Electrolytic Caps [Good or Bad?]

[guitician]

I have many old amps and they mostly have the original electrolytic caps in them. They have very little hum and sound great. But, every time I read about people suggesting pulling the filter caps out of old amps, I scratch my head and think...why. If you think about it, the only time they really go bad is when the amp has sat for many years in a poor environment, and then, plugged in and they fail right away, with little doubt about what has failed.

[booj]

Exactly. I've replaced 40 - 50 year old caps in amps and checked them with a cap checker only to find out they are absolutely perfect. No leakage, no change in value, although they generally read OVER the rated value.
If the amp is used occasionally, theres no telling how long they will last.
This is just an opinion, of course, since I've only been working on amps for 40 years or so.

[guitician]

Thank you. I knew I wasn't imagining this. I think techs confuse the "Shelf life" of an electrolytic cap with their "Running life". In an amp that is designed correctly, and is used regularly, for how ever long, the caps wont be an issue. There seems to be a lot of "You may as well pull all the filter caps because they will go eventually" posted by people and it's just not true.

[tedmich]

Surely you have had to replace many bad old ELs right? Granted they may be replaced too often but they go south more than any other component except tubes. Perhaps recapping is the equivalent to re-immunizing your pet every year even though once is generally enough, cause vets have to make house payments too. And it usually does no obvious harm, which is the common criteria for many misspent dollars.

[Resonator Guy]

I am of two minds on this one, but generally I agree that if the amp sounds good, leave it alone. I recently got a 1977 Fender Silverface champ that was completely original. It even had the original Fender tubes, and it sounds magnificent. No hums or buzzes, just a real chimey sound.
I did recently recap a 1957 Supro Super that I have had for years. It still sounded good, with very little hum, however I thought I would try some new high quality caps in it and I was amazed. It now sounds so much better and is a good deal louder.
So I guess it can make a difference to recap, but I try to do it only when necessary.
It is all great fun though, isn't it?

[guitician]

Actually the most EL caps I've replaced were of low voltage types used as signal coupling caps. They are lousy for that application. Changing parts in any amp will make a difference, just like changing types of strings on your guitar. If they made a huge difference, then your old parts were of poor quality or damaged a bit by environment.

[guitician]

Potentiometers are worse than electrolytic caps for failure, especially the cheap low cost ones that are being used more and more by manufactures.

[Diablo]

I think it makes economic sense to replace all the small e-caps in the amp after so many years of service. They are inexpensive, easy to change out, prone to age-deterioration (like all e-caps), and some of them (like the bias supply caps) are critical to the amp not destroying itself.

[guitician]

prone to age-deterioration (like all e-caps)

Not when they are operated under optimal conditions. Also, all e-caps are not created equal. Yes, replace garbage, 5 cent, econo brand stuff. Quality e-caps should never need replacing an any amp..ever.

[Diablo]

Not when they are operated under optimal conditions. Also, all e-caps are not created equal. Yes, replace garbage, 5 cent, econo brand stuff. Quality e-caps should never need replacing an any amp..ever.

Do you have a link to any manufacturer's data that shows his e-caps last forever when operated under "ideal conditions"? What conditions do you consider optimal? How does one identify a quality e-cap?

[guitician]

Be serious, no mfg is going to state that. Optimal would be either; always having voltage applied, or at regular intervals, and kept at temps below 77 degrees Fahrenheit. The better the seal, the better the cap.

[R.G.]

Not when they are operated under optimal conditions. Also, all e-caps are not created equal. Yes, replace garbage, 5 cent, econo brand stuff. Quality e-caps should never need replacing an any amp..ever.

It may make for good wishful thinking, as in "... oh, Toto! Quality e-caps SHOULD never need replacing in any amp... ever!", but it's not realistic. Fact is, some portion of any brand, any amp, any age, will fail. Period.

