On Fri, 12 Mar 1999 08:50:00 -0000 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>
> >>Any problem using 500 volt ceramic disc caps for the larger
>values
>and
>>>silver mica for the smaller ones
>
>
>I'd worry about the current rating of disc ceramics. If you have 100
>watts
>in 50 ohms, that's 70 volts rms. The reactance of 3300 pF at 1.8 MHz
>is
>about 27 ohms, so there's getting on for 3 amps in that capacitor. If
>you
>don't run contests, SSTV, RTTY, or AM, you may well get away with it.
>Doorknob ceramics should be OK; I'd be a bit dubious about using
>silver
>micas. It depends on whether you want to push ratings in the good ole'
>ham
>tradition, or go conservative for reliability - a choice that gets a
>bit
>like the transformer discussion we had earlier on this week. The big
>mica
>compression trimmers should be OK, though, even CCS AM.
I have never had to replace an input network silver mica in a commercial
amp that was driven with <200W. Case size is important, the CD15 and
CD19 500V units work very well. The smaller CD10 is not adequate. Using
multiple caps on the lowest bands will spread the current load if a
150-200W type exciter is used.
ARCO 42xx series trimmers are also just fine.
73 Carl KM1H
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