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[Amps] Ameritron AL82 @ 17 meters

To: <amps@contesting.com>
Subject: [Amps] Ameritron AL82 @ 17 meters
From: W8JI@contesting.com (Tom Rauch)
Date: Mon, 13 May 2002 09:01:03 -0400
> 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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