by amandarae » Sat Nov 13, 2004 4:41 pm
In addition to the excellent suggestion above, if all you tried does not eliminate nor moved the sound, then it has to be the quad filter caps which un-modded Dynaco ST-70 is notorious for. A weak section of the caps will almost certain to introduced hum to a certain channel where it is being use.
In the event that the filter cap was already replaced or modded, but the proper star grounding was not cautiously observed, hum of the nature you described will emerge periodically or sometimes permanently.
Also, make sure that the rectifier (5AR4) tube is not faulty. But of course, try to isolate if it is really the amp giving the problem and not other component.
Goodluck!