[Amps] grid resonance

[R L Measures]

On Aug 7, 2006, at 2:09 AM, Ian White GM3SEK wrote:

> Peter Chadwick wrote:
>> Steve said:
>>> In a finite universe I figure there's always going to be some
>> capacitance and/or inductance and/or resistance between the 'open'  
>> ends
>> of a series L and C, so everything is a parallel circuit. The  
>> issue is
>> whether it's significant.<
>>
>> All that does is complicate the circuit. The point I'm trying to get
>> across is that the current induced in the coil produces an EMF in
>> series with the coil. Not in parallel with it - if it was, a non
>> resonant coil in a strong RF field would get hot. So the circuit is a
>> series circuit of the EMF generator, the coil and the capacitor.
>
> Surely the important thing is whether there is a CIRCUIT, in the  
> strict
> literal sense of that word - a closed loop around which current can
> circ-u-late.
>
> Without a closed loop, no current can flow from one part of the  
> circuit
> to another, so there can be no exchange of stored energy between
> electric and magnetic fields, and therefore no possibility of  
> resonance,
> no coupling with the coil of a GDO, and no "dip".
>
> Also the resonance belongs to the whole circuit - not just the single
> component that you're trying to test. This is where the problems  
> begin,
> when the circuit is completed by unknown factors like distributed
> capacitance, or stray inductance in other parts of the loop.
>
> The grid in a "grounded grid" amplifier is a case  in point. It 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.

Well put, Ian.
>
> 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.

It tells me that the grounded grid is no  longer acting as a  
perfectly wondrous shield between the anode and the cathode.
>
>
>> I must admit that I've never had a lot of use for GDO
>
> Me neither, because it's so difficult to understand what a dip  
> actually
> means, except in the very simplest cases.

The dip means that the entire circuit has a resonance at that frequency.
>
> The GDO is deceptively simple instrument. The trick is to tell when it
> stops being simple, and starts being deceptive.

Can a dipmeter indicate resonance when there is none?
>
>
> -- 
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org