[Texasrocker]

A good friend wants me to restore his childhood era family 1959 Zenith mod G 730 ( chassis 7C05). I dont have a schematic, but really dont need one, as these are fairly straight ahead circuits. I have it fully recapped & working again on both AM & FM. Getting low voltages from the old orig selenium rectifier. Its been quiet awhile since I had to replace a old selenium rectifier with a diode/ resistor sub. Seems if I remember correctly, I used a 1N-4007 type diode with a 50 ohm 5w wirewound resistor in series last time I had to replace one to get the voltage back up? This is a old half-wave rectifier with "paint-bubbling" on the plates.Its needs changing. This circuit is a ac/dc type 7-tuber ( all miniature tubes, usual compliment for AM/FM receivers of that era). One side on the ac cord connects to pin # 3 of the 35C5 audio tube, runs to the (-) negative side of the old selenium rect with a old bumblebee cap to chassis grnd. I figure a .01/600 will work there. The (+) side of selenium rect feeds a 22 ohm 1-watt carbon then to one side (40uf/160v) section of the B+.
I have all the parts here if the 1n4007 works for this, and if I remember correctly, I can remove the old selenium assy, mount a terminal-strip in its place, add the (-) side of ac line to one end of the 50 ohm resistor, soldered in series to the negative end of a 1N4007, then + side of diode to the 22 ohm that feeds the B+. Does this sound correct? Thanks in advance for refreshing my memory!I can bring the set up slow with my variac & check filament voltages after I swap out the parts & see if the tubes are operating at normal current ratings. Any suggestions would be appreciated!! Im getting absent minded in my older age!! Randal

[codefox1]

Last time I changed out a selenium I did just what you did but I used 100 ohm 5 watt instead of 50 ohms. If the capacitance of the filter cap replacements you used were larger than the originals, you might have to go higher on the resistance value to keep the B+ under control. I personally wouldn't want to see anything above 130 volts plate voltage on the 35C5, or whatever you have in your set. I'd also replace that 22 ohm "fuse" with a new one. They tend to become unreliable because of decades of heat.

Bring the set up slowly with the variac and monitorthe B+. If it is too high, add more resistance, and of course vise versa.

As an aside, I have a Philco 50-620 on the bench now with the same problem. The B+ also feeds the filament string I'm going to be a lot more careful since this design feeds the delicate 1.4 volt filaments. With new filter caps, and the higher mains voltage we commonly now see, these sets need to be tamed down. Even with the tube rectifier versions of portables, the filaments are usually way over stressed.

[Texasrocker]

Thanks very much Codefox! Your info has been most helpful! I will proceed with the replacement, and monitor voltages carefully when bringing the set back up on todays higher ac line current! I will go ahead and use the 100 ohm resistor you suggested, instead of the 50 ohm. Thanks again for refreshing my memory!! Sincerely, Randal

[FStephenMasek]

I did not know that a resistor was needed. I have replaced the selenium rectifiers in three radios with diodes, so will have to go back and measure B+ to determine how to proceed.

[Arran]

With replacing a selenium in a portable you do need something to drop the voltage, in other sets where the selenium was strictly for B+ I sometimes never bothered with the dropping resistor, the tubes usually drag the B+ down far enough and the seleniums were already instant on so the circuitry was designed around this. With regards to the size of the filter condensers there seems to be a law of diminishing returns in terms of larger capacities boosting voltage, going from a 2mf to a 4mf causes a boost, as does going from a 4mf to an 8mf, but when they get larger 8mf, 10mf, etc it doesn't seem to matter so much. I have an AC5 (Canadian AC/DC) which has a pair of 160mf filter condensers in place of 40 or 50mf ones, the B+ is dead on to where it should be, according to the schematic.
Best Regards
Arran

[37-670]

Thought I'd jump in with a question of my own on the same relative subject. I will be replacing a tired copper oxide 1/2 wave rectifier on my Triplett 3413A emissions tester with a 1N4001 diode. Am I correct in assuming that I won't need a voltage dropping resistor as the B+ is fed to the meter and the overall voltage is controlled by the "line adjust" wirewound pot?

[codefox1]

The line adjust pot may not give you enough range. I think you will still need a series resistor. I'd start with 50 ohms, and measure what you get. If you're low, change to 22 ohms, if high, try 100 ohms. Somewhere in there you will come to the sweet spot.

[exray]

I'm not familiar with the device so I can only guess that changing the line adjust is also going to affect the filament voltage. Therefore you'd want to have the B+ be correct at the corresponding filament setting. If you have the manual the correct B+ should be indicated. May or may not be terribly critical.