Ed, your ballast may be alright if you still hear the local BC.
You say R390A in your first post, but then R390 in the second.
Very different beasts, do not abbreviate as a nickname.
Maybe a stage tube has gone out. The 6DC6 in there seem to go soft, but gradually. So does the 6AK6, and there's only one of that type in there.
If tubes all check good, try swapping like tubes into different sockets. This may reveal something the tube checker does not. Also sounds like you might want to check the internal TR relay for dirty contacts, and don't forget the fuse that protects against a destructive RF input to the receiver. (or is that only on my R390, I forget)
A quick RF check is to use the threaded C chassis connector rather than the twinax to see if it restores signal.
As for the ballast, you wouldn't hear anything except hiss from the audio stages if the PTO fils were dark.
Frank that's good on switching to 12V BA's, why didn't I think of that.
Ed, if you get stuck, Chuck WA4HHG is an old friend of mine and perhaps can talk you through some areas to explore:
http://www.r390a.com/html/technical.htm