Dear Doc,
You need to be careful about mica caps. They are used widely in
BC applications in AM service because they are effective below 1.0MHz.
Above 1 MHz., they tend to heat up too fast and drift value
as the KVaR going through them disrupts the molecular
composition. Above 1MHz it's necessary to use capacitors
rated for the voltage and RF amperage (KVaR) that the circuit
will impose.
Ceramic capacitors are manufactured for High Voltage service
and for RF coupling: Different capacitors completely, and
using a HV cap in RF service where KVaR runs through them will
again cause heating and value drift. Case in point: the Emtron
amps use HV caps as tank padders. The old Alphas used '50's
as coupling caps and padder caps and many a beautiful amp was
rendered useless when the wrong caps fried on 160 and 80 service.
Doubling caps to increase voltage or capacity can be a tricky
pastime, too. The old Alphas used a set of two paralleled '50's
as a plate blocker, with silver strapping. This forms a VHF-resonant
tuned
circuit just primed for a parasitic oscillation and onward destruction.
Good capacitors for RF service will carry a KVaR rating. Look at Rich
Measures
web site SOMIS.ORG for understanding how current capacity is important as
well as value. If you're building an amp with thousands of dollars
investment
and hundreds of hours of time, purchasing cheap capacitors will only
serve to
bite you in the tuchas later on.
Fondly,
Hal Mandel
W4HBM
> I am building a switched low pass filter for my full legal
> 160-10M solid state amp (1500W).
>
> I have access to a number of free (from a friend) older surplus
> Mica caps rated at 2,500V (a few are 3KV and 5KV).
> Questions:
>
> 1. Are the Micas likely to have drifted significantly in
> value over time?
>
> 2. Is 2,500V a sufficient rating for this application?
>
> 3. Are series-stacked Ceramic Disk caps of the same value
> (e.g. four 100pF caps for a 4KV rating) a good alternative
> to achieve high power handling capacity?
>
> 4. Are Micas and Ceramics of the same capacitance, voltage
> rating, and tolerance % equivalent components in this app?
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