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Are you SURE the tube is bad? Likely it is just ASLEEP...after many decades of non-use. Unless it has gone to air--I would NOT give up on the tube yet. Give the fil. about 7.5 volts for maybe 10 mins...and see if that wakes it up some.
I have repeatedly stated and will continue to do so...at LEAST 2/3 of old CRT's I come across are at least uesable. This includes a 7JP4 tube, and a predicta 21" tube I recently acquired. It is not perfect....but IS useable. both emissions and cutoff are good--after a wake-up. lfe test does NOT hold long though...but that may improve after time.....and good 21" predicta tubes do NOT come along a dime-a-dozen.
They are NOT makin 'em anymore--NOR rebuliding them either--AFAIK--so we MUST do our very best--to use the CRT'S we come across.
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