[NJARC] [NJARC Selenium Rectifier replacement mea culpa

Mon Apr 18 21:18:22 EDT 2016

BillNot a problem and no need for the mea culpa, we are a friendly bunch here and all opinions are welcome; not one of us here is above impertfection!
We both agree, the most important thing is is to protect the tube filaments from over voltage.  We just differ on how to do it and that is OK.  
Selecting a resistor to make up for the higher voltage drop of Selenium makes little sense to me.  Using a Zener diode to limit how high a voltage that can be applied to a series string of filaments is, for me, the way to go.  This way, the tube filaments are protected for all line voltages.  As for higher B+, the few added volts you get by going to silicon diodes is just a tempest in a teapot.
Now on to other things,Jim

      From: Bill Zukowski <n2yeg at optonline.net>
 To: antqradio at sbcglobal.net; "njarc at mailman.qth.net" <njarc at mailman.qth.net> 
 Sent: Monday, April 18, 2016 8:17 AM
 Subject: Re: [NJARC Selenium Rectifier replacement mea culpa

 OK, egg on my face.  It wasn't a TO, it was a Hallicrafters S-72, kinda sorta like a TO.  

 I had restored the S-72 last month, then worked on a H500 that I had purchased for myself.  Old age being what it is, I said TO when I meant the Hallicrafters S-72.  BUT THE REST OF THE STORY IS TRUE!

 Realizing that the 1 volt tubes were very fragile to higher filament voltages, I used that as a gauge to determine the correct resistor value.  I used 1.3 volts, as that's listed as the normal operating voltage for these tubes.  Surprisingly the voltages were not all equal, but off by a few hundredths of a volt, I guess to to slight variations in filament resistances (never would have noticed it with an analog meter).  The B+ did drop a few volts, but operation wasn't affected (I didn't document it, but it was still well within 10% of specifications).

 Sorry for any mental anguish that I may have caused.  

 Gee, it's 9:15 AM already, time for my nap.

 Bill
 N2YEG

 On 4/17/2016 3:30 PM, antqradio at sbcglobal.net wrote:

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   Bill
     Which Transoceanic?  I checked the G500 and H500 schematics, which appear to be identical with respect to power supply, but didn't see a 20 ohm resistor associated with the Selenium rectifier.

  I see your point though, the TO is a much different radio then the AA5 AC/DC radios which I was initially  referencing.  Looking at the "H500 Restoration Guide" ( http://www.renovatedradios.com/articlePages/H500%20Restoration%20Guide.pdf ), John Kopp suggests, among other ideas, adding a 9.1 volt Zener from the junction of R23 and S3 to B- so that the filament string never sees anything above 9.1 volts when operated from the AC line.   
  The Zener diode was added after John had already replaced R21, R22 and R23 in, what seems to me, to be an  unfocused attempt to set the filament voltage across each tube to 1.3 volts.  He seemed to have replace those resistors because they were out of tolerance but nonetheless changed the  resistor values from those specified by Zenith.   
  Interesting to note that the 9.1 volt Zener will allow about 1.5 volts across each filament section at a  filament current of 50 mA.  But that said, the Zener diode is much saver option then adding a few ohms to the 2k or so ohms already in the  power rectifier circuit.  This in an attempt to drop the B+ supply a bit when changing from Selenium to Silicon to power the filament  string when operating from the AC line. 
  Again, the filament voltage is much more important to tube life than a somewhat higher B+, (see http://www.bext.com/eimac2.gif )  note that this graph is for power tubes but it is still applicable.  It would be prudent to also add a 24 ohm, 1/4 watt resistor between S3 pin 5 and the junction of R19 and  pin 7 of the 3V4.  This would drop about 1.2 volts and bring the filament string voltage to about 1.3 volts across each tube, assuming  equal voltage drop across each tube filament section.  This will be helpful whether operating from AC line or 9 volt battery.  A lot less work and a better outcome then changing all of those power resistors willy-nilly. Jim 

          From: David Sica <dave.sica at njarc.org>
 To: William Zukowski <n2yeg at optonline.net>; Jim Whartenby <antqradio at sbcglobal.net> 
 Sent: Saturday, April 16, 2016 4:04 PM
 Subject: Re: [NJARC] Selenium Rectifier replacement

   Thanks Jim, and Bill.  I wish this email reflector had a "like" button as I always appreciate hearing all the  knowledgeable opinions.  
  -- Dave  

 New Jersey Antique Radio Club      
 On Sat, Apr 16, 2016 at 2:43 PM, William Zukowski <n2yeg at optonline.net> wrote:

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   I replaced a selenium rectifier in a TO with a  1N4007 and had to increase the series resistor from 20 ohms to  47.  The series filaments are fed from the rectified DC, and  the 1 volt tubes needed the additional resistance get them to the normal 1.3 volts.  So yes the resistor is necessary.

 Sent from my iPhone 
 On Apr 15, 2016, at 17:38, Sal Brisindi via  NJARC <njarc at mailman.qth.net> wrote:

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   You should put a resistor in series with the silicon diode as the B+ will be higher when replacing the  selenium diode.

 Sal   From: Matt Reynolds
 Sent: ‎4/‎15/‎2016 5:10 PM
 To: NJARC Reflector
 Subject: Re: [NJARC] Selenium Rectifier replacement

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    I'm probably the least knowledgeable\qualified  person on this thread thus far when it comes to radio repair but I was always under the impression the  resistor was added to the diode to help account for the  differences in how the selenium rectifier works\"acts" in relation to the silicon diode  replacement.  It was my understanding the resistor was not added  to protect the diode, but the resistor was to protect "the radio" from being "harmed" by the different  characteristics of the silicon replacement.  I don't ever think I've heard of fear of  the modern part being damaged/burnt out.

 Matt

________________________________________
 From: NJARC <njarc-bounces at mailman.qth.net> on behalf of Raymond F Chase <raydio862 at verizon.net>
 Sent: Friday, April 15, 2016 4:35 PM
 To: 'David Sica'; 'Jim Whartenby'; njarc at mailman.qth.net
 Subject: Re: [NJARC] Selenium Rectifier  replacement

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 The predominant failure modes for  silicon diodes are: 1. Overheating the
 junction by gross current overloads and  2: Exceeding the peak inverse
 voltage (PIV) rating.  Silicon diodes with 10 times the expected  current
 rating and several times the PIV cost  pennies so they will be the last to
 fail.
 Ray

 -----Original Message-----
 From: NJARC [mailto:njarc-bounces at mailman.qth.net] On Behalf Of David Sica
 Sent: Friday, April 15, 2016 4:03 PM
 To: Jim Whartenby <antqradio at sbcglobal.net>; njarc at mailman.qth.net
 Subject: Re: [NJARC] Selenium Rectifier  replacement

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