[Boatanchors] restored radio question

[jeremy-ca]

----- Original Message ----- 
From: "J Forster" <jfor at quik.com>
To: "jeremy-ca" <km1h at jeremy.mv.com>
Cc: <Boatanchors at mailman.qth.net>
Sent: Wednesday, October 10, 2007 8:31 PM
Subject: Re: [Boatanchors] restored radio question


> 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.

WW2 Military caps are of higher quality than some of the junk found in 
consumer radios.
I have several that are still good.

I use a 0-500V regulated lab supply that I can switch current readings down 
into the microamp range. I can usually tell within the first hour if the cap 
is worth bothering with based upon decades of experience.


>
> 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.

Totally incorrect. The cathode will be poisoned only when it exceeds its 
emission capability. This becomes a major problem with large and expensive 
transmitting tubes that lack sufficient protective circuity.

The accepted way to degas an oxide cathode tube is to first run filaments 
only for a few hours and then apply HV and adjust the bias until the rated 
anode dissipation is reached and let it cook with plenty of airflow. Big 
tubes do this in stages to minimize the chance of arcovers.




>
>> 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.


You may think it does.


>
>>   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.

That was just a figure of speech as the anode wouldnt be operationaly 
poisoned.


>
>> 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
>
>