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A few years ago, I fired up a 14" DuMont portable to find that the tube had almost zero emission... in a dark room a very faint fuzzy picture could be observed. On a tube checker the needle did not move, even when the heater was cranked up to 8 and 10 volts. At 10 volts, I could occasionally get "normal" emission for a bit by tapping on the neck... hmmmm loose cathode connection. I tried re-soldering the base pins several times to no avail. Next, I tried to re-weld the cathode connection by discharging a capacitor charged to a few hundred volts between the cathode lead and the grid and anode tied together (with the heater turned up to 8 or 10volts)... No joy there either. I replaced the tube.
Later, I autopsied the tube, a DuMont 14SP4 and found the worst looking carbonized weld that I have ever seen at the connection between the cathode tab and the base pin... Somebody must have been having a bad day at the old DuMont factory.
jr
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02-22-2015, 11:39 PM
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