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I replace the seleniums with a silicon diode with a 100-ohm 10 watt resistor ahead of it. The silicon has less resistance than the selenium diode so the resistor helps to keep the dc output voltage at the same level. The later zenith tube-type AM-FM sets do use a silicon diode. Make sure the selenium is disconnected out of the circuit when you install the silicon. The seleniums tend to develop more internal resistance and will drop the B+ voltage. They can also short out by themselves.
One time when I was 10 yrs. old or so I had a Zenith AM-FM where someone had installed a headphone jack. The jack came loose and fell on the selenium and cause a short. The smell and smoke is awful, it is said to be toxic also. Still have the radio but never got around to fixing it yet!
The 35C5 produces a little less max. power than the 50C5, maybe cause the filament doesn't have as much power. The 35C5 can do 1.5 or 1.6 watts, 50C5 is 2 watts or a little more.
If you can't find the schematic I can check my collection of photofacts if you'd like.
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