[grandntl]

Hi everyone, my 91-121 has a hum that occurs after tube warmup. It is not changed by the volume or tone control and is superimposed on the very slight hum at turn on. When I pull either one of the 42 output tubes, the hum disappears and replacing with other 42 tubes makes no difference. The radio was recapped and had many resistors replaced before I bought it  and the job looks well done. I tried reversing the field coil connections and the radio acted the same as if one tube was removed (no hum but lower volume). The field core wires were originally correctly connected according to the schematic: 
http://www.nostalgiaair.org/Resources/884/M0013884.htm   I can find no wiring errors and the only capacitor not replaced was the no. 47, 0.001mf that was bolted to the chassis by the speaker wires. I replaced it (connected to the 42 plates) with no effect. Where should I look for a solution? The sound is never distorted, just an excessive hum. Thank you.
Bob