The tube fuse on the amps blowing is indicative of a bad tube, not of a problem with the amps. I used to have a few go as my tubes grew older or went ballistic.
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It is a tube failure, but the original JA-80, which I owned, was VERY hard on tubes because of the high bias current. The original MO Valve KT-88 could last several years, but other tubes would not. The secret is to lower the bias by increasing the value of the cathode resistor (the output stage is cathode biased, not fixed bias).
Hi, It should be a dark green 150R Resistor across the base of the tube
Just take bottom cover off and you'll see it--replace and test--though it sounds like a dicky tube, but I have replaced them and the units operate OK.
So give it ago it is easy to resolder plenty of empty real estate in their!
Agree with Gmuffley about the high bias and tube failures. The old MO Valve and GEC KT88s lasted well in them, but every 6550 tube I tried started going bad within a year, and EL34s about the same. Too bad I didn't know about the trick with the resistors to lower the bias, might have saved me some money on retubing.