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Regarding the open end of the rheostat, that should be to A+, normally. On the AK 20 I have on the bench one end of the wire had detached. There was enough in the winding that I could carefully pull out 1/4" and solder that to the lead.
One of the rheostats controls the filament for detectors and the other for amplification. Depending on signal strength you may have to adjust one or both for best results. If one of them is open then some tubes won't light at all. If you needed to temporarily you could run a jumper and run all filaments off of the one good rheostat. For that matter, you could just run 6v direct to the filaments, for testing purposes.
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Bryan
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