<The Alpha 78 manual states "A load VSWR of 2:1 or better is required for
<safe manually-tuned operation of your Alpha 78. For safe and efficient
<operation in the bandpass (no-tune-up) mode, a load VSWR of 1.5:1 or
<better is desirable". The fixed tuned "bandpass" mode preset capacitors
<in the Alpha 78 are pretty small compared to the variable tune and load
<capacitors used for manually tuning mode. Also, the 8874 tubes used in
<the Alpha 78 are sensitive to grid overcurrent. Both of these things may
<factor into the more limited VSWR range for "bandpass" mode. It may be
<possible to accommodate a VSWR range greater than 1.5:1 in a "bandpass"
<mode amplifier with more beefy tank components and more forgiving tubes.
<73, Mike W4EF.............
Points well taken. The only possible ....'fix' to increase VSWR range
would be to lower the loaded tank Q slightly, on all bands. (more L, less
tune + load C). That method works on all bands....except upper HF like
10m..where min C will be an issue. You end up with slightly less harmonic
suppression, but that is not high on my list of priorities...esp if using
monoband ants. Even my F12 40m loaded yagi doesn't resonate on 15m.....
it resonates on 12m (24 mhz)...but I don't operate 12m. IE: swr on both
even and odd harmonics is sky high..and typ, that will result in another
9-13 db of harmonic suppression.
Jim VE7RF
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