Black beauties
Brad Thompson, Contributing Technical Editor -- Test & Measurement World, 8/1/2004
Disassemble test instruments or consumer electronics from the late 1950s and chances are you'll encounter one or more "Black Beauty" capacitors. Introduced by Sprague Electric sometime in the 1950s, these capacitors originally featured a molded Bakelite case surrounding a foil/paper-wound capacitor. A filling of oil added through a brass tube later soldered shut around an inserted wire lead (note the solder blobs in the photo) provided extra insulation. Over the years, these capacitors earned a reputation for poor reliability. Traces of acid in the dielectric paper provoke electrical leakage, and the brass filling tube leaks oil. When overloaded, a Black Beauty's case can crack open like a peanut shell.
Later-model Black Beauties didn't include oil-filler tubes, which reduced oil leakage and thus improved reliability. Still, those of us who restore vintage test instruments routinely replace, or "shotgun," every Black Beauty capacitor we find.
So, why do new and used Black Beauty capacitors sell at auction for as much as $4 each?
Some electric-guitar players and audiophiles swear that Black Beauties lend desirable sonic coloration to amplified sound, and when pressed, offer subjective comparisons with modern capacitors. I agree that when used for input or interstage coupling, certain capacitors introduce audible distortion, but not when serving as AC-line bypasses!
Designers of low-performance consumer goods can get away with choosing barrel-of-nails capacitors, but audiophile-grade equipment demands better—or at least idiosyncratic—components.
To investigate capacitor-induced anomalies, prolific Electronics World author Cyril Bateman designed and constructed equipment for ferreting out extremely low levels of distortion (see box below). He explodes several persistent myths, including the following:
- all ceramic capacitors introduce distortion;
- dielectric absorption compresses dynamic range and "smears" audio;
- polypropylene dielectrics are lossy and inefficient;
- capacitors look inductive at audio frequencies;
- a capacitor's ESR (equivalent series resistance) remains fixed at all frequencies.
Space doesn't permit a detailed discussion of Bateman's findings, and alas, he never mentions Black Beauties. Now, if you'll excuse me, I have to go "shotgun" some capacitors.
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