Date: Sat, 5 Mar 2011 13:39:13 -0600
From: Rob Atkinson <ranchorobbo@gmail.com>
Subject: Re: [Amps] door knob capacitors
I'm looking at those doorknob caps for sale from Ameritron. Actually
they are listed as MFJ but it's all the same now.
They have two types:
500 pF 7.5 KV and 170 pF 7.5 KV. They are made by High Energy (type
HE50) and sell for $22 each from MFJ. I wanted more information
about their current rating v. frequency so I went to the HE website
and found this data page:
### No bargains there. The HT 50/58 are $9.00 to $22.00 each from SSON.
You can get an even
better price right from Richardson's.
http://highenergycorp.thomasnet-navigator.com/viewitems/ht50-ht58-series-ceramic-capacitors/ht50-series-ceramic-capacitors?
For the 170 pF cap the fL and fH give me some confidence but as value
goes up, frequency range lessens so one has to de-rate the current
capability for the high-bands. but for load cap padders for a ham amp
even the 500 pF is probably okay at 30 MHz. I'm still sticking with
what I have on hand for now though, my 2 buck micas because I am a
cheap ham.
### The problem with ceramic doorknob TX caps is,.,,., as freq increases,
the RF current rating decreases.
Mica caps are exactly the opposite.
## The 500 pf current rating at 30 mhz is higher than at 160m... a LOT
higher. It's a moot point since you are NOT
padding any load cap on 10m anyway ! The preferred method is to use the
biggest load cap you can get your hands on...
then the least amount of padding will be required..and usually only on
160m...and maybe 80m as well.
## Alpha tried using a single 1000 pf @ 5 kv ceramic doorknob cap as a
padding cap on their load cap..on their 77-dx/sx.
The 1000 pf cap has a lousy temp coeffient..and will drift badly. It's also
only good for 1A of RF.. at which point it runs warm..
and drifts, The 'fix' was to replace with a pair of 500 pf ceramic
doorknob caps. The 500 pf units are NOT NPO type... will still
drift, but no where near as bad as the 1000 pf units. The kicker is..the 500
pf units are good for 3 A on 160m... and with 2 of em in
parallel, that's a total of 6A.... which is 600% higher than the single
1000 pf case. The end result is...no more drift. Imo, depending on
the application, and how much of the total current the padders have to
handle..VS the main loading variable cap, it would be better to use
smaller vlaue caps, like the 170-300 pf variety..and more of them. I think
the biggest NPO cap you can get in the ht-50/58 series [ in either
5/7.5 kv] is 170 pf.
Later... Jim VE7RF
Rob
K5UJ
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