[AMRadio] Capacitor Source


Jim candela jcandela at prodigy.net
Tue Aug 3 08:07:01 EDT 2004


Ron,

	I have found the hard way that replacing tubular bypass capacitors from old
rigs with modern ceramic disks will often change the circuit performance
towards the worse. The average ceramic bypass capacitor has a ceramic
dielectric coded by either a Z5U or Z5V characteristic. These caps offer a
great capacity versus size ratio but that comes at a cost. The dielectric is
often lossy, varies significantly with temperature, and applied voltage.
Their use in critical bypassing, and audio coupling may have adverse
effects.

>From http://www.audience-av.com/on_capacitor_dielectric_material.htm  :

"Based on polarization/dielectric constant considerations for minimization
of dielectric absorption, the best films for audio applications are Teflon
and polypropylene. Runners up would be polyethylene and polystyrene, based
on these considerations alone. Throughout this discussion, I have assumed
that dielectric absorption and the dielectric constant are directly
correlated."


	An example is in my QRO'd Central electronics 20A's is the B+ bypass
capacitor for the 6BA7 mixer circuit. The plate tank coil is high-Q (after
cutting out the parallel resistor), and the bypass capacitor has to "RF
ground" a circuit with high circulating current. Early 20a's used a 0.005 uf
mica bypass capacitor. Since these capacitors sometimes short out I once
arbitrarily replaced it with a nice fresh 0.005 uf Z5U 1 Kv ceramic disk.
The mixer gain dropped by about 3 db, and the output showed some distortion
(AM positive modulation peak compressed). I pulled my hair out trying to
figure out what the heck was going on. In the end I have found that
polypropylene did the best. Currently I have 0.001 uf 630 vdc bypassing in
parallel with RC series bypass consisting of a .5 ohm carbon resistor in
series with a 0.047 uf 630 vdc polypropylene. Yes the ESR of the
polypropylene is lower than a mica, orange drop (715), Z5U ceramic, or
anything else I had. I had to add the series resistor to limit the stage
gain because I was getting a little regeneration without it (on 75 meters).

	For those wanting to use ceramic, the usual sleazy bypass types (Z5U, Z5V)
are fine for non critical bypassing in high impedance circuits. I would
otherwise stay away from these like the plague. The ceramic NPO, or in more
modern times COG are excellent capacitors, but don't usually come in the
higher capacity values; like for bypass applications.

	For me, Mouser has a good selection of Polypropylene capacitors, that are
cheap, and come in a large range of values. I use them for most everything!

Regards,
Jim Candela
WD5JKO



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Edward B Richards
Sent: Monday, August 02, 2004 3:45 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Capacitor Source


Hi Ron;

I have and have been well satisfied. I like the axial leads over
ORANGEDROPS with their radial leads. They are much smaller than the
originals . They even send some spaghetti tubing to go over the leads.
They have been very prompt and friendly. Good people to deal with.

The value numbering system is not unique. It keeps changing over the
years. This one has been in place for a number of years. It is a 3-digit
code similar to resistor color coding. The first two digits are the
significant numbers and the 3rd digit is the number of zeros, expressed
in picofarads. e.g. 503 is 50+3 zeros or 50000 pfd. or .05 mfd. To change
from pfd to mfd move the decimal point 6 places to the left.

73, Ed Richards K6UUZ


On Mon, 2 Aug 2004 14:53:42 EDT Rivpapa1 at aol.com writes:
> Have any of you used the capacitors from the Canadian source
> Justradios? I
> have two receivers I want to recap and am considering this source. I
> understand
> the capacitors have a different value identification numbering
> system...Tnx e
> 73's Ron W6MAU
> ______________________________________________________________
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> Home: http://mailman.qth.net/mailman/listinfo/amradio
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