[AMPS] SB-220 Smoke Test

[measures]

>
>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


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