[NJARC] Fw: Selenium Rectifier replacement

Sun Apr 17 15:30:26 EDT 2016

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.

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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.

SalFrom: 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

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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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