[Amps] grid resonance

[Ian White GM3SEK]

Phil Clements wrote:

>> >Ian,
>> >Are you saying that a GDO can only dip a "circuit?"
>>
>> I'm saying you can't even have an L-C resonance without a circuit -
>> literally a closed, hard-wired loop.
>>
>> For example, if you simply connect an L and a C in series, with opposite
>> ends floating, you don't have a closed loop so you don't have a resonant
>> circuit yet.  Before you can see a resonance and a dip, you must make
>> some other connection that closes the loop and completes the circuit.
>>
>> But the resonance then belongs to the entire circuit you have made - not
>> just the obvious inductor and capacitor, but also all the strays that
>> you don't know about.
>>
>> That requirement for a hard-wired circuit only begins to break down if
>> components are physically large enough to have significant
>> electromagnetic interactions between different parts of themselves -
>> antennas being the obvious example, and large anode chokes being
>> another.
>
>I agree with all of the above. Someone should have said this about 75 posts
>back to enlighten the multitudes of readers on the reflector. About 50% of
>the questions would not have had to be answered.
>
>Now here is the biggie that caused this thread....is the grid on a 3-500Z
>physically large enough to fall into "significant electromagnetic
>interaction" category, or is it in the "hard-wired" category, or none of the
>above?

In between - not long enough to be a significant fraction of a 
wavelength, but end-loaded by capacitance to other parts of the tube.

As I said in the previous posting:

"The grid in a "grounded grid" amplifier is ... behaving as an inductive 
length of wire, terminated inside the tube by some distributed 
capacitance to the anode and the cathode... which in turn are connected 
somehow back to ground. Certainly you have a circuit there, with 
inductance and capacitance in the loop, so it will have a resonance; and 
if you can couple a GDO into it, you'll see a dip.

"But that resonance belongs to the whole circuit, involving the anode 
and cathode and all their associated components. There are too many 
unknowns in that loop to understand what the frequency of the observed 
dip might be telling us about reverse feed-through from the anode back 
to the cathode."




-- 
73 from Ian GM3SEK