[ARC5] Hello Smart People: Output Network "L"

[Dennis DuVall]

Ran into the same thing in a 3-1000Z linear I built a while back,  
wouldn't dip or load on 15M.  Some kind of a parasitic resonance in  
the 160 - 10M taped PI network tank circuit.  Hung a 25pf 5KV mica to  
ground halfway down the total tank inductnce and solved the problem.   
Did change the tuning slightly on 160, 75/80 and 40M.

Dennis D.  W7QHO
Glendale, CA

*****************
On Apr 15, 2012, at 7:58 AM, Ian Wilson wrote:

> I've seen similarly puzzling effects with an 813 in a pi network
> using an inductor with switched taps. The thing would dip weakly and
> not produce expected output. Changing to another tap close by would
> make more change than I would expect.
>
> I have tried taking all the power away and putting a resistor to  
> ground
> in place of the tube, then looking into the pi network with an antenna
> analyzer. These results also indicated that something was not right.
> Replacing the RF choke made a noticeable difference, and at the time
> I thought there was a resonance.
>
> On reflection, however, I suspect that what is happening here is that
> there is a grounding problem and/or a stray capacitive or inductive  
> path.
> Changing the pi network coil slightly changes the phase shift around  
> the
> network and this in turn combines with the stray to produce odd  
> effects.
>
> This sort of effect would be cancelled out by neutralization although
> unless it's a pure capacitance that would only be effective over a  
> small
> frequency range.
>
> This is still a theory, but since the observed effects can't arise  
> from a
> simple pi network, there have to be other L/C in the circuit  
> somewhere.
>
> 73, ian K3IMW
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