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Unplug the amplifier. Using a voltmeter, measure for the presence of DC voltage on every pin of the rectifier tube. If there is DC voltage on any pin, drain it away with a 10K reistor at the end of a clip lead, being sure you are insulated from the resistor and clip lead.
Pull out the rectifier tube and replace the fuse. Before replacing the rectifier tube, measure the resistance from the rectifier tube cathode pin to chassis ground. This should deflect the meter at first, but then get to a higher and higher resistance as the filter capacitors charge up from the meter. Once you get to over 50K or so, you can be sure that the filter caps are not dead shorted. If the resistance never comes up, check the filter caps.
If you're getting a reasonable resistance on the cathode, fire the amp up without the rectifier tube in place. If the fuse still blows, the rectifier tube was not the problem - keep going down the debug trail. If the fuse does not blow, replace the old rectifier tube with a new, known good rectifier tube. Try it again. If the fuse now blows, you have an overload somewhere on the load side of the rectifier, perhaps in the power filter caps, perhaps in the inductor or decoupling caps in the preamp.
The state of tubes being what it is today, NEVER trust a new Chinese or Russian tube until it has had some burnin time. Once the amp is working, let it idle for a few hours to see whether the new tube is going to go south, too.