On 4/11/2013 2:37 PM, Robert Mcgraw wrote:
BINGO on difference! Do check the RF current rating for the caps of
choice. It is not the voltage that frys them, it is the heating from RF
current that makes them go pop!
Yep."Equivalent Series Resistance" (ESR) is a very important
specification for a capacitor that carries significant AC current, and
in a capacitor anywhere in that output stage is going carry a lot of
current. ESR is exactly what it says -- an imaginary resistor in series
with the capacitor that, when carrying the current that is present,
accurately predicts the power lost (and heat produced) in the capacitor.
One of the reasons those caps are cheap may be that their ESR is high.
We still have a nice, old-fashioned electronic surplus store in Silicon
Valley that's a big warehouse full of all sorts of electronic
components. When I first moved here, I needed caps to rebuild my Ten Tec
tuners, and to use in antenna matching networks. I bought a good
selection of HV caps. No specs are listed, of course, only voltage
ratings and cap values, but at average costs of 5-10 cents on the
dollar, you can afford to stock up and smoke test them to find the ones
that can handle current.
I stuck them in series with the feedpoint of my 160M vertical, ran legal
limit to it for a few minutes, then went outside to check for heat (and
smoke). It didn't take long to find the bad ones. :)
73, Jim K9YC
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