I saved 22 electrolytic capacitors I had changed during the recent Altec Extravaganza. You may have read me writing that virtually all old capacitors - especially can-type electrolytics have a life expectancy of about 25 years maximum. Here we will prove that fact.
Periodically I test them for leakage just for the heck of it. It's really an academic exercise, but it gives me a chance to look at the cool eye tube on my Heathkit capacitor checker.
That cap is leaky.
Note that all of those caps physically look good. In fact, they look really cool with their blue coloring.
But they're leaky - which I knew going in and that's why they were all changed.
All of the caps I tested were those aluminum cans, except for 3 sealed some-sort-of-plastic caps used in the Altec microphone transformers as input caps.
Overall, almost 82% of the capacitors total were bad. The three plastic caps tested good, as did one newer (maybe from the late 80s) Mallory cap.
If you calculate the percentage of bad can caps only, the percentage of bad ones goes up to about 95%.
And this is on a small sample. You still want to leave those caps in your gear?
I have a couple dozen old (as in 1930s-50s) paper caps I've been wanting to test, again, just as an exercise. One day I'll do that. Those are legendary for going leaky.
This is why restorers replace old capacitors wholesale. They have a finite life. Sure, your XYZ tube radio/amp is "working fine" now with its original caps. But if you continue to use it, the caps will expire. Guaranteed. And possibly take something expensive like a power transformer with them.
The original power supply caps on my Hacker RP-38A failed while I was listening to it! Made a terrible racket and hum.
What makes the caps go bad is that they absorb moisture from the air, and the dielectric goes bad. Capacitor manufacturers realized this since the begininng of radio - they've made caps that will be more moisture resistant - early radios used capacitors placed in a tar-filled can or block (RCA and Philco come to mind). Later, paper capacitors were dipped in wax to ward off moisture. Larger electrolytics were placed in sealed cans. And finally plastic types were developed. Those tend to have a longer lifespan, but they also fail eventually.
Older types with PCBs or other hazardous waste get taken to the hazardous materials dump run by my local gummint.