Maurizio Panicara wrote:
>The resonance of SB220/TL922 grid-ground circuit is given by the parallel of
>few mH and few hundred pF, actually that circuit is never really "open" at
>any frequency because of the low Q caused by the consistent Ohmic resistance
>of the long inductor (litz) wire.
Since I own neither of those two amplifiers, I'm only asking a question
here.
>I've also some trouble to understand why a Gg mounted triode with some
>series resistance between grid and ground should have a bigger tendency to
>oscillate at VLF (or any other frequency) than when the grid is straightly
>grounded and gain increased.
As you say, there are two opposing effects and you don't know "a priori"
which one will win at any given frequency. What direct grounding does is
to remove some uncertainty.
>In any case, plate choke (expecially in a VHF/UHF amp) has very low
>reactance at VLF (bypass capacitors have neglectable reactance) so, unless
>you have a PS with high internal resistance, the tube plate is dynamically
>closed on a very low load expecially at DC and VLF.
If the PS has a low internal resistance, that *increases* the Q of any
LF tuned circuits formed by bypass capacitors and external RFCs and lead
inductances. On the other hand, the presence of a surge ("gitch")
resistor effectively kills this resonance.
>VHF/UHF half wave lines don't play any role than any other lead wire inside
>the amplifier.
>
(At the lower frequencies, that is correct - that's what I said.)
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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