First I'd say 680 pf , two parallel, is not at all adequate for a 160M
KW amp. More like 4 would be expected for about 2800 pf total required
for 160M depending on plate Z. Also with 3200 volts or so, again
depending on the plate Z, 7.5 KV again is not much margin of safety.
Ep x 4 is about right.
Just my comments and data from a Pi L calculator.
73
Bob, K4TAX
On 1/14/2016 9:44 AM, Dave Olean wrote:
I bought a bunch of those Russian TX caps. Mine were 680 pf and looked
like the 7.5 KV Centralab ceramic TX caps with threaded holes on each
end. I put two in parallel and used them for the plate blocking
capacitor in a 160 meter KW triode amplifier. Big mistake. They cannot
handle much current at all. When I keyed the amp, I saw the power
output drop very fast. Those caps got very hot and failed miserably. I
complained to the eBay seller and he advised that they were only for
use as bypass caps and should not be used directly in the TX line.
(Silly me!) Their heating cycle reminded me of what happened with
Vic's antenna system of late.
I still have them. They might make good target practice subjects.
Buyer beware.
K1WHS
----- Original Message ----- From: "Bob McGraw K4TAX"
<rmcg...@blomand.net>
To: <wb5...@centurytel.net>; <elecraft@mailman.qth.net>
Sent: Thursday, January 14, 2016 2:47 PM
Subject: Re: [Elecraft] OT antenna question
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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