I have a Silvertone model 4 that I am working on for my girlfriend's father. It was totally dead when I started on it. The schematics call for a 50B5 tube, but the radio fires right up when I replaced it with a 50C5. I seem to recall reading that this was the correct tube to use as to meet UL requirements, the 50B5 was replaced with the 50C5. Is that correct? Also I changed all caps but the radio has a slight distortion after it plays for about 5 minutes. What could be causing that? Any input would be appreciated.

A 50C5 is not a sub for a 50B5. Different pin out. See this link.

http://www.nj7p.org/Tube4.php?tube=50C5

After look at a 50B5. You can wire the set for a 50C5 but I would not advise it.

If the radio is working, most likely the 50C5 is supposed to be in the radio. Radio manufacturers made slight changes along the way regarding this and the 50B5. This has been noted elsewhere in this forum. If one tube was swapped for the other, you would not get full performance, and you might just smoke the cathode resistor or burn something else up.

Regarding distortion, if you are sure that you replaced all capacitors with new units (most importantly audio bypass capacitors), you may have some other problem such as a partial grid short that increases as the tube warms, or a gassy 50C5. Given the extreme temperatures of this tube and the cramped spaces within, it isn't too uncommon to run across one with a thermal grid short problem. This can be indicated by an individual element shorts test with the heater lit, if your tube tester allows for this. This can be corrected (if you know how to) by blasting the offending grid with short bursts of high current.

Both a shorting grid and a gassy tube can be revealed by noting voltage across the cathode bias resistor when the radio is first turned on, and after the distortion begins. If the voltage goes up, the tube should be swapped for another known (hopefully) good one. Be sure to use a 50C5 if the radio is currently using a 50C5.

You might also check resistor values, most importantly in the output section. If they are rather high (above 20%), this may allow the tube to run away with current.

T.

Going along with Thomas about this. If it did have a 50B5 before, it would not play at all with a 50C5. It may be modified to use a 50C5 or factory production change.