[AMPS] Shorted Pi-Network Turns

[Phil Clements]

> >I believe the purported reason for using a progressive shorting switch is to
> >reduce or eliminate the tesla coil effect of having the unused portion of
> >the coil step up the voltage.
>
> I Agree.  The safest way to remove L is to short it out.
>
> >In certain cases, an extreme high voltage
> >could be developed.  In the Heath, something was always connected to each
> >end so that effect could not happen.  ...
>
> Amen
> >
> >The progressive shorting of the coil presents problems for the amplifier.
> >In particular, shorted turns will act like a shorted turn of a transformer
> >and can lead to large circulating currents.
>
> I do not believe that this problem exists in the Heath SB-220.  Perhaps
> the reason why is that the two sections of L in the tank are not
> electromagnetically coupled to each other.

The more bands covered, the greater the risk of the "Tesla effect."
I have mentioned my experiences in past posts. Basically, when a
160 meter coil and/or WARC band taps are added, fireworks can start.
Several construction projects in ARRL Handbooks and others used band
switches with a single wiper to cover 6-9 bands. I have personally destroyed
several 20-30 amp switches with only a single 8877! The problem seems to
be worse with a Pi-L. If you look at modern amps, you will see that the band
switches have multiple wipers that progressively short out 3-4 contacts  at a
time, leaving no coil taps "floating" between the selected position and C-tune.
This reduces Mr. Tesla to nil.

The problem in the Heath SB-1000  switch arcing IMO
is due to the layout and the Tesla effect.  If the 80-160 meter tank coil had been
wound on a torroid, the problem would have been solved. Just too much
coil in too little space. As the inadequate padder door knobs change value due
to heat, the problem will raise its ugly head by burning out the band switch. The
addition of the washer on the switch contact is not enough to overcome the
inevitable when values start to change in the padders. Ameritron has solved
this problem by using robust mica caps as padders, which fit nicely into the
SB-1000.

IMO,  the Tesla effect is the culprit in arcing switches.

(((73)))
Phil, K5PC





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