[Amps] door knob capacitors

[Jim Thomson]

Date: Sat, 5 Mar 2011 13:39:13 -0600
From: Rob Atkinson <ranchorobbo at 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