[Boatanchors] restored radio question

[J Forster]

jeremy-ca wrote:

>
> It all depends upon what you consider old. If a cap tests leaky then it is
> not a good candidade for reforming. Very few caps of the 30's have survived
> to today without developing leakage.

In fact, when reforming an electrolytic, the current drawn decreases with time.
It eventually reaches an asymptote. If that asymptote is too high, the cap is no
use in most circuits. In general, I reform caps with stepped voltage increments,
keeping the dissipation under 100 mW for typical caps of roughly 1" OD x 2"
long. Larger ones can take a bit more.  Typical WW II caps may take several days
to reform.

[snip]
>> Receiving type tubes, if not gassy when stored, wont be gassy in another

> >> 10
> >> years or more. Some also attribute the glow from some beam tubes as gas
> >> when
> >> in fact it is simply charged electron flow.  It is a different story with
> >> some transmitting tubes that have an established reputation of having
> >> seal
> >> problems.
> >
> > By operating old, slightly gassy, tubes in a tube tester with both
> > filament and
> > plate applied, it is possible to degas receiving tubes. (The plate voltage
> > is
> > needed because it's necessary to ionize neutral gas molecules before they
> > will
> > react with some getters.) I've done it with several hundred WW II tubes.
> > About
> > 90% are recovered using this technique. Occasional use will keep them
> > happy.
>
> There is a huge difference between normal outgassing from the anode alloy
> and a slight drop in vacuum due to seal leakage.
> The best way to recombine outgassing is to operate the tube at its rated
> dissipation or even slightly above for an hour or so.

If you do this, you will poison the cathode. Typically, you want to operate the
tube at about 20% of the rated current at the rated B+. This combination will
ionize the gas and permit the getter to scavenge it, but not poison the cathode.

> An easy way to do this
> is to use a unimportant radio that already runs its tubes hot and switch to
> SS diodes in the PS. If the xfmr fries its no big loss. This procedure will
> last for many years if not decades. A tube tester provides nowhere near the
> power necessary for many tubes.

My Triplett works just fine for tubes as large as 807s.

>   As
> > to 'old wives' takes' I made careful engineering measurements on this
> > topic
> > about 5 years ago. I wanted to restore some WS 19 sets to original
> > condition and
> > had no desire to realign them, hence wanted to keep the original tubes in
> > their
> > original location. Oh, and helium (present as a trace in the atmosphere)
> > diffuses through most glasses, so even a tube  with perfect seals can go
> > soft.
>
> Very true but a trace of helium does nothing to most receiving tubes except
> possibly some audio distortion.

He ionizes and increases leakage which is problematic for many circuits. Cooking
the anode will free up He and other gases that are adsorbed onto the interior
surfaces, but will not getter them. When the surfaces cool, the atoms will
stick, when heated they will go into the vacuum. There is no such thing as
poisoning the anode, just the cathode.

> Cooking the anode will eliminate the helium
> and slightly poison the anode. I have no idea how many cycles it would take
> to ruin performance. Probably longer than anyone of us will live.
>
> Carl
> KM1H