[R-390] Black Beauties

[Drew Papanek]

Kenneth Crips wrote:

>I have a question about the so called black beauty, red stripes, etc.
>As I understand it these where made by Sprague. I have a bunch of these
>capacitors that are brand new unused.  Do these things deteriorate while
>in use over time, or by age regardless.  I would love to use them but
>if they are always bad I suppose it's a waste of time.

Yes, these Black Beauties are bad by definition.  I have a few NOS Black 
Beauties of Death and they test leaky.

Leakage testing using a milliammeter in series with a high voltage power 
supply will certainly work, but a milliammeter is not sensitive enough to 
detect leakage current levels which would  be detrimental in vacuum tube 
circuits.   If the capacitor under test has extremely high leakage or shorts 
out, destruction of the milliammeter is certain.

A more sensitive and robust test is to use your VTVM or DVM set to the volts 
scale, in series with cap under test and a power supply of appropriate 
voltage (preferably the cap's rated voltage).  The meter serves as a 
sensitive microammeter with a built-in current limiting resistance.  To 
calculate a cap's leakage tested in this way, divide meter reading by 
meter's input resistance (typically 11 meg).  A typical BBOD might show 100v 
reading with a 300v power supply.  This works out to 9 uA leakage (with 200v 
across cap).  If you want to get really picky, adjust the supply voltage so 
that the difference between it and the metereading equals cap's rated 
voltage.

Heat the capacitor slightly (hairdryer) and watch leakage current soar.  
Shows what happens as the radio gets hot.

Modern plastic dielectric caps such as the higly esteemed Orange Drop will 
show no discernible reading, just a fluctuation of a few tenths of a volt 
positive and negative (power supply noise).

Believe it or not, I have read banter in some of the AudioPhool forums 
extolling the "sonic virtues" of NOS Black Beauties and Vitamin Q's.  You 
might sell your BBOD's on one of those forums or on E-Pay and make a bundle!


Drew



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