If you examine numerous commercial linear amplifier schematics, you will
find that many use a progressive shorting pi-network switch. An equal
number use a non-shorting pi-network switch.
I begn researching this interesting fact a number of years ago when a friend
damaged the pi-network switch on his SB-220 by turning the bandswitch
instead of the plate tuning control while tuning up. (No, Rich, it was not
a parasitic).
I went through my switch storage facility (junk box) and found a few wafers
that had dual contacts - but none that were progressively shorting. So, the
question was whether or not I needed a progressively shorting switch for a
replacement. I mention this problem, because it is one that every builder
may face.
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. 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. Since I did not have a shorting
switch, I began to question the use of it in the first place - to justify
the installation of a non-shorting switch.
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 found an article in the May,
1963 issue of 73 magazine by Bert Green, W2LPC. Bert wrote the article
while working for Amperex. He made distortion measurements of the amplifier
after construction and stated that "the distortion was decreased even
further by leaving the unused turns on the plate tank coil unshorted when
switching from band to band instead of shorting them as was done when the
amplifier was first constructed".
I installed the replacement wafer with no shorting contacts and had no
problems.
So, the question for home builders is " to short, or not to short?"
73, Colin K7FM
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