Time For A Cap Job?

[Beck-Ola]

Ok, got an almost 28 year old JCM800 2204 that still seems to work fine. Never had a problem with it. I've always read that the electrolytics in an amp need to be replaced after, say 20 years or so. Different sources give varying numbers of years. The amp still seems to work fine. It might hum a tiny bit more than it used to, can't be sure. It might be a little softer in the bottom end. Hard to be objective about it.

Anybody think it needs a cap job? Physically there are no signs of leaking or drying or whatever.

I suppose I could go ahead and replace them and head off any future problems since it will need it eventually. I wonder if it's just the large filter caps for the power supply that get replaced or also the smaller electrolytics like on the preamp?

Thoughts?

[surfnorthwest]

The amp still seems to work fine.

How many guys have kicked themselves in the ass for screwing with their amps, just asking.

[jboyjams]

It's a good question Beck-Ola. I'm in a similar boat with a couple of amps.

How do you know when the caps need replaced?

[JCM900MkIII]

Pretty confusing stuff indeed.
I´ve been advised to replace mine on my 900MkIII (1992).
But there seem to be a couple of important factors to keep in mind.

High temperatures shorten the lifetime. No idea what ours are rated at, or at what temps they run in our amps.
There supposedly is a "law" dictating capacitor lifetime. (http://en.wikipedia.org/wiki/Arrhenius_equation). But I don't know anything about electromagic or chemistry.
"As a rule of thumb, the life doubles for each 10°C lower operating temperature." <---(http://en.wikipedia.org/wiki/Electrolytic_capacitor)

And some people say they dry out, regardless if you actually use them.

[Beck-Ola]

Thanks, guys. Looks like I'm not the only one wondering about it. Let's keep our eyes and ears open and share whatever we find.

Hopefully SteveD can shed some light on this as it is common to all tube amps.

[SteveD]

It's preventative maintainance to replace your electrolytics maybe every 10 to 20 years or so, just like the recommendation to replace the drive belt on a car engine every so many thousand miles. It never does any harm to change them but I would say if your amp sounds OK and is not particularly noisy or hummy then it's not essential to do it.

The consequences of not replacing a car's drive belt can be far more serious though.

[Beck-Ola]

That's good to know. Thank you very much!

Almost forgot, every electrolytic or just the big filter caps?

[SteveD]

Mainly the big (power supply) filter caps. It's up to you whether you do the small signal ones as they can get leaky too, but in general they take far less abuse. Even other capacitor types can get leaky and cause noise. A give away for this is when you get noise when turning a pot and cleaning and even changing the pot doesn't cure it - 9 times out of 10 it's a leaky cap that is supposed to be decoupling the high DC voltage from the anode.

[Beck-Ola]

Alright. That's very helpful.

As always, it's a real blessing having you around.

[Wildone]

A lot has to do with how often the amp is played, and if it was stored at what temp. If you have been using it often and don't hear much noise its fine, it will let you know when its time as they get real noisy.

[Beck-Ola]

A lot has to do with how often the amp is played, and if it was stored at what temp. If you have been using it often and don't hear much noise its fine, it will let you know when its time as they get real noisy.

Yeah, I agree with that. I've always heard that the ones needing caps replaced tended to be the immaculate amps in perfect condition (indicating not being used) whereas in ratty, beat-up amps that looked like they'd been to hell and back the caps were more likely to be fine. Makes sense.