[Boatanchors] And Now for Something Competely Different: Zenith

[J. Forster]

On old radios and capacitors:

Audiophools hit DELETE now.

Some people think that old capacitors have unique 'sound' that is somehow
absent from nerer components. IMO, this is BS.

A real capacitor can be represented by a model like this:

         C     Rs
  o--o---||---xxx---o--o
     |------xxx-----|
             Rp

An ideal capacitor, in series with a resistor (the ESR) and the sewries
combination paralleled by Rp a shunt resistor.

There is no reason to believe that Rs is anything but linear and Ohmic.

In an Ideal capacitor, Rp would be oo ,  but not so in fact. The Rp may be
a function of temperature, and possibly slowly changing voltage, but there
is no reason to believe it is variable on a mS time scale.

C is Epsilon*a/d. There is no reason to believe Epsilon is a variable. A
is pretty much fixed by geometry. The d can vary and deoends on the
compliance of the material separating the plates. However, there is
nothing to suggest that the compliance has changed over time. Taken
together, the C-V curve has probably not changed since the part was made.
(Note, that some ceramic capacitors, hi C & low V are significantly
non-linear).

Bottom line, I can easily believe that changed capacitor leakage, Rp,
could influence the bias of tubes, but nothing else in a capacitor will
alter the 'sound'

YMMV,

-John

=====================





>
> ----- Original Message -----
> From: "Joe Connor" <joeconnor53 at yahoo.com>
> To: "Glen Zook" <gzook at yahoo.com>; "Charlie , W5COV"
> <cvest at cox.net>; "David Stinson" <arc5 at ix.netcom.com>;
> <boatanchors at mailman.qth.net>
> Sent: Saturday, March 30, 2013 11:02 AM
> Subject: Re: [Boatanchors] And Now for Something Competely
> Different: Zenith
>
>
> The quality and sound from these radios is pretty amazing.
> My theory is that after we restore them, they perform better
> than they have since they left the factory.
>
> Here's why: back in the '30s, capacitors were a high-failure
> item even when new. A lot failed quickly and a lot more
> probably suffered leakage even then. A radio with a dozen or
> so caps that are leaky to one degree or another isn't going
> to sound as good as it should. It's sort of a cumulative
> degradation. If there was one particular cap that shorted or
> caused a noticeable problem, the repairman would replace it
> but I've never heard of any repairman back in the day
> replacing all the paper caps. It would probably have cost
> more than a new set. Instead, the set was written off as
> "old" and "tired" and relegated to a dusty attic.
>
> Now, we come in and replace all the caps with high-quality
> modern caps. We replace the out-of-spec resistors and align
> the IFs with precision (thank you, frequency counter). The
> end result is that the radio performs the way it was
> designed to perform, probably for the first time since it
> left the factory.
>
>
> Joe Connor
>
>
>      It would be interesting to find any actual measurements
> on paper caps from that time. I suspect the dissipation
> factor or ESR would have been higher than on modern film
> caps where its so low as to be difficult to measure. Mica
> caps were used where low ESR was necessary.  The old
> handbooks say that paper caps degraded from applied voltage
> and for long life the rated voltage had to be very
> considerably higher than the applied voltage. I think that's
> why you often find 400V caps in applications where perhaps
> no more than 200 volts is across it.  The plastic
> impregnated caps which began to become available after WW-2
> such as the notorious Black Beauty caps were supposed to
> have much longer life than untreated paper. So were
> oil-filled caps.  The BB's should not be used to judge the
> average performance of caps of this type because I think
> they were defective in manufacture.  I found that the flat
> Micamold caps have a similar looking dielectric and were
> much longer lived.  Most of the old caps were impregnated
> with wax to seal out moisture. It must have been fairly
> effective since we are replacing caps that are often sixty
> or seventy years old.  I've measured many modern plastic
> caps using a General Radio bridge to measure dissipation
> factor and various methods to measure parallel resistance,
> AKA, leakage, both are so low that its hard to even detect
> them.
>     I think around the 1960s that manufacturer's wanted
> something better than the usual paper caps so we began to
> see both plastic impregnated paper and ceramic caps being
> used.  Ceramic caps are very long lived but those with high
> dielectric constant material are not stable and can also
> degrade with time. Low K caps, like the zero temperature
> coefficient type, are extremely stable and can be used for
> virtually any application but tend to be expensive and are
> too large for modern surface mount use.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
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