Replacing 40 year old filter caps

[Randall]

I have a 76 Champ on the bench that had a dead shorted cathode bypass cap on the 6V6. Now it sounds great with all American tubes in it. Question is, it has the original Mallory 20/20/40 can. I will typically inform the customer that it is prudent to replace filter caps that old even if the amp is working well so as to head off problems further down the road. But, with this one, the amp is so clean and quiet, and the caps measure close enough with only 1.5vac ripple on the first node, I'm not sure I want to give that talk to this owner.

What do you think?

[TomCarlos]

Due diligence... have the talk, let them decide :-)

[Enzo]

And while we are at it, also tell teh customer that a cap job is like buying new shoes. Old shoes have softened leather, the leather wears thin in spots, the sole has worn down under parts of your foot, the shoe is overall more flexible. New shoes correct all those things, but no longer feel like old comfotable shoes. Just so a cap job. When we replace old caps, the amp will return to the old days of tight filtration, etc. In other words the amp dynamics will change.

[Randall]

Enzo, yes I agree. But he has also purchased all new tubes, installed a new speaker, and brought a folder full of Gerald Weber internet forum posts about how to "fix" this amp. I talked him off the ledge with regards to that stuff, and convinced him to get the amp working the way it was intended first before we go doing Weber mods. I think I got him to where drilling holes in the chassis on a clean vintage Fender because of youtube is not a good idea. Especially on an amp as simple as this.

[Enzo]

You are right, I just feel that is part of telling the customer what to expect. One large problem in retail repair is unrealistic expectations in customers.

[Justin Thomas]

Personally, my 38-year-old Champ still has the original Mallory. It's quiet as can be til I plug into it. I don't take it out gigging, so I'll fix it when it dies. Given the cost of new can caps and the failure rate I've had with new ones, I'm just gonna leave it in. It gets played regularly, and as I said, I don't gig it, so I don't worry about it embarrassing me. But Champs are cheap enough (kit or old) that if I was <REALLY> worried about it, I'd just make another for myself.

Justin

[Mick Bailey]

The shorted cathode bypass cap probably also worked right up to when it failed.

I really don't like replacing those Mallory cans, but failure in the end comes down to probability. If it was my own amp, I'd leave it alone until it began to get noisy. But that assumes the cap would get noisy as it gracefully began to fail. Maybe it would short out - maybe it wouldn't, but that would be my own problem. Caps are rated (among other things) in hours service life. I bet those caps are orders of magnitude over the original lifespan prediction set by Malllory.

You can't see what's happening inside that particular cap. You could take ESR and other more involved readings but those won't tell you if that cap would fail abruptly. I've re-stuffed many can caps and it's often clear that there are substantial changes inside the can even if ESR is low and there's little leakage. What we're now looking at is the survivors; the caps that outlasted the others and defied statistical predictions where others failed and got replaced. These are the elderly 80-a-day smokers who beat the odds and can still climb stairs quicker than a 20-year old. Question is, who would you put your money on not dropping dead tomorrow?

[Enzo]

You cannot apply statistics to individuals. Simple as that.

[Randall]

Interestingly, this customer is an EMT and when explained the options, went with let's do it now.

[gbono]

You cannot apply statistics to individuals. Simple as that.

Mmmm tell that to the pharmaceutical marketing and R&D folks....

Here's are some cap lifetime calculators to play with..

https://sec.kemet.com/tools

[Enzo]

I had a discussion with a doctor friend the other day, and he was calling something normal that usually affected people between the ages of 50 and 100. I told him that made "normal" a useless predictor.

The average family has 2.4 children. Ever actually meet an "average family"?