[NJARC] Selenium Rectifier replacement

Fri Apr 15 15:52:58 EDT 2016

JoeI 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.pdfPassword 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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