Need help finding cause of hum

Sounds like you've investigated pretty thoroughly and logically. But I'm not sure what is meant by this statement:

When I reversed speaker placement (Right to Left & Left to Right) - cables staying in place - the hum stayed on the right coming out of what had been the left channel speaker.

Does that mean that each cable remained connected to the speaker it was originally connected to, and you moved the left speaker and its cable to the right side of the room, and moved the right speaker and its cable to the left side of the room? Or does it mean that the cables remained on their original sides, but the speakers themselves were moved (with the cables being disconnected and then reconnected to the other speaker which was moved to that side)?

Also, I think that Sid's suggestion sounds well worth trying. And re your last question, I wouldn't worry about damage resulting from this hum. Whatever is causing it is producing the equivalent of a very low level signal, which would seem unlikely to be harmful in any way that I can envision.

Regards,
-- Al

Ghosthouse -- thanks for the clarification. You've gotten some good suggestions to pursue. I'll just add a couple of thoughts:

1)If the problem is caused by magnetic field induction, I doubt that it would be induction into the speaker cables, due to the low impedances which are present there (especially the amp output impedance, as well as the speaker load impedance).

It could, however, conceivably be magnetic field interaction directly into internal amp circuit points. It's probably worth trying to move the amp, even though it appears that nothing had changed in the house when the problem arose.

2) While you are pursuing Newbee's good suggestion, keep in mind that a very common cause of hum, assuming your interconnects are unbalanced rca's, results from the fact that the shields of those interconnects serve two functions. They serve as the return path for signal currents flowing between the connected components, and they also tie together the chassis of the connected components, which in turn are connected to ac safety ground (if the components have 3-prong plugs). AC safety ground, in turn, is connected to ac neutral back at the service panel in the house. What that all means is that if the ac safety ground potentials (voltages) to the two components are slightly different, unwanted currents will flow through the interconnect shields as a result (at the 60 Hz power line frequency and its harmonics), which the destination component will have no way of distinguishing from desired signal, thus resulting in hum.

The ways of minimizing that effect, if it is the cause of the problem, are to use interconnects with the lowest possible shield resistance (meaning they should be both short and high quality), and by keeping the safety grounds as common as possible (meaning don't have the power plugs isolated from each other by the power conditioner or by being plugged into ac outlets that are separated by significant wiring). You could also try isolating all of the safety grounds (other than the one for the amp, which should remain connected for safety purposes) using cheater plugs, or 3-prong to 2-prong adapters with the safety ground connection not connected.

There are advantages, as you realize, to having some degree of isolation between the ac to the amp and the ac to low-level components, and between digital and analog components, but that trades off against this increased possibility of hum. Trial-and-error is commonly the only way to optimize this trade-off.

Hope that helps,
-- Al

Al - Thanks for your reply. A lot to digest, but I'm thinking a ground loop isn't the cause. Wouldn't that cause hum in both channels? and at a higher volume? ALSO - I can hear the hum with only amp and speakers connected - no sources connected to the amp. Thanks again.

Both channels -- yes most likely. Higher volume -- not necessarily; it could be just about any level depending on the ground voltage differentials, impedance levels, etc.

Also, when you disconnect all the inputs it would be a good idea to put a pair of shorting plugs on the inputs. Otherwise your results may be confused by hum pickup from stray electromagnetic fields, to which the amp would be susceptible when the inputs are open and hence high impedance.

Basically, at this point I'd suggest disconnecting (and unplugging from the ac) everything but the amp and speakers, putting shorting plugs on the amp inputs, and plugging the amp directly into the wall, without the power conditioner. If you still hear the same symptoms, try moving the amp around physically. If the symptoms still remain the same, I would say that pretty much rules out everything other than the amp itself, even though the previous amp appeared to do the same thing.

And if that's how it turns out, I'd just leave it alone. As I said earlier, I can't envision any possibility of damage due to such a low level hum.

Hope that helps,
-- Al

Hi Dan -- That sounds like a tough one. If I understand correctly, the problem developed after a long period of use of exactly the same setup without any hum. Given that, and given the rest of your description, the one thing that comes to mind would be that the amp has developed a leakage path between ac neutral and chassis, within either a power transformer or a capacitor.

Not sure why only one channel would be affected, though, even if that theory is correct. But it seems to fit a lot of the other facts. Turning on the other amp would worsen the problem due to its current draw increasing the offset between ac neutral and ac safety ground (which presumably is tied to chassis). Lifting the ground on the humming amp would then let its chassis float to or toward the ac neutral potential.

Also, my understanding is that in general it is not a good idea to run a tube amp without a speaker load, which it sounds like you have done briefly while investigating the problem. It can result in higher than normal internal voltages, possibly over-stressing capacitors and other components.

Not sure what else to suggest.

Regards,
-- Al