Hi yall, I believe that a switch that shorts out the unused taps on the tank
cct.
is the best way to go. The shorted coil in the tank next to the used tank
portion
will create a field that forces more of the circulating current to stay in the
used
tank portion, sort of like a shield. It also minimizes the Tesla effect on the
higher bands and keeps the voltage down on the unused tank portion. I built the
8877 amp using Jerry's K8RA design which uses an air coil for 10M-40M and
toriods
on 80M and 160M. I had an arcing problem on 20M. I thought it was between the
contacts of the band switch (Radio Switch model 88 non continuos shorting) but
after closer examination I found it was arcing from the 40M tap on the switch to
ground. That contact was at the bottom of the switch which made it difficult to
see
and looking at rf arcs is not the best thing for your eyes. The gap from the
switch
to ground is 3/4" so you can imagine the voltage present. I did some snooping
using
a NE2 neon bulb on a stick and found high current or voltage on the 80M toriod.
I
changed the phase on the 80M toriod and the arcing went away. Amps like the
Commander use a continuos shorting switch which I believe helps clean up the
signal
by forcing more current to stay in the used tank coil. This just my opinion and
some practical experience.
gl es 73, Steve
Bob & Linda McGraw K4TAX wrote:
> Phil: I agree.
>
> Anyway, as I understand things, it's best to short the un-used turns out
> to prevent a high EMF on un-terminated taps and thus causing a
> flash-over or arc in the bandswitch. At the same time, I figured out
> that by doing so, shorting the turns, this created a "shorted turn"
> situation and some lost energy and heating would result. I guess it
> makes a difference which end is shorted. Short from the output to the
> input side of the coil or from the plate end toward the output.
>
> I'm sure someone can straighten me out on this.
>
> 73
> Bob K4TAX
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