[ARC5] Capacitor Types

[Richard Knoppow]

On 1/24/2015 8:03 AM, Mark K3MSB wrote:
> Hi Guys
>
> I have this info squirrled away somewhere but can't find it, so I'll
> ask.....
>
> When replacing capacitors I thought it was recommended to use mica for the
> RF sections, and for all sections downstream of the IF (towards the audio)
> to use orange drops.
>
> Is is advisible to use mica or orange drop type caps for IF stages?  I'm
> working on an HQ-129-X and I was planning on using mica for the RF and 1st
> IF stages and my supply of orange drops for all the other stages.
>
> I've typically just used orange drops for everything (as I have them "in
> stock"), but I'm wondering if it's better to go mica for the RF side of the
> house.
>
> Thanks!
>
> Mark K3MSB
>
       Be careful about what you call "orange drop" caps.  The name 
"orange Drop" was originally a trade-mark of Sprague for radial lead 
caps in dipped cases with the same type of dielectric as the infamous 
Black Beauties.  Both are described in Sprague advertising as having 
plastic impregnated paper dielectric.  The BB was an axial lead cap in a 
molded case, filled with oil through one of the end connections, the 
Orange Drop was a radial lead type in a dipped epoxy case.  BB's 
suffered with a manufacturing problem which caused them to fail after a 
rather short time.  There was a second series of similar caps, sold by 
Sprague as TeleCaps, that were evidently much more reliable.  The Orange 
Drop caps seem to have been quite reliable but have the usual life limit 
that applies to all paper caps.
      Modern caps may resemble the old Orange Drop and I think the trade 
name is still used, but plastic dielectric caps are far superior to the 
old paper caps even when the paper was impregnated with plastic.  Modern 
plastic caps are much smaller and have much less parasitic inductance.  
They are probably equivalent to mica caps even at fairly high RF.
      A rough idea of capacitor self resonance can be gotten with a grid 
dip meter.  The leads of the cap are shorted together to make a loop and 
the resonance found.  Since the leads have a certain amount of 
inductance there will always be a resonance. The lead inductance can be 
calculated. Since any internal inductance is in series with the leads 
the resonance will be lower than it would be for the lead inductance 
alone.  I have compared a number of plastic caps with ceramic caps of 
similar value and found that their resonance was either about the same 
or _higher_.  Also, plastic film caps are very stable.  Probably for 
some applications silvered mica caps are superior and I think probably 
stacked mica caps may be for high voltage applications but for the 
general run of by-passing and coupling caps I think modern film caps are 
entirely satisfactory at least up to any frequency where the leads 
themselves do not cause problems.