You might have had a poor contact on a critical contact on the tube pins and the socket, but your description is also consistent with a shorted tube also. Check the bias on that tube. If it is still showing proper bias then every thing is probably well. If not you may have a damaged resistor in the bias circuit. FWIW the KT77's were discussed at some length on AA tubes forum. It seems that there have been a lot of them with undersized pins - you might do a little research on this forum for the background on this problem.
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