This is good advice. Ceramic capacitors can have very low loss - it
depends on the dielectric used. Smaller value NPO (zero temperature
coefficient) types tend to have the lowest loss. Large value X7R, Z5U,
etc. types have very high loss and are generally unsuitable for any
high-current RF application.
The type of doorknob ceramic caps that you can find in old tube-type TV
sets are the high-loss type. OK for bypass capacitors but not for a tuner.
Using several capacitors in parallel to split the current is a good
idea. It also reduces the stray inductance.
Alan N1AL
On 01/14/2016 06:47 AM, Bob McGraw K4TAX wrote:
When I updated my 2 tuners, I used some doorknob style capacitors, of
Russian source I presume, which I purchased via E-Bay. Something like 5
for $20. These were 470pf units rated at 16KV. Don't be mislead
with the voltage rating, as it the current handling ability being the
key for RF usage.
In most all cases one will need to provide a somewhat different mounting
as they have screw terminals on either side and are larger physically. I
just happen to have one in the desk drawer. Typically 1.0" dia x 0.75"
tall with, guessing, #6 or #8 screw terminals on either side. Could be
Metric screws as I don't recall and can't tell without my thread gauge.
Another approach is to divide the total current by using 3 or 4 caps in
parallel, if you wish to use disk ceramic caps. Thus if your circuit
requires 330 pf then use 3 pcs of 100 pf or 3 pcs of 120 pf in parallel
and keep the voltage rating the same as the originals. This way the
total current is divided between 3 devices as opposed to one device.
This also works.
In various transmitter updates, I have used dipped silver mica caps.
These work well. Just keep the voltage rating the same as the
originals. And it won't hurt to use 2 or 3 units parallel by taking the
original capacity value and dividing it by 2 or 3 for the pf value.
This approach was used in restoration of some of my old boat anchor AM
transmitters. These caps are more stable, meaning less tuning drift,
thus work much better than the original disk ceramics units.
73
Bob, K4TAX
On 1/14/2016 8:27 AM, Al Gulseth wrote:
Bob,
Are silver(ed)/dipped micas (CDE etc.) the preferred type for a tuner,
or is
there a superior newer technology available?
TNX/73, Al
On Wed January 13 2016 2:52:52 pm Bob McGraw K4TAX wrote:
If your tuner uses disk ceramic caps, as many do, these can be
heating thus
not being able to handle the RF current. They heat and cool and change
value which in turn changes tuning.
Replacing them with suitable RF current rated units is the solution.
Bob, K4TAX
Sent from my iPhone
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