V5, a 12AU7, is the output amplifier stage. This tube draws a lot of current and, as a result, the heat generated (155 degrees F on the glass) has cracked the original socket and scorched the Repro circuit board. Click on the pictures to see better detail. By the way, the lint in the socket is from a Q-Tip used for cleaning.
Tube Socket Replacement
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The way to stop this damage from getting worse is to replace the original socket with one made of ceramic.
Click on pictures to enlarge