[Boatanchors] SB-220 with parasitics

[Keith Densmore]

Safe Holidays to Everyone,
I have agreed to look at a Heath SB-220 amp for a friend. The amp, tubes
look like new but one of the 3-500's has had a parasitic bang, at least
once and maybe twice. (On the same tube.)
I cannot find much info on the web but the question is, can such a tube be
reformed? Run the filament for some hours? Power it up with lower plate
and/or filament voltage? Any hope or is it dead?
If anyone has experience in this area, your advice is welcome.

Thanks,
Keith ve3gem