"Mike Knudsen" wrote in message
...

This makes very good sense.
I suspect that back in the old days, manufacturers would
throw in up to 100%
extra foil plates area just to make sure they at least met
the rated
capacitance. So you would get caps well over the ratings.

But yes, once they got the process down really tight, why
toss in extra
material. In fact, shaving it on the low side is just
what the front-office
bean coutners probably tell them to do nowadays! --Mike
K.

Oscar loves trash, but hates Spam! Delete him to reply to
me.

I don't think I've ever seen an electrolytic capacitor read
more than 20% high, and even that range is very rare. Even
on the few low ESR survivors from the late 40's - early
50's. I'm not using a lab quality bridge, or checking large
numbers of electrolytic capacitors, so I can't come to
really firm conclusions. But I'm thinking the manufacturing
process was reasonably precise by 1950.

I don't know if the comparision holds, but carbon
composition resistors were getting more precise all through
that era, as well. The 5%ers were pretty common around
1970. I have to wonder how much expense was added to the
more precise resistors just for keeping extra inventory.
There's about twice as many values for 10%ers as 20%ers.
Double it again for the 5%ers. Assuming the cap makers
could reliably come with 5% electrolytics, would there be
any value to stocking 4 times as many values? I can't think
of any. They are used almost entirely for power supply
filtering, or audio coupling. So maybe they kept the old
20% spec on 5% tolerence caps only to keep inventory simple.

Frank Dresser