Hi Sasha:
The terminology of shorting v. non-shorting switch may not be correct. The
"non-shorting" amplifiers do short the output circuit. The entire pi-nework
coil is connected one side to the plate circuit (with a coupling cap) and
the other side to the output. Then a band switch rotor shorts out the
unused turns. So, it really is a shorting bandswitch since neither end of
the coil is left unterminated. in that respect, the tesla coil effect of a
step transformer would not occur. I was trying to distinguish this circuit
from a similar circuit that is varied by using a band switch that
progressively shorts each coil tap that is "outside" of the operating pi
section.
Therefore the questions is shorting v. progressive shorting switches. Both
configurations are used in commercial amplifiers.
I think Tom pointed out that the problem is that when the amplifier is used
on 10 or 15 meters, the remaining inductor could end up being a back to back
L network at some frequency causing high voltage somewhere in the coil. As
I understand it, there would never be an arc in the coil itself, because the
voltage between turns would not be high enough to arc. But, if one of the
high voltage points occurred at the switch contacts with an adjacent switch
contact or a grounded point, the voltage may exceed the voltage breakdown of
the switch. By using a progressively shorting switch, this problem would
not occur.
Most home built amps use large switches that have a high enough voltage
rating to prevent any problem - so any switch could be used. The question
may be more difficult to answer in the SB-220. It used a receiving type
band switch wafer. So, by using a non-progressive shorting switch to
replace the original progressive shorting switch, the question arose whether
the voltage rating of the switch would be exceeded. I did replace the
original switch (which had failed) with a non progressive shorting switch
and it has worked fine. The answer may be that I was lucky.
For commercial manufacturers, the question may be more important than for
homebrewers. For Heath to have used a larger bandswitch may have been tens
of dollars increase in the cost of manufacture and could have represented
the difference between making a profit or losing money.
In answer to your question about the Pi-L, we still may be talking about
terminology and what is a non-shorting switch. I think most Pi-L
configurations I have seen in practice have a shorting - but not
progressively shorting Pi switch and then a shorting but not progressively
shorting Pi-L switch. Since that configuration has more total coil, and
depending upon where the switched turns, there may be a greater possibility
of a high voltage point occurring at one of the switch contacts.
Otherwise, you are correct that the volts per turn in the Pi section would
be less than in the Pi-L.
For homebuilders, it is simple. Use a big bandswitch with lots of voltage
and current and it will not fail. For commercial manufacturers, either you
need to put a lot more engineering into the design or take the Microsoft
approach and design an amplifier quickly and put it out into the field to
see what fails. After the failure points are determined, come out with an
improved model for more money to solve the original weak points. And good
engineering will then build newer problems into the "improved" amp requiring
an ever further improved model.
It is reassuring to know that at least Dick would never sell Alpha Power to
Microsoft!
73, Colin
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