[guf]

I had my preamp open today and was using my headlamp to inspect some things. I thought noticed a noise when i moved my hand closer to the tubes. Which would make sense. It took me a few minutes to realize that it was my headlamp's spot light hitting the tube and making the sound. It was a higher pitch noise. It was really weird.

Why does this happen?  It was a mullard 12au7, but not the other tubes right next to it. This is not really a problem because I don't normally shine a spotlight on the tube. Just curious.

 

[bladesmith]

Light is a frequency. It's just very high in the spectrum. Only speculating when I say. It could be some kind of harmonic distortion developing, when the light frequency is combined with the audio frequency, very odd.

[FullRangeMan]

Vacuum tubes emit a large electrostatic field that exceeds the glass tube and this torch electric field connected with the valve electric field causing an audible result.
IMO

[avahifi]

I can see the effects of that headlight LED light on tube circuits on my test bench with the scope, especially with the headlight in the strobe mode.

It appears that an undocumented "feature" of vacuum tubes is their photo-receiver capability?

Does this mean for best results your audio system should be run in the dark?  And what then from the light from glowing tube heaters interfering with adjoining tubes and parts?  Build every separate tube into its own separate dark box?  The mind boggle!

Of course maybe this effect led to the development of IR senders and receivers in the good old days.

Frank Van Alstine

[Speedskater]

Some types of solid-state diodes are light sensitive.  But with an LED lamp, it may have a simple SMPS that is generating EMI/RFI.

[mgalusha]

Some types of solid-state diodes are light sensitive.  But with an LED lamp, it may have a simple SMPS that is generating EMI/RFI.

That was my first thought as well, some sort of nasty emissions from the LED lamp. May not be the case but would not be surprising.

[sebrof]

I'm NOT crazy! I knew my system sounded better in the dark 

[Roger A. Modjeski]

just for fun get the noise going and then put some opaque paper between the light and tube and see what happens.

It is possible that your headlamp has a switching regulator in it.

[avahifi]

My LED "headlight" is run from a battery pack so its not regulator noise.  I can shine the intense lamp on the circuit board and see the very low lever effect (maybe at a couple of millivolts out of the X10 gain stage) and just shine the light away from the circuit but at the same distance and see the noise effect stop.  Something in there is slightly photo sensitive, I am guessing a tube as there are no diodes or solid state devices in the vicinity of the light beam.

So, don't listen to music with a high intensity lamp shining right down on your equipment.  Pretty easy issue to deal with. 

Frank