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Ralph Mowery

In article ,
says...

Myths: A variac will help reform caps (with specific reference to tube/
valve radios).

Fact: Only on those vanishingly few radios with solid-state rectifiers
that pass B+ current at any voltage, and only if done over more time than
one cares to consider. Bench DC supplies are best for this typically futile
purpose in any case.

In the case of a tube rectifier, most of them do not start to pass DC
until the filament voltage reaches between 65% and 75% of nameplate rating. If the B+ on the particular radio is say.... 350V, that means that the first voltage the caps-to-be-reformed would see would be something between 227V and 263V... not exactly a soft-start.

There is a 'more correct way" to do the reforming if one wants to try.
If a tube rectifier you pull that tube or tubes and install solid state
rectifiers. Then pull the other tubes to prevent cathode stripping.

Most electrolytic capacitors are formed at the factory at about 10 % or
so over the rated voltage. As they sit on the shelf they will degrade
somewhat. It sometimes makes sense to reform even new ones if they have
been on the shelf for many years. That is where a bench supply comes in
handy.

I wish I could find the magazine I had around 1965 give or take a few
years. In it was an automatic restorer. I think it used a bunch of
neon lights, but could be wrong. It may have been Electronics
Illistrated. Don't think it was Popular Electronics. Maybe Radio
Electronics.