when do you replace capacitors?

I have read that capacitors last around 20 years. So, do you just have caps replaced as they go bad, or at what point do you replace all the caps in an amp? Do resistors or other parts (other than tubes) degrade, so replacing caps will just lead to some other failure? Is it like trying to keep an old car running?

Asking because I still enjoy the tube amp (Baron) I've had for 16 years. Not the best, but American made, still supported by Mesa, and I am not convinced that newer is better re: tube amps, or at least, not within semi-rational budgetary constraints. Equivalent performance from new production could cost dearly.
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Do ALL the caps at one time when you decide to have them done.
Twenty years is a safe period.
It might not be neccessary to have all of them done then, but better that, if you really like the amp, then have one blow up on you suddenly.
I you want to do them early, no problem. Especially if the company is still able to do it for you, instead of some third party.
Think of it this way: if you do them now, that gives you 20 MORE years of service from now. And in four years, heaven forbid, maybe they will not be able to redo them for you.
Does a stored or "retired" component experience aging of the capacitors and resistors at the same rate of speed as a component actually being used over the years?
Thanks. Will plan on doing it before long. Naturally, I want better capacitors than the manufacturer would use, at least in some spots, so a third party will be needed.
As for a long time use/non use, I would say the time is time. used or not, for caps. Capacitors actually LIKE to beused some. Not too hot, not too seldom.
So the twenty years is used a lot or not much.
Some discussion is given to theory about high temps will cause premature aging of caps in hot running electronics, and that they dry out faster. But little real evidence to show that is really true IMO.
Some caps just leak or die no matter what.
The one bad thing is a LONG time and never used, is very bad for capacitors when they get turned back on for the first time. KaBOOM!
A super no cost way to reform them (what the process of getting them back after yearss of no use): plug the unit in. Turn on for the briefest possible time fast switch on/ off under one second. Wait ten minutes. Turn on again for only two seconds or so, then off, (don't stretch out the time) leave off for one half hour. Turn on for five seconds, then off. leave off for several hours to overnight. Then it should be good to go.
Cheaper than other ways to safely reform caps. Works too.
Thank you for your reply.
Seeing your name on a posting here on Audiogon is cause enough to check out the words of wisdom and experienced opinion to be discovered.
As Elizabeth mentioned, unused is worse.

But the correct way to reform capacitors is with a variac.
i replace them if they re crap caps. I like v-caps if you have room, the REL RT, or Theta caps. Big sound difference over stock caps if they used junkers.
Resistors are inert and don't really change over time (they can drift in value from thermal stress, but that's another matter).

Capacitors are a different story. Electrolytic capacitors are the ones to change; film types tend to last a very long time and rarely need to be changed. Electrolytics contain a semi-liquid dielectric compound that dries out over time (used or not) leading to catastrophic failure. Electrolytics tend to be larger in value (over a couple of microfarads); film type caps tend be much smaller in value (under 1 microfarad).
Variac is the porper way to restore a cap. If your electronics item is worth a lot of money, and you have the money to spend getting it variac'ed. Do it. Otherwise the turn on style will do "in a pinch" or if you are just too cheap to get it done right. It is a 'dirty' method, but works. (than means it is not approved by anyone who actually does this stuff for a living, because it cuts into the bottom line)
Not because it cuts into the bottom line, because it's much safer.
So, the equipment is going to "explode in boiling fire"? Or is it that the cost of replacing a blown cap is less than renting a variac?
I can understand if the item is a bit of medical equipment attached to your heart pump. but usually old stuff that was stored for ten years is not the usual Ferrarri material.
I would not be afraid to use the cheap and dirty method on ANY product i owned.
Any yeah, if the product means a LOT to you, and it is really valuable, then splurge if it makes you feel better. But the item is not going to explode with the turn on method any more than with the variac.
This is like do you take your car to the dealer? or a local guy who you know.

