[AMRadio] Capacitor Source


David Knepper cra at floodcity.net
Tue Aug 3 07:10:27 EDT 2004


Jim, this quality and most informative discussion is deserving of serious
consideration by all of us.

Thank you.

Dave, W3ST
Publisher of the Collins Journal
Secretary to the Collins Radio Association
www.collinsra.com
----- Original Message -----
From: "Jim candela" <jcandela at prodigy.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Tuesday, August 03, 2004 9:07 AM
Subject: RE: [AMRadio] Capacitor Source


>
> 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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> >
>
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