[Boatanchors] Disc caps vs paper caps

[Richard Knoppow]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: <jfor at quikus.com>
Cc: "Boatanchors at mailman.qth.net" 
<boatanchors at mailman.qth.net>; "Mark Foltarz" 
<foltarz at rocketmail.com>
Sent: Thursday, September 27, 2012 8:28 AM
Subject: Re: [Boatanchors] Disc caps vs paper caps


I don't have a "golden ear"! However, I can often tell a 
slight difference between using a disc ceramic and other 
types of capacitors in audio coupling circuits. Now my 
ability to hear high frequencies is better than most. Back 
when a number of television remotes used ultrasonic tones 
for control, I could easily hear those tones. That ability 
may have some effect in being able to tell a slight 
difference.

However, I definitely agree that most people cannot tell any 
difference and, for all practical purposes, replacing audio 
coupling capacitors with disc ceramics will have no apparent 
effect on the quality of the audio. This is especially true 
of "communications" audio which is generally limited to the 
300 Hz to 3000 Hz segment.

As for the snubbing capacitors: Back when vibrators were in 
widespread use, every new vibrator came with replacement 
snubbing capacitors. If the capacitors were not replaced, 
the warranty on the vibrator was void. I believe that this 
was because failure of the snubbing capacitors played a 
significant role in the fact that the vibrator needed to be 
replaced.

Glen, K9STH

     I used to have hearing like that, ultrasonic sensors 
and burglar alarms drove me crazy, like a knitting needle 
stuck through my ears, TV flybacks were also almost painful. 
Can't hear much of anything now and certainly am hors de 
combat as far as judging Hi-Fi.  I think the problems with 
ceramic caps depends on the type of ceramic although all 
share the general effects.  The higher the K or dielectric 
constant the poorer the cap is from the standpoint of 
stability and other factors.  For surface mount components, 
where size is of paramount importance, high-K caps are 
necessary but for replacing paper caps its of no importance 
because even relatively large low-K ceramics are smaller 
than the originals.  Because of their relatively small size 
ceramic caps work very well for RF purposes and are nearly 
as stable as silvered micas.  For some purposes plastic film 
caps are better.  Curiously enough paper caps have very good 
dielectric storage properties so are good for timing 
circuits.
    Evidently the problems with paper dielectric were well 
known long ago.  There were derating charts showing the 
expected life of a capacitor for applied voltage vs: rated 
voltage.  I think this is why one finds 600V caps in places 
where they never see more than a couple of hundred volts 
even with surges. I don't know for certain what the failure 
mode was but certainly moisture was part of the problem. 
Oil or wax impregnation was supposed to prevent moisture 
absorption as were the molded plastic cases found on later 
caps.  They may have worked for this but impurities in the 
paper itself was probably the main cause of failure.  In any 
case, plastic film and ceramic caps do not have these 
problems.
     BTW, my HQ-129-X is filled with caps made by Astron in 
molded green plastic cases and paper labels.  I was going to 
replace all but tested a few and found them as good as new 
plastic caps!


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com