Perhaps I just like to live dangerously?
I deliberately bought a SinglePower MPX3 because the notorious 'exploding with brimestone and boiling fire, killer/blinding headphone amp was just too good a conversation piece to pass on. Aparently some dude over at headfi got in a tizzy over his product being shoddily made and declaired all SinglePower products to be timebombs waiting to maim and kill the owners, and everyone needed to stop using them immediately, or risk immolation.
I have used my SinglePower now with it on the higher bias setting and so far have not been killed. (though some wish I was dead..sigh.)
Or is it I just am practical?
I stand up for cheap HiFi. and the turn on is fine by me: cheap.
A qualified technician can apply a variac and reform the caps over time if they have been sitting for awhile. However, the tech must first inspect the caps to see if leakage has occured. Caps can be different. There are paper caps, oil filled caps, etc. So, they will age and fail differently. This is why the best caps are stupidly expensive vs. cheaper caps. Personally, for a device that has sat unused for many years, I would apply a variac and slowly bring up the voltage over a period of days to reform the caps and see if some other component has failed. If everything is okay, I would not change the caps unless they are leaking or won't reform. Also, another area people absolutely don't know about or forget is that power transistors typically use silicon transistor heat sink grease between the transistor insulator and the heat sinks. This grease definitely hardens and fails over time and for older amps is the main reason (other than the owner switching interconnect or speaker cables without turning it off first) why amps blow. For a really nice older amp, I typically will test all the pre-driver and output driver transitors, see if they are within spec and if not replace them (or replace them on general principle with newer more linear transistors) and absolutely replace the silicon grease. Of course I am an Electrical Engineer and also work on amps/components and upgrade them all the time. However, a good technician can do this also. Again, if you manage to grab a Mark Levinson ML3, 23.5, Bedini, etc. amps absolutely check the caps and change the silicon grease. But, if the caps reform and aren't leaking, I wouln't replace them just yet.

Thanks Minori, I was unaware that the silicone grease can degraded over time.
The silicon grease will dry out and causes the transistor body to come into direct contact with the heat sink and shorts out the transistor and may cause a cascade effect with the other output drivers. For TO3 type metal can transistors there is a rubber or mica, or really cheap plastic insulator between the transistor and the heat sink and silicon insulating grease is applied between the device and the heat sink. the cheap insulators will crack causing a short to the heat sink also. So, I typically take all the output drivers off and replace them with more linear better transistors and replace the insulators and the silicon grease, check the capacitors for signs of leakage and replace if necessary. More detailed upgrades can turn a very good amp into a really great amp.

What transistors are best and more linear
I have a pair of DNA1 dlx amPs
Minorl, what thermal silicon grease would you recommend best for this application? Would Tech Spray 1977-DP be a good choice?

Thank You
I'll get back to you shortly on this. However, the older transistor amps have very old non-linear transistors. The best of their day, but much has changed since then. Several companies have come through with much better sounding, much more linear high power transistors. It depends on the transistor package. TO3, etc. and whether they are available in that package or what other transistors would fit that meet or better the specs and would sound great. People have spent years swaping tubes and found that there are many that are better than the stock tubes. While it is very easy (although expensive) to do this with tubes, the homework has already been done. But, people don't really swap transistors on newer amps because, hopefully the designer has already done their homework and selected the best transistors they could find. Not always the case. However, in the case of older amps, the field is wide open. If I'm going to open it up and upgrade the power supply or some such anyway, then I check the specs on the transistors and if there are some better ones, I replace them also, rebias, add the threshold type heat sinks if they are seriously lacking proper heatsinking and they are wonderful. I upgraded a bedini 250/250 recently by making two power supplies (one for each channel, including rectifier diode brides, capacitors, transformers, etc.), then replacing the older output and pre-driver transistors with newer, more linear ones,added Threshold type heat sinks, got rid of the speaker fuses, upgraded the internal wiring with Cardas wiring, upgraded the smaller caps to much better ones and this amp seriously blew away most others I compared it with. If I add a regulator, it would be a monster.

I'll get back to you soon on the transistors, but first tell me which ones you have in your amp.

Smoffatt the thermal grease is not an insulator in case you got that from Minorl's comments. But if it is dried out, heat will not transfer and so the transistor can overheat and possibly short.

The best kind of grease is white. There is a clear variety also, but the white grease seems to be more efficient. It should be applied with as little as can do the job; a very thin but even coat on both sides of the insulator seems to work best. One of the better compounds is made by AOS Thermal Compounds, part number 520221E, although that part number is for a one pound jar, which takes a very long time to go through even if you are replacing power transistors every day. I am sure they sell it in lesser quantities.