>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
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