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Unless you have a direct heater to cathode short of which a isolation transformer will cure, other type shorts are usually a minute speck of debris lodged in the gun assembly somewhere. Many times you can clear a short of this type by using a tesla coil "sparker" by running it around the pins on the base. In rebuilding tubes you would normally do this if a tube was showing signs of internal arcing due to impurities. (Not a continuous arcing, just a flash every now and then) Try sparking the pins and see if this will work. These types of tesla coil hand sparkers are normally used in vacuum work to look for leaks. This same type of device is also found in 1930-40's medical cases that had ultra violet tubes of various sizes to rub on your skin. If you have access to one, just stick a wire in there and run it around the pins.
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julian
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