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The mains plug contacting the speaker terminals will definitely not be anything to do with it.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
What would cause the power valve to fail?
Things where the valve itself is not the cause:
Bias being too hot - common on old Marshalls when the bias caps have started to leak.
No screen resistors in the amp - not fitted on most 70s Marshalls.
Shock/extreme vibration, especially when the valve is hot.
Being used with an attenuator like a THD Hotplate which has the wrong impedance curve.
It's also possible that it's not a valve - a faulty impedance selector (also common on old Marshalls, if the original pull-out type is left in place), speaker cable or jack can also blow the HT fuse, as can an arc in the OT or a failing filter cap.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
But more likely just a valve. Get yourself a multimeter and learn how to bias it yourself, then you'll know how hot/cold it's been set.
Read up about biasing and give it a go. If the fuse blows again with the new valves you'll have ruled out that as the problem.