Premier Guitar

The Life and Death of an Electrolytic Cap

May 12, 2008
It was a day like any other in the amp, everybody doing their job, the resistors making the electrons run obstacle courses, the tubes making the electrons line up and act nice, and us capacitors doing the heavy lifting, sifting out DC from AC, holding off the high voltages like power lifters and letting the AC through like yapping poodles on short leashes.

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.

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.

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.

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.

Here’s what Bennie went through.

Tech Views 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.

The second photo shows a pair of cardboard tube caps, a GE and another Mallory. In this case, the tech caught them before they vented. The bulge on the terminal and vent at 5 is easy to see. It’s harder to see in 6, but the terminal is just lifting a bit and there is a thin discoloration of leaking fluid.

Electrolytic caps are formed by putting thin sheets of very pure aluminum into a chemical stew and running electricity through it. The 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.

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, 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.

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.

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.

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 gradually 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.

Just like humans, a capacitor’s life is best used actively. Play those amps and fire up those pedals.

Don’t let Bennie die in vain.




R.G. Keen
Cheif Engineer
Visual Sound
www.visualsound.net