Univox ; fuse issue

[Latole]

The story began here ; https://el34world.com/Forum/index.php?topic=31632.0

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Summary; Univox U 1246 head

New 6L6GC
50% ( and less ) bias and blown fuse.
Filter caps to be replaced?  They are original
Output transformer
Power transformer ?

The customer has always used an old guitar wire between this head and the cabinet.
Then he noticed that the wire was cut........

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The story in detail;

Used Univox amp only worked a few hours after purchase. The fuse blew.
A 6L6GC vintage short power tube caused the problem.

Put 2 JJ 6L6GC specified 30 watts on the Tubes Store
Did the bais at 50%; 22 ma / 716 vdc plate.
The amp worked at idle and playing for hours at a time at home; not at maximum volume.

The customer (once at home) played very loud, bridged channels and a fuzz pedal.
After 30 minutes, the fuse blew.

I went to his place; the bias had not moved.
Adjusted it to 35% cooler (?) 8 to 10 minutes later; fuse blew Still playing very loudly.
Put in other tubes; no change except that the fuse was blowing faster and faster.

What do you think ?
Replace filter caps and / or....?

[Latole]

I remind you that the amplifier works perfectly at "reasonable" volume.