I wish to second Bob and also add that Russian TX capacitors are usually
labled not only with max voltage but also with max power they can handle. In
Russian that is printed as XX KBAP which stands for Kili Volt Amp Reactive
power. And XX is the number.
73, Igor UA9CDC
----- Original Message -----
From: "Bob McGraw K4TAX" <[email protected]>
To: "Dave Olean" <[email protected]>; <[email protected]>;
<[email protected]>
Sent: Thursday, January 14, 2016 8:56 PM
Subject: Re: [Elecraft] OT antenna question
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"
<[email protected]>
To: <[email protected]>; <[email protected]>
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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