Can you give a make and model number of the tube tester you used to test a 3-500z. Thanks. Now that we are beyond the tube being bad the next step would be to "at least" check the rectifier diodes and see if there is a shorted one. Very easy to do. Then the HV secondary can be easily isolated from the rectifier and then once again see if the fuse blows. If it does then probably a transformer short if not then probably a capacitor.