[Milsurplus] 51J-3

[J. Forster]

The objective is to avoid localized damage due to spot heating within the capacitor
during the reforming of the dielectric film. That said, I'd choose either ramp up
with a current limited source or full voltage from a high impedance. Opinions vary
though.

I prefer voltage increases of about 10% of the operating voltage rating of the cap,
with current limited to a milliamp or less for a typical 30 MFD at 450 V capacitor
typical of WW II gear.

I prefer to let the capacitor tell me how it's doing, rather than follow a preset
schedule. I watch a current meter in series with the C and when the current stops
dropping (flattens out) I push the voltage up another 10%. BTW, I usually reform all
the caps in a set at once, by applying the B+ to the circuits near the rectifier
output, with the filaments off.

 If there is a dyno, it should be disconnected. An easy way is to pull the HT fuse or
remove a HT brush.  Reforming a typical WW II set can take several days in my
experience.

When the reforming is complete, I like to let the set soak for a day or two, again
with filaments off, and normal HT applied.

Just a couple more things more:

In measuring the current for doing a whole set, remember to account for the various
resistors between B+ and B-.

Also, cathode bypass caps will NOT be reformed by this process since there is
typically no voltage applies across them.

Finally, metering circuits can draw significant B+.

Good luck,
-John

Peter Gottlieb wrote:

> It's been a while..
>
> What is the best way to reform caps?
>
> Full voltage with high source impedance?  half voltage?  ramp up?  over what
> time period?
>
> J. Forster wrote:
> > telegrapher at att.net wrote:
> >
> > [snip]   I won't apply any power till i've looked underneath and then
> >
> >
> >>will do so only with a variac in line.  The main tuning knob is from a
> >>BC-348 so now i've got a nice spare for one of mine.  This will be
> >>another item i'll need to find for this receiver.
> >
> >
> > If you mean to reform the caps by applying power slowly from a Variac, I'm not
> > convinced this is the best procedure for tube gear.
> >
> > Certainly reforming is essential, IMO, but without full filament on the
> > rectifier(s) the actual reforming voltage may be somewhat unpredictable. It may
> > also be hard on the rectifier tube(s).  usually leave the set unpowered and
> > apply voltage from an external PS.
> >
> > FWIW,
> > -John
>