[NJARC] Selenium Rectifier replacement

[David Sica]

I have often heard people recommend that the resistor be added, to reduce
the voltage and also to limit inrush current through the new diode.  But
I'm not the engineer here, so I don't know how much validity there is to
their fear of the diode popping.  I'm gonna keep your experiment in mind
for the next time I find myself in that situation.

-- Dave


New Jersey Antique Radio Club

On Fri, Apr 15, 2016 at 3:52 PM, <antqradio at sbcglobal.net> wrote:

> Just remember
> Reply = Poster
> Reply All = Everyone
>
> _________________________________________________________
>
> Joe
> I personally think the issue of adding a resistor in series with a silicon
> diode to make up for the "voltage drop" of a selenium stack is just a waste
> of time.  Another five volts should not make any difference at all as long
> as you don't end up over volting the electrolytic filter capacitor.  More
> important would be to insure that the tube filaments were operating at or
> below rated voltage.  This will at least guarantee longer tube life.
>
> As an experiment, when you do the swap, could you measure the current draw
> of the radio and the B+ voltage with the series resistor in circuit and
> then with the added resistor shorted?
>
> If you think about it, adding a resistance in series with the rectifier
> makes B+ voltage regulation in the radio worse.  In other words, at volume
> peaks, the B+ voltage should droop a bit more with the resistor in circuit
> and less so when out of circuit.  Current draw should not change at all
> since most of the tubes in the AA5 are pentodes where plate current is less
> affected by plate voltage.
>
> Interesting to note that the resistor used in series with the selenium
> rectifier was there to limit peak current and to act as a fuse in case of
> selenium rectifier failure!  Selenium stacks were crappy rectifiers;
> working on the edge of performance because they were limited by how much
> current they could pass.  But they were more efficient then a tube
> rectifier and soon replaced by silicon diodes.
>
> Have you seen this pdf book on radio repair?
> http://www.richardmcwhorter.com/vacuumtuberadio/The%20Vacuum%20Tube%20Shortwave%20Radio.pdf
> Password is "allamericanfiveradio".
> Jim
>
>
>
> ------------------------------
> *From:* TrainBee via NJARC <njarc at mailman.qth.net>
> *To:* njarc at mailman.qth.net
> *Sent:* Wednesday, April 13, 2016 7:58 PM
> *Subject:* [NJARC] Selenium Rectifier replacement
>
> Recently, I've come across a few 50's vintage radios that should have
> their selenium rectifiers replaced.  I'm thinking safety first over
> authenticity.  This is uncharted territory for me.  What are some of the
> guidelines to follow for their replacement.  I have some 1N4007 diodes, but
> I'm not sure of the math to follow.  When it comes to the corresponding
> resistor, is it best to use a 10 w resistor after you figure out the
> necessary resistance.
>
> I don't recall seeing an article in our newsletter over the last 15 years
> or so.
>
> Thanks in advance for your help.
>
> Joe Devonshire
>
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> Reply = Poster
> Reply All = Everyone
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