Photosensitive tubes

[Bolivar]

I have a Cifte 12au7 tube that reacts to a specific led light when used in my bottlehead crack. When I shine the light on the tube I can hear a sound through my headphones. It sounds like a fairly pure tone. Anyone else come across tube behaviour like this? I'm no expert on tubes and have no idea how common this is. I don't have any other equipment to connect it to and it's the only tube that acts this way.

[Laudanum]

Maybe it's possessed  ...  J/K

[porcupunctis]

Tell me about the red light.  How is it powered?  Is it LED, compact flourescent, incandescent, neon, Xenon?  How close does it need to be?  Is it a high tone or a low tone?

My thinking is that it isn't the light but some electrical noise that the light source is producing.

[Bolivar]

It's actually not a red light but a LED light. It's a light on my phone that acts as a camera flash/ flashlight. At first I thought it would be interference from the phone, but if I hold the phone next to the tube facing the other way or just hold my finger in front of the light, no sound is emitted. No electromagnetic interference would be that uni-directional(well apart from the light of course).

The sound itself seems to be somewhere around 500hz and it changes from the left to the right channel depending on which side I point the light. The sound becomes audible with the light source at about 20cm or so.

[Grainger49]

Randall is on the right path with his questions.  He is asking about other known interferences that cause tube noise.

Phones often cause interference with tubes if they are too close.  The same can be said of many computers.  Can you try the phone without shining the light?  That might nail it down.

[Bolivar]

The phone with no light on doesn't cause any sounds no matter how close it is or whichever way it's oriented. Data transmissions to/from the phone do cause that pulsating noise we all should be familiar with. But during these tests I put the phone on airplane mode. I think I've exhausted all other possibilites, the light has to be the cause.

[Grainger49]

Hmmmmm....  Interesting!

[elcraigo]

500 Hz is 2 mS -- if the LED is pulsed ON/OFF at appx 2 mS the LED would appear to be on continuously.
It is likely the LED is pulsed to save battery power. That could explain the 500 Hz.
If you have a LED flashlight, with a similar color light as your phone, try that.
The flashlight likely is not pulsed, and should not make a sound.

[Bolivar]

Good idea about the pulsing! Indeed my other led flashlights make no sound.

[elcraigo]

The cycling On/Off of a LED in a product that has some kind of smarts (a CPU or such), like a cell phone is a very common design technique. It a software/firmware solution to save battery power. Adjusting the frequency of the pulsing is a way to adjust brightness also (to a limit), or pulse slowly to indicate an error or alert.

[Grainger49]

The pulsing is what allows the range to be found.  Of course, a flashlight doesn't need range, so no pulsing.  I'm guessing it is Doppler effect, the same as RADAR and LASER speed indicators.

[elcraigo]

I didn't think of the camera flash working better if the distance to subject was known, but for a plain ol' light source just to save power. Killing two birds with one stone so to say. Wonder how much battery life you would save if a flashlight had a simple circuit to pulse the LED? Or would the power required by the circuit to save power be a break even?

[Grainger49]

Craig,

You got me there.  With the current that an LED draws I would guess, and it is only a guess, that the pulse circuit would wipe out any savings since it would always be on.

[2wo]

You see a pulse circuit used in things like car tail lights. You get a bright light and limit the duty cycle to control the heat...John

[porcupunctis]

Spock would say "Fascinating!"

If you have a camera or flash unit with a range finding red LED, you might be able to replicate the effect and confirm (to some degree) the pulse hypothesis.

This is definitely worth experimenting with.