by dcgillespie » Tue Apr 27, 2010 1:00 pm
Hi Derek --
Yeah, I kind of keep tabs on the Yahoo Forums, AK, and here. Lots of new folks getting into the "old" technology today and are still working up the learning curve. I was there as a young boy, and could only wish I had something like the internet when I had all the same questions so many years ago, so I try to help folks when I can now. I have all the 'bibles" of the industry that I learned from, but the internet is invaluable for those learning today.
So is this a new build that has had this problem from the start, or something that just developed in a unit that has otherwise worked great? That will determine a lot here. If it's new, could it be transformer orientation so that the power transformer is coupling into the left output transformer? I take it the heaters are balanced to ground with 100 ohm resistors (as opposed to 100K resistors, which will not allow a proper reference to ground), although I'm not clear on how the .1 caps are connected into the heater circuit. If that is the general scheme though, then that should not be the problem. A couple of other tests might help narrow things down as well:
1. Does the hum go away with the driver tube is removed? If not, then the problem is centered in the output stage, although that is unlikely.
2. Does the nature of the hum change when the input is shorted to ground? If so, and if this is a new build where the problem has always existed, the problem could possibly be a ground loop, although they are almost invariably a 120 Hz event.
Solving a problem like this starts with getting as many facts as possible, and then using a logical process of elimination to help pin-point the area to target. It will be interesting to know if moving the heater wiring makes a difference.
Dave