> Sounds like a neat trick, Tom, but I am not quite clear on the
> connection between the light bulb and the T connector. Is the
> lightbulb connected between one leg of the T connector and chassis
> ground so that the bulb is lightly coupled to the choke via mutual
> coupling, or do you actually connect the top of choke in series
> between one leg of the lightbulb and ground?
One connection goes directly through the T to the load.
The T is ground directly to the chassis as close to the top of the
choke as possible.
A small low current bulb with wire leads connects between the open
port of the T and the top of the choke.
Everything else is left in place as close as is reasonably possible,
including sheet metal, except all connections between the top of the
choke and the rest of the system. Only the lamp (or an impedance
meter) connects there.
The amp is cold. The tank is set below or above the test frequency
range so it does not act like a loosely coupled high-Q "suck-out"
trap.
You just sweep frequency slowly while keeping power low and any
inadequacies in the coke show up as an increase in brightness. If you
substitute a resistor for the choke you can get a very rough idea of
the impedance, but that isn't really a necessary thing. At series
resonances the bulb will glow very very brightly even with low
exciter power.
Now if you move the tip a screwdriver up and down the choke, you
will find a point most sensitive to the tip being near the
screwdriver. This is where voltage is highest, and where if you add
capacitance or remove turns (to reduce capacitance at the that area)
choke series resonant frequency will change the largest amount.
The amount of inductance overall change can be quite small, but the
removal of the tiniest amount of wire (reducing stray
capacitance) can move the resonance quite a bit. That's because the
impedance at that point is extremely high, with the choke acting like
two high-L low-C back-to-back L-networks. That's what causes the
series-resonance, and that's how you can find where to remove turns
to park unwanted resonances outside desired frequencies.
73, Tom W8JI
W8JI@contesting.com
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