TECHVIEWS
The Life and Death of an Electrolytic Cap
It was a day like any other in the amp, The second photo shows a pair of cardboard
everybody doing their job, the resistors tube caps, a GE and another Mallory. In
making the electrons run obstacle courses, this case, the tech caught them before they
the tubes making the electrons line up and vented. The bulge on the terminal and vent
act nice, and us capacitors doing the heavy at 5 is easy to see. It’s harder to see in 6, but
lifting, sifting out DC from AC, holding off the terminal is just lifting a bit and there is a
the high voltages like power lifters and let- thin discoloration of leaking fluid.
ting the AC through like yapping poodles
on short leashes.
unused equipment is prone to dying when
you zap it with fresh power after many
years. The thinned oxide punctures under
the sudden voltage pressure.
Bennie’s the cap next to me. Him and me
came from the same lot, put into the amp
side by side. He’s got the tough job, the
first power cap. I back him up.
Electrolytic caps are formed by putting thin
sheets of very pure aluminum into a chemical
stew and running electricity through it. The
Today I see Bennie don’t look so good.
He’s hot, hotter than normal, and I think
there’s a whiff of electrolyte fluid in the
air. I guess that it’s coming, and so does
Bennie, and there’s nothing we can do.
Inside Bennie there’s a hot spot, a leak
in his oxide. The voltage is pouring current through the hole, and in better
times, that would fix the leak. But this
time Bennie’s run out of oxide-growing
copolymer. This time, the current eats
the hole deeper, faster than the oxide
can grow, letting in more current until
a break happens, and the current leaps
through the gap unopposed.
If a capacitor has a supply of electricity
every so often and it does not get too hot
(which damages the internal chemicals that
help repairs), it can last for a good fraction
of a century, as we know by some older
Fender amps. Sometimes we just get lucky
when we pull out an unused pedal or
amplifier and it works. In those cases,
the caps may be leaky but have not
reached a meltdown point yet and
repairs can catch up with any accumulated aging.
I see it happen. Bennie’s pressure skyrockets and his vent bulges. There’s a
snap as the puncture goes critical and
Bennie vents. Then the lights go out as
the main fuse opens. Well, at least we
won’t go down in flames.
There’s a nugget of knowledge here:
using the amp (or effects pedal) that a
capacitor is in can keep the caps going.
Remember that it was electricity that
formed the oxide layer to start with, so
keeping the right voltage on the caps
as much as possible will keep the caps
in good condition. If you’re a collector,
or just have older equipment that you
value, power it up as often as you can
and let it run for an hour or so – signal is
not necessary. Doing this once a month
might add useful years to the end of its
life or put off the inevitable cap job.
See ya on the other side, Bennie.
Death is never pretty, even when it’s
over-dramatized. As we know, electrolytic
caps have a life cycle; they’re born in an
oxide-forming bath, live a more-or-less useful life and die when the stress gets to be
too much.
chemicals and current are set so that it grows
a layer of aluminum oxide on the sheet. That
layer is the real insulator. Electricity coming
through the chemical stew forms the oxide,
and the presence of voltage on the cap in
the correct direction is what keeps it there.
Reverse the polarity of the voltage and the
electricity will work to disassemble the insulator, causing the cap to die quickly.
If you wonder if your caps are in good
condition, look at them. If you see signs
of leaking or dried up fluid, or bulges
on the ends or sides of the cap, then
you know that the capacitor is dying.
Get it replaced before it fails on you.
Over time, every electrolytic cap gradu-
ally gets worse; eventually, they all give
up. If you depend on your amp for a living,
replace all the electro caps every 10-15
years, whether you think they need it or not.
Here’s what Bennie went through.
Just like humans, a capacitor’s life is best
used actively. Play those amps and fire up
those pedals.
The caps are Mallory axials. The vents in
these caps have opened as the electrolyte
forced its way out (shown at points 2 & 3).
You can see the first symptoms of failure at
points 1 & 4, where internal pressure forced
the terminal to bulge out.
Don’t let Bennie die in vain.
Over time, with no polarizing voltage on
the cap, the oxide will dissolve back into
aluminum and chemical gook. This is why
electros have a shelf life and why old,
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
