[NJARC] Fw: Selenium Rectifier replacement

[Robert Bennett]

Jim -
        I think Zenith may have been concerned with varying amounts of house
voltage and current (my guess) because if you look at a schematic for a
miniature tubed T.O., some noted the voltage after the rectifier was 105
volts. Next the 130 resistor drops it more. Then if you really wanted to get
weird, zenith would use that .047uf bumblebee cap (that I've seen blow up)
beyond that. I've done at least a dozen or more of these radios and I think
that only once there was a problem with the multi-section 950 ohm resistor
riveted to the chassis. Yeah the battery new will reach 9 volts, but
remember a battery will not have severe possible voltage spikes like when
using home power. I have a homebrew battery pack I made several years ago
and I use 6 "D" for the "A" or 9 volts, and 60 "AA" batteries for the "B"
side. I usually use used flashlight batteries so I'm maybe between 8.4 and 9
volts. It will run any of my T.O's conservatively 40-45 hours before the "A"
side gets down to around 7.2-7.4 volts. There's a seller on eBay that makes
a power pack to run your T.O off DC, rather than using what came from zenith
but never tried one (I'm too cheap).
 
 Bob B.
 
BobOK, how does one not exceed 8.4 volts applied to the tube filaments when
the TO battery pack has an A battery value of 9 volts (or higher when the
battery pack was brand spanking new)?
Getting back to the original discussion, adding another 25 to 50 ohms to
make up for the Selenium stack internal resistance does not significantly
change B+ voltage.  In the TO, that 25 to 50 ohms is in well within the
tolerance of the 2000 or so total ohms that drop the B+ to the A+ level
needed for the TO tube filaments.  
Actually the three resistors in the filament circuit are 130 ohms (10%) +
950 ohms + 950 ohms (unknown tolerance) for a total of 2030 ohms give or
take.  So the internal resistance of the Selenium stack is about 2% of the
total dropping resistance at face value without considering resistor
tolerance.
Adding that little bit of resistance might work for one specific line
voltage but there is no guarantee that the line voltage won't vary day to
day or hour to hour.  So what does one do then?
Funny, as you imply, NOS 1L6's are worth much more than a TO in average
condition!Jim
 
      From: Robert Bennett <dino66 at optonline.net
<http://mailman.qth.net/mailman/listinfo/njarc> >
 To: antqradio at sbcglobal.net
<http://mailman.qth.net/mailman/listinfo/njarc> ; njarc at mailman.qth.net
<http://mailman.qth.net/mailman/listinfo/njarc>  
 Sent: Sunday, April 17, 2016 5:45 PM
 Subject: RE: [NJARC] Fw: Selenium Rectifier replacement
   
When you replace the selenium rectifier in a trans oceanic, there was
usually a 130 ohm 3 watt resistor in line to drop the voltage. Regardless of
which type of diode you change to, the focus needs to be on the filament
string or "A" voltage. Zenith specification was not exceed 8.4 volts. You
may want to install a 1R5 installed in the socket of the 1L6 unless you have
a bunch of them or "Bill Gates" funding. I've used varying resistances of
3-5 watt resistors for the drop after replacement. 

 



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