Zenith C845 - FM works, no AM

[Jeffhs]

I replaced the 12AT7 converter tube in my Zenith C845 AM-FM radio. Now I get FM (all local stations in Cleveland and many distant ones), but almost nothing on AM. I say "almost" nothing because I can hear a local station (however, the sound from that station is very weak and garbled), but nothing else. I also replaced the AM loop antenna with one I salvaged from a Zenith MJ-1035; no luck. I am sure the loop is connected properly (high side to the tuning capacitor, low side to the feedthrough capacitor terminal on the chassis). I considered replacing the 6BJ6 RF amplifier tube with a 12BA6, but now I'm not sure I want to do that after reading that this is not a good substitution--the reception on both bands could be worse than it is now if I tried it (as someone in this forum, or maybe it was on ARF, mentioned recently). Because the radio receives the local AM station (about two miles away) at all, I am sure the AM section must be working, if only just barely. Where could the problem be hiding? If it is anything worse than a bad tube, I am not going to bother with it, as AM radio in this area isn't worth listening to anymore--except possibly for AM 740 in Toronto.

Something else puzzles me about this radio. The volume control works normally, but I can still hear some sound from the speaker when the control is set at minimum. I read somewhere, again I think it may have been here or on ARF, that this can be caused by excessive B-plus voltage due to the selenium rectifier having been replaced with a 1N4007 silicon diode, but no dropping resistor. The selenium stack in this radio is probably as old (53 years) as the radio itself, so I'm thinking it may have been replaced some time before I got the set.

Thanks for any and all replies.

73 (best regards in ham radio talk),