Attention Gurus- New Unused Caps- Shelf Life and Should I Charge Them

[Joe L]

Over time, the state of the art in terms of aluminum alloys and electrolyte chemistry progresses. You can't buy caps like they used to make 'em - thank God. Modern etched foil electros are a fraction of the size and much less prone to internal chemical failure as well as drying out and have lower ESR and ESL, as well as better heat dissipation.

Great write up RG! I'm building a Mark IIc+ repro right now and anywhere I can use film caps instead of tants or electrolytics, I'm doing so.

I have found that Mallory and older Spragues are more likely to reform and test good after light, intermittent duty even from the '80s. But I agree that it is easy to find new caps that are of better quality for cheaper. Especially with radial leads.

One thing I'm fuzzy about. I know that you can make a non-polarized electrolytic by putting two in series with the negative terminals connected but just how does that work? It seems to me that either direction of the current flow would be reversed polarity for one of the caps.

[R.G.]

how is it that many reforming protocols for stored EL caps simply have them heated in an oven with no applied voltage?

As Chuck correctly points out, this is probably a step in a multistep process; probably-possibly to get the electrolyte gook into a receptive condition for the electrical reforming.

It's worth noting that modern practice with non-aqueous electrolytes may well have diverged into being better for the normal/first application of the caps but be worse at supplying what's needed for re-forming. That's something I have not run across.

My approach to re-forming in a tube amp is simple: Remove the tubes, hence removing the loads. If it's a tube rectifier, sub in a SS diode so you can generate DC, and temporarily insert a 100K in series from the rectifier to the first filter cap. Watch the voltage across the 100K. When this drops to less than 5-10% of the total DC on the cap, you're done. If it never gets down that far, either you have some loading resistance across the filter caps that you didn't remove, or the cap leakage won't let it get there. If it doesn't get there in a day or two, the cap's beyond repair. Using the equipment itself for re-forming is so much handier.

One thing I'm fuzzy about. I know that you can make a non-polarized electrolytic by putting two in series with the negative terminals connected but just how does that work? It seems to me that either direction of the current flow would be reversed polarity for one of the caps.

It takes advantage of a quirk of oxide film insulators. They insulate in one direction much better than the other. Remember copper-oxide rectifiers? Neither do I, but I looked them up. Oxidized copper plates conduct one direction well enough to be used as rectifiers in stacks. Radio hams used to make rectifiers by toasting one side of a copper plate with a torch, then cleaning the other side and stacking them in series. It wasn't great, but it was cheap and do-able outside a semiconductor fab back in the early 1900s. Aluminum oxide does much the same.

If you reverse polarize an electrolytic cap much more than a few volts, current flows. It can be used as a crude rectifier, at least until the current flowing in the conducting way electrochemically eats away its ability to turn around and block in the other direction. Dry copper oxide is much better this way. If you put two electro caps head to head, then no matter which way the voltage points, one is low resistance and one is high resistance. The one that blocks DC flow also blocks the currents that would eat away at the oxide film of the one that's being polarized backwards. And you only paid a 2:1 penalty in bigger volume for the privilege.

[tedmich]

ooops wrong Weber! It was GERALD Weber who talked about ovening old caps.... not Ted (RIP), add 3 moles NaCl!

Some more details on electrochemical reforming here:
Reforming Electrolytic Capacitors

[nickb]

....temporarily insert a 100K in series from the rectifier to the first filter cap...

If a cap a fails the 100K will bake pretty darn quick. Might be a good idea to use a flameproof or a five watter.

[nickb]

ooops wrong Weber! It was GERALD Weber who talked about ovening old caps.... not Ted (RIP), add 3 moles NaCl!

Some more details on electrochemical reforming here:
Reforming Electrolytic Capacitors

I heard of this being done at low temps to remove moisture from paper caps.

[R.G.]

If a cap a fails the 100K will bake pretty darn quick. Might be a good idea to use a flameproof or a five watter.

Actually, using a 1/4 W will make a pretty good fuse . Or an olfactory alarm indicator, I guess.

Yeah, a failing cap will make that get really hot. I've only ever had one of those happen on bring-up, but then I don't do very many bring-ups of old gear.