[Bill Baker]

Okay, I have an odd one that I cannot figure out. I had a customer call me tonight with a wierd experience.

 He has a set of tube mono blocs. They are sitting in a rack, side by side, and are about 4 feet off the ground.
 After having the amps on for about 2 hours today, he noticed a "bbzzzzz" noise coming from the rear of one amp, near the IEC unit, when he would very lightly tap his foot on the floor 8 feet away.

 He then turned them off and a half hour later, he would still get the noise from it even with the amp off.

 He then removed the power cord from the amp and the noise would fade away getting lighter as he taped on ANYTHING in the room.

 Anything? This one has me stumped.

[Bill Baker]

Two other factors he mentioned.

 The amps have been running fine foe nearly 2 months.

 There is a TV on the shelf above the amps, about 6 inches and today was the first time he ever had teh TV and amps on at the same time. The TV does have a lot of static in it as it will zap his fingure from about an inch away from the screen.

[Lensman]

From the clues provided, I'm inclined to think it may be mechanical vibration rather than an electrical problem (static, etc).
I would ask him if he can consistently induce or alter the buzzing sound by stomping his foot or tapping on other surfaces in the room. 
There may be something in or near the amp that is creating a sympathetic vibration, in much the same way as you can induce vibration in a guitar string by holding a vibrating tuning fork near it. 

What a mystery.

Lensman

[Lensman]

From the clues provided, I'm inclined to think it may be mechanical vibration rather than an electrical problem (static, etc).
I would ask him if he can consistently induce or alter the buzzing sound by stomping his foot or tapping on other surfaces in the room. 
There may be something in or near the amp that is creating a sympathetic vibration, in much the same way as you can induce vibration in a guitar string by holding a vibrating tuning fork near it. 

What a mystery.

Lensman

And I should add...If it is mechanical and not electrical in nature, he would still need to find the energy source:  a washing machine running, computer case w/fan on or near the rack, you(!), etc.  Anything that might be producing periodic vibrations, even if he doesn't sense them himself. What is different today from the past when you didn't have this problem?


Lensman
Lensman

[Bill Baker]

Thanks for the thoughts Lensman. The amps run beautifully without noise. The noise occured only when he taps an anything in the room. Even if it's several feet away.
 He could tap the floor 8 feet away very lightly with his foot causing the noise.

 He says the only thing different this time is having the TV on (again, sit's only 5-6 inches above the amps), I have never heard of this happening but.............

 The amps haven't been moved since setup up. I cannot imagine it being an electrical issue inside the amp as it does not effect the functionality of the amp at all. No noise through the speakers either.

[Lensman]

Bill,
If it were me, I'd induce the buzzing sound (as he is apparently able to do), and then press a finger on the amp chassis.  Does the sound stop?  Does it change? 
I would also try putting something like a folded towel between the rack and the amp to see if that stops the buzz.  I'm thinking one of the rear amp feet is not making contact with the rack and the vibration that he induces causes the amp to rock.  Is it a glass shelf?  Are the feet cones? Hmm.

Lensman

[Bob in St. Louis]

Bill, ditto to what Lensman said. Isolation between the amp and the rack.
Maybe some squishy rubber feet.
Have an assistant taping their feet on the floor while he's fingering the tubes/chassis should quickly find the culprit. Maybe just a loose screw/nut on the chassis causing the buzz??

Bob

[Bill Baker]

 I thought about these ideas but he was concerned about the chassis being "hot" and getting shocked.
 I will have him reposition it as well as put something under the amp. It has stock feet.
 Thanks again for the participation.

[JoshK]

Does the TV have a lamp chord 2 prong plug or earthed 3 prong plug?  Did he use any cheaters anywhere?   Can he try moving the TV out of the system to see if it is something with the TV being introduced?   If the amp is unplugged and still makes noise, is the TV unplugged too?  I am just wondering if the TV's tx is causing some EMF reaction in the iron in the tube amp.  Kind of like how unshielded speakers react with a TV. 

Does he live in a cold & dry climate?  Those climates tend to have a lot of static electricity build up in the winter which could be exacerbating the issue if it were EMF related. 

My guess is he has a number of smaller issues leading to the affect.  Rack isolation, EMF isolation, static, etc. 

[Bill Baker]

Does the TV have a lamp chord 2 prong plug or earthed 3 prong plug?  Did he use any cheaters anywhere?   Can he try moving the TV out of the system to see if it is something with the TV being introduced?   If the amp is unplugged and still makes noise, is the TV unplugged too?  I am just wondering if the TV's tx is causing some EMF reaction in the iron in the tube amp.  Kind of like how unshielded speakers react with a TV. 

The TV is an older tube TV so I would imagine it is a 2 prong. When he was experiencing all this, the TV was on. I am starting to wonder if the TV is not causing problems with the mains transformer as it is in this area where the noise is coming from.

 I have never come across what he is describing.

Does he live in a cold & dry climate?  Those climates tend to have a lot of static electricity build up in the winter which could be exacerbating the issue if it were EMF related.

 

 He lives out in the country and we do usually have very dry houses due to gas heat. However, the past few days it has been raining so I do not think this is the problem at the moment.

 It's hard diagnosing a problem over the phone without being able to experience the noise first hand.

[JoshK]

It's hard diagnosing a problem over the phone without being able to experience the noise first hand.

Amen to that!  This is why I am just asking questions and trying to eliminate plausibilities.  I would start with eliminating the TV as the problem since it is the *most* likely possibility given your description of events.

I asked if it were two prong because I wondered if the insulation was wearing down somehow creating EMF.  I think what you described with the TV & the mains tx is a likely suspect, so control for that first.

[Bill Baker]

I just got off the phone with him talking about some other ideas to try. He did indicate that when he pressed down on the mains transformer cover, the noise would go away indicating that we do have a mechanical vibration being cause, I suspect, by interaction between the TV and mains transformer. I would suspect this would excite the chassis as well as everything inside. Even the slightest loose piece of solder would then resonate at a very high rate of speed causing such a noise.

 He is leaving the TV off for the day and will let me know more this evening.

[Bill Baker]

Latest update. He turned the TV off and left it off last night. Today he has been running the amps all day with no recurrence of the problem.
 Looks like it was an interaction between the TV and the amp's transformer.

 Thanks again to everyone who chimed in here. Greatly appreciate your thoughts.