>
>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.
>
Direct grounding:
1. slightly lowers the frequency of grid resonance.
2. makes virtually no detectable difference on the air or on the
wattmeter.
3. removes the grid fusing mod.
>>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.
... defending the VLF oscillation theory carries a high price.
>
end
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|