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Drive cap needs to be fairly tight. Most of the change is going to happen when it is very tight t0 snug. That's only about a quarter to half a turn. The tighter it is, the less drive signal you have. Too much drive and the horizontal output stage is pushed too hard and you'll get distortions.
Have you checked/replaced the 68, 82, 100 and 1M resistors on the 6BG6?
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