>
>I haven't the article with me here at work, but I don't remember seeing any
>advice about oiling fan/blower motor bearings, which I would have thought
>was likely to be desirable, depending on the fan or blower.
>
***  Amen.  Sleeve/ball/Oilite - bearings (and radio operators), need an 
occasional drink.  

cheerz, Peter