I appreciate everyone's suggestions. Per Richard's great advice, I
verified that the NC-57 I own is indeed a NC-57B. Also, the slight
hum does change in volume. However, I noticed that my 6SL7 (AF amp-
BFO) was only slightly warm to the touch while the other tubes were
obviously quite hot after several minutes. Perhaps I have a filament
going out or the tube has failed. I'll replace the 6SL7 and update
the group. Hopefully this will work!

Chris, N4JOY