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Original poster: Frank <fxrays@xxxxxxxxxx>
There has been much discussion about the "right" cap for a SGTC or
even a SSTC.
Having been in electronics for the past 45+ yrs and restoring early
electrical and electronic items as well as being taught by various
folks who had been in the electrical/ electronic business since spark
gap days, there is much that has been forgotten about capacitors (or
condensers in the old days).
The advent of solid state has all but rendered HV caps and design
obsolete and much has been forgotten.
What is known is a cap has a certain "Q" and subsequent frequency
range it is effective in. Outside this range, the cap effectively is
an open circuit and has little or no effect on a circuit.
High power circuits of frequencies say from 100 khz to 30 mhz require
not only caps of high voltage ratings but also large surface areas
to handle the RF currents generated.
A RF system can have much higher currents circulating than the DC or
AC voltages present.
A good example of this is the early medical Tesla systems. he AC side
is typically 1 to 5 KV at 1000 to 250 ma yet the RF side of the
machine can generate 5 to 10 amps of RF current!
The caps in these machines is typically a stacked mica sheet type
with copper shim stock as the plates. The typical cap is 3"L X2"W
X1"T. The capacity is low, around .01 to .05 ufd but the current
capacity is very high!
Modern dielectrics have made large value caps in small packages but
they have also made them worthless in large power applications
because the plate area is so small, the cap cannot handle large
currents and will either overheat and/or break down.
Additionally, any dielectric is frequency sensitive and proper
selection is necessary for the appropriate frequency it will be used in.
Tesla discovered this in makes his experiments. Enameled wire,
shellac and varnish were among a few of the common insulators he
found worthless for his coils.
These same materials are commonly used in coil making today.
Tesla found mineral oil, kerosene and the best , beeswax were far
superior in insulating his coils. He even stated a properly designed
and wound coil could be made of bare wire using no insulation what so ever.
Varnished wood or cardboard is a superior coil form over PVC or ABS
pipe. Selection of varnish is also very important.
Today, most folks will use a varathane type varnish which is not an
effective insulator for a TC.
A good tung or linseed oil or marine spar varnish will yield much
better results. An added bonus those finishes are much more resistant
to oxidation (ozone) and UV radiation as compared to the varathane
plastic finishes and will not crack with use.
Now, by far the best cap for a TC is a good old fashioned large sheet
stacked mica cap with at least 0.005" thick metal sheets.
Unfortunately, they are almost impossible to make anymore as the
materials are too expensive. They are voltage limited and need to be
under oil for voltages typically over 5 KV.
The second best are the old mica transmitting caps, fairly easy to
find but also horribly expensive.
So, what next? As the group has found, there is predominantly one
mfgr and style of cap that works for a TC.
Since it is not a HV type, a bank is made of series/ parallel
combinations to obtain the correct value.
This bank has one great impact on the coil, the large number of caps
will yield an effective large plate area to handle the RF currents
generated to make the coil work.
If you fall outside this particular cap, you find the bank overheats
and fails, sometimes rather rapidly.
Example, consider the following caps with identical values, voltage
not important as we will drive them with a signal generator.
0.1 mdf non polarized
0.1 mfd tubular wax foil
0.1 mfd mylar or polystyrene
0.1 mfd disc ceramic
0.1 mfd silvered mica
They are in order as to the best frequency for operation from low to high.
The first two caps also exhibit an inductive effect.
The disc in my favorite for high frequency noise blocks from
induction coils or spikes.
The paper and non polarized are the best for audio to standard IF frequencies.
The mylar is good for most ham band, CB and FM sets.
The last 2 are for uhf and VHF and above RF front ends.
They will not interchange for operations.
The wax caps are still the best for the old tube radios, just cannot
be beat and one of the secrets to the wonderful sound. Unfortunately
they are not made anymore and the "modern" substitutes work but the
sound quality cannot be repeated. The radios sound flat.
I salvage old wax caps always for restorations!
Electrolytics are a DC sensitive cap and have a very limited
frequency range of operation, basically 0 to 20 khz. Higher
frequencies tend to puncture the dielectric and short the cap out.
Their best use is as filters for a DC power supply or DC switching
supply. They will not work at higher frequencies and tend to short
and electrically look like an open circuit to the system while they
are still good.