Just noticed a mild humming in headset when S.E.X. 3 C4S is off but not when on

[12AU7-6SN7]

The used, professionally built amp was shipped across the states about a month and a half ago. Not sure I ever really intently listened to the amp with the headset when the amp was off since I normally don't wear the headset when it is powering on (tubes zapping on and such.)  I can't say if this is new or not, but I did notice it after moving from one AC outlet to another in a different room where it seems to be worst after trying a few different circuits in the house.

I just checked if the noise was there with or without an ac powered DAC or a non plugged in phone to isolate and it seemed to stay no matter what was or wasn't plugged in. The noise disappears a few seconds after hitting the power switch as soon as the tube heaters fully heat up and the tubes power on to make clean sound in both channels.

No humming noise while listening to music or when paused. Edit: It doesn't seem to be there now, for a few minutes so far after powering down. edit: now hearing very short buzz pulses every once in awhile. edit 3, a few minutes later it seems to be there when amp is off, but now so faintly that I have to have the HD650 on my ears completely sealed and pads not resting on the ear lobes at all.

Could switching headsets with the amp on but turned down all the way have done any harm? I had read this was ok somewhere.

Amp is wired for 4 ohms.

I appreciate any insight on the subject

[Doc B.]

Sounds like the amp is working as designed. A lot of tube amps hum until the heaters warm up and the tubes start conducting.

[12AU7-6SN7]

That's good, it just spooked me since I never noticed it before. It buzzes even more after putting it back in the original room it was in. Touching the transformers seems to mute it. Again, this is when completely off and not being powered on in any way. Just to verify.

If everything is working as normal I guess there is nothing to worry about. Thanks for the quick reply.

[Doc B.]

You may have issues with noise on your house wiring if grounded equipment is buzzing when turned off.  The pulsing noise description sounds like GSM buzz from a cell phone.

[12AU7-6SN7]

The noise is constant in the original room where I had never noticed a problem before. It seemed to have stopped right after powering it off after being on while set atop the dining room table and had a few pulses afterwards, but not patterned like cell interference. It was constant when off before I turned it on at that test location too.

I guess I will pack it up and try it elsewhere later tonight.

[12AU7-6SN7]

The buzz seems to be gone at another location with different wiring.

 I wonder if it is something new as far as interference or if it was something that was occurring unnoticed. Weird stuff.

I will keep an eye on it here... the wiring very well could be it since the wiring at the initial house is old; I just find it odd I never noticed it for a month and a half until moving the amp to a different room. Who knows if it is a new gremlin or old.

Thanks for your help Doc

[2wo]

I use a Powervar isolation transformer in my system, mostly because it was free and has enough outlets on it. Whenever there is a D.C. Conponet on the incoming AC line the transformer  moans and complains. It can get pretty  loud.  Fortunately it usually only last a couple minutes ...John