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Unless you're concerned about 'absolute' authenticity, and still leery about running the 6W4 without heater isolation, why not run a 6AX4 on the common heater supply? it has the same basing and heater current (1.2A) as the 6W4.
If still concerned about H-K breakdown, put a fuse in the B+ line to the damper.
6AU4 is a little beefier, has the same basing, but higher H current (1.8A).
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