Humans have the problem that they're more willing to believe anecdotes like stories typed into an internet forum than they are willing to believe the underlying math, especially statistics. That quirk is what keeps insurance and gambling industries in business.

The observed life for electro caps is predictable, statistically, just like the life expectancy of humans. Sure, there are humans who live to be over 100 years old. But the insurance companies can tell you pretty closely what the odds are of you personally making it there.

The fact is, repair techs replace failed e-caps a lot. That's where the predictions of life on caps come from at the cap making companies.

The issue is not really one of "how long can I run this amp before an e-cap vents and pours out smoke?" Instead, for the working musician, it's "This d#*d amp had better finish this set!" A working musician wants reliability and predictability more than long life. Or at least the ones smart enough to think about it do.

So people advise other people to change out caps. I do that, frequently. And I'll keep doing it, explaining if needed that you may be removing perfectly good caps. But those perfectly good caps may last perfectly good for five years, or ten more years, or five *minutes*. Recapping an amp nearing the conservative life expectation of the caps in it is not about draining every minute out of the old ones - it's about knowing that the amp will not have a cap failure on the new ones for a good long time.

Exactly. I've replaced 40 - 50 year old caps in amps and checked them with a cap checker only to find out they are absolutely perfect. No leakage, no change in value, although they generally read OVER the rated value.
If the amp is used occasionally, theres no telling how long they will last.
This is just an opinion, of course, since I've only been working on amps for 40 years or so.

Good opinion. I defend your right to have it. However, I also defend my right to think it's incorrect.

What you typed is factually correct.

If the amp is used occasionally, theres no telling how long they will last.

There is, literally no telling. It may be 50 more years, it may be five minutes, or the next power-on surge. The statement is literally correct, but not true in the semantic sense that it implies. The only way to tell is to look at the sum of the usage conditions and the statistics for that type/brand/lot number of caps and then ... guess. Caps on a shelf go bad. That's why they're a shelf life item. Using them with a voltage in the upper third of their specified voltage tends to keep the oxide layer repaired by leakage currents electrochemically where shelf sitting would make it degrade more. The occasional use also mitigates against short life by having few power on/off cycles and less overall hours of thermal stress. It's like heaven on earth for a cap.

But musicians want to PLAY these things, and working musicians want them to work every time. That's a little different.

This is just an opinion, of course, since I've only been working on amps for 40 years or so.

I've been working on amps on and off for only about 35 years, and haven't made a living at it. I made a living designing and supporting manufacturing of power supplies, so I've seen a LOT of caps used in power equipment, and I've had to understand the math and producers' data on projected lives of power components. I've also talked to a lot of other techs who repair amps, and I get a lot of agreement with doing recap jobs where an owner wants reliability.

I've also had a lot of feedback from owners who took my advice to have the amp recapped. Very often the comment is that the amp sounds amazingly better. Old caps, even ones that don't leak, have drifted values and high internal resistance. That changes the power supply impedance and the sound of the amp. Changing the caps out restores the power supply operation to the way it was when the caps were newer.

I recommend you borrow or rent an ESR meter. It's pretty easy to pick out which caps will make a big difference in sound by testing ESR, not capacitance or leakage. Old caps have higher ESR even if they don't leak and have the same capacitance. It makes a difference.

No one can predict exactly which cap is going to live for another decade. But if you want your expectation that the next time you turn on your amp it's going to play just fine, and the time after that, and the time after that, it makes sense to replace parts which have a built-in failure mechanism, *as admitted by the makers of those parts* every so often so you don't have to have the unpleasant opportunity to play air guitar in front of a crowd.

One of my favorite paraphrased proverbs runs "The race is not always to the swift, nor the contest to the strong; but that's the way to bet."

[tedmich]

point made!

[guitician]

Most of that I already knew. My opinion remains unchanged. You like to type, and you have lots of experience. But you are not as knowledgeable as you would like to be on electrolytic cap physics.

[guitician]

point made!
[img][/img]

Yeah