> >? The MLA commits one of the worst layout errors known --too much
> >distance between the anodes and the Tune-C
> >
> >You have got to be kidding! The distance is 5 inches..
5 inches is a long distance, unless there is no other option.
> >The other day you mentioned Alpha 77SX's using non-RF rated padding
> >caps...I have had several SX's and repaired several. They all use RF
> >rated door knob caps.
>
> ? RF rated caps come in two flavors, for use in tanks (-750ppm) , and for
> bypass use. Bypass caps are not suitable for use in tuned circuits.
Transmitting doorknobs are all RF rated. The temperature
coefficient is a offshoot of the amount of capacitance and voltage
rating.
At about 200 pF and 5 kV a single-layer ceramic capacitor is
forced into the next higher range of N3300. The greater the
capacitance, the higher the temp coef.
Below 200 pF, you can normally purchase N-750 and at very low
values NPO types. None of this has anything to do with the
capacitor being a "bypass type".
> > The trouble is that the 5 kw output is too much for the 160m
> >load padders and they get hot and drift.
>
> ? ... tank circulating current with two 8877s is about 20A. The 160m
> padder cap that is used is rated at 1.4a max @1MHz.
The padder cap gets a portion of the current, and the current rating
increases with frequency. The rating is also only for a given
temperature rise, and can be substantially increased if the
capacitor is mounted to heat-disippating surfaces and if the
capacitor drift is not a problem.
The ratings explanations in the Jennings/HEC catalog cover all of
this.
73, Tom W8JI
w8ji@contesting.com
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