Bathtub capacitors.

[Joe W1GFH]

In a message dated 8/6/05 2:54:14 AM, brian.goldsmith at ECHO1.COM.AU writes:

>Grettings to all.
>Some time ago, I came across a large number of ex equipment one,two and
>three section bathtub capacitors at the best possible price-free!! :-)
>  I needed a 3 X .05uF  600 Volt bathtub for my AR88 and found one in the
>junk box with the intention of gutting it and fitting three modern polyester
>caps into the case.Out of curiousity,I pulled out the high voltage megohm
>meter and checked each section of the used bathtub.Suprise,surprise,over
>2000 megohms resistance @ the rated 600volts.Not only on the first
>section,the other two as well!!! Capacitance checks out OK. I found many
>others,2 X.1uF and 3X .22uF etc. and these were also in excellent
>condition.I know the common failure mode is for the seals to shrink and
>let
>the oil escape (and thereby gumming up large areas of chassis) but have
>not
>tested non oil leaking caps for electrical leakage.I gutted a 2 X .05 to
>examine the dielectric which was,as expected,paper.
>  What experience of these much maligned caps has any other member had?
>
>Brian Goldsmith.

I have a 1940's era Hammarlund BC779 which experienced a serial failure of 
these caps. Most people don't replace them because they are riveted to the 
chassis wall. When I first got the receiver, someone had already replaced one 3 x 
.05 bathtub (with another bathtub!) but left the old bathtub's carcass in. A 
foreshadowing of trouble to come? Nevertheless, the other bathtubs began to fail 
shortly after I got the rig. One of them was across a low wattage resistor, 
which in turn is across the T/R switch with 400VDC on it. The cap began to 
physically leak oil (PCB's I assume!) and electrically short. The resistor fried 
and 400V became "available" on the chassis. Needless to say, I drilled out ALL 
the old bathtubs from the rig and replaced them with modern poly caps.

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