Quote:
Originally Posted by jr_tech
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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Dumont CRT's were some of worst CRT's out there. Several different manufacturers used them and there were all junk. That is, the later production. Those old CRT's, from the late 40's or early 50's, seemed to last as long as the other makes.
