Bottlehead Forum
General Category => Technical topics => Topic started by: Bolivar on November 28, 2011, 12:03:45 PM
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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.
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Maybe it's possessed ... J/K
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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.
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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.
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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.
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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.
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Hmmmmm.... Interesting!
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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.
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Good idea about the pulsing! Indeed my other led flashlights make no sound.
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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.
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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.
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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?
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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.
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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
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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.
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I hope to remember this when I sit down for some listening today. I'll get it all on and then focus my camera at different tubes, one at a time. My Canon has a green focusing light.
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Ok,
I tried it with two cameras just now and both flashes do cause a sound, a short popping sound this time. One of the cameras has orange colored range finder led, but that didn't make any noise. Perhaps the led needs to be of the bright white variety.
I have a ne555 timer circuit connected to a red led, so I thought if I change the led to a white one and add a potentiometer to control the pulse time, I could perhaps create varying sound frequencies through the tube.
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I think it's hard to say if the camera flash interference was from light or the electromagnetic field or both.
With this in mind; is it the light or just the electromagnetic field?
Try optically filtering the light. Maybe dark paper.
555 timer and different light color:
Red to Blue would be pretty close to opposite ends of the visible spectrum.
If it really is just light; going just beyond visible, IR to UV could be interesting:
The data carrier frequency is probably too high to hear from an infra-red (IR) remote. I think the frequency is around 35 kHz.
If a Blue led works better than Red, then a ultraviolet diode could be interesting.
I have low power ultraviolet diodes at work (around 350-250 nm) I could bring home to try for grins.
OK that's why I listen in the dark - just kidding.
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Tried my camera on all 6 tubes, nothing.
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Ok, I did more experimenting.
I filtered the light with different colored paper. Results:
Plain white - sound still there
Red - nothing
Green - nothing
Blue - sound comes through, same as white paper
Inspired by this, I soldered a blue led to the timer circuit as I don't have any white ones. Success! Varying sound frequencies as I adjust the potentiometer. And this simple circuit should not be giving off any other electromagnetic interference like my phone possibly could have done.
As to why only this one tube makes noises, I heard an interesting theory. Perhaps there's a small amount of gas in the tube that is ionized by the light.
Maybe some you should get yourself these Cifte 12au7 tubes. Even if they don't all react to light, they're still very good sounding tubes : )
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I can't say I've had any experience with photosensitive twin triodes but I have with gas regulators (OD3).
I worked on a buddies amp which had a string of OD3's in the power supply. The last pair were difficult to "strike" since the voltage was marginal at that point. A trick that worked with them was to use a camera's flash on them. It worked about 90% of the time (every once in a while, you had to do it twice).
When I mentioned this in another forum, it was suggested that this was the result of something called the "Compton Effect".
Well, I ain't no freakin physicist but maybe some of you guys are.
All I know is it worked.
http://en.wikipedia.org/wiki/Compton_scattering
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kind of like kick starting a light-cycle
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My newest ebay purchase just arrived: 13 different 12au7 types. And I found another light sensitive one. A GE 5963 tube. The effect is quieter on this one, but still evident.
I can't be the the only one finding tubes like this. Unless I'm quite insane, of course. But the voices assure me that I'm not :)
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I find those voices can't be trusted. The music helps drown them out.
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It suddenly pop into my head, I work with a physicist (and I talk to him everyday) --- a duh-huh moment.
So I bounced this discussion off him. First thought was some kind of gas. He thought, yes, a tube like a OD3 would do this, that's easy to understand.
But, a tube that is supposed to be a plain ol' vacuum, was puzzling. I gave him some info on getter materials.
Comment was: "strontium, caesium and phosphorus are reactive especially phosphorus".
It popped into my head seeing blab about tube testing, and 'no gasses' as a good thing.
So, the guess is; the tube that reacts to light, has a higher level of gas in it. Maybe that is a indication of a the getter not doing all it's job?
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Well, there are photomultiplier tubes that are designed to respond to light. Photons are just radiation a bit higher up the scale than electrons. doesn't seem that farfetched to me.
Have you tried placing an opaque shield in front of the LED?...John
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Yes I did try filtering with some opaque/semi-transparent material. Similar result as with a white sheet of paper. Lowered amplitude, but sound still there.
I also finished testing all my new(old) tubes. Out of the 16 different driver tube choices I own, 5 show some reaction to light.
And I went out and got some IR and UV leds. Strong reaction to UV, none to IR.
My next idea at mad science is to build a pulse width modulating circuit and feed an audio signal through it. Music from light perhaps?
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OK -- going out on a limb here. I have a Physics degree, but I haven't done any experiments with this particular apparatus at all.
It is plausible for light to have some audible effect, particularly if the amplification factor is high. The relevant physical process is the Photoelectric Effect. Very well known and characterized -- a single photon can excite and dislodge a single electron, which is then effectively free to move under the influence of electric fields.
This effect will happen whether or not there is gas in the tube. What you're thinking about with a gas-filled tube, as in a Geiger-Muller tube, is in fact something different -- in a Geiger counter, you are relying on the photon (or other radiation) to ionize an inert, otherwise insulating gas between plates at very high voltage, and when it does so it briefly creates a circuit path through the gas. What happens here is not that. Here the photon would strike the cathode directly and dislodge electrons to create a current, and once the electron is free it doesn't care if it's in a gas or in vacuum; vacuum is probably slightly better.
Every material has a "cutoff frequency," i.e., a minimum photon energy below which no electrons will be emitted. Metals, in general, require fairly low energy -- light in the UV or even visible light are often enough to do it. Light in the IR range, probably not. The metal in vacuum tubes is also (a) heated, and (b) doped with metals to encourage electron liberation, both to encourage thermionic emission which is how tubes work in the first place. I'm going to guess (don't know for sure) that these steps also make it easier for the photoelectric effect to take place.
If electrons are being shaken loose by the photoelectric effect, they will create a current, as the free electrons will then proceed directly to the anode. Our tube systems are forward biased, creating an electric field which will always push them in that direction, so they really have nowhere else to go. So there will be an actual signal. How strong, I don't know, but it's not much of a stretch to think it could be audible.
Supposing this is at the heart of it, there are other factors that might amplify the effect. Like amplifiers, for instance. Or really sensitive speakers. There could also be things like instability in an amplifier downstream that magnify the effect, but I can't see this upsetting any decent amplifier, particularly a zero-feedback design at a good operating point. It's also possible that a photoelectric effect could create enough current for other effects, like microphonics, to become audible, particularly if those other effects on their own are not strong enough to start current flowing, but together they can.
If you want to try it further, here's a couple of experiments:
1. Does the sound vary with light intensity? Try holding the light closer and farther away.
2. Is there a "color" below which there suddenly is no sound? LEDs are pretty monochromatic but glass filters and such sometimes let weird frequencies through.
3. If you create a narrow beam of light, does the effect change if you shine it on different parts of the tube? Shining it on the anode should do nothing at all, shining it on the grid shouldn't have much effect, but shining on the cathode should have the greatest effect.
4. (Advanced) Does the effect change with heater current?
And finally,
5. If you're an audio purist and worried about light screwing up your music (!), tubes with dark glass, reflective interior, or screens should be immune. Try mechanical ways of blocking the light.
In my own system, I keep it all in a wooden cabinet, and I rig up LED power indicators always using deep, deep red just because I like them... but maybe they sound better, too...
Sounds like a fun experiment. All I can tell you is that physically it's plausible. I have no idea how strong the effect is under normal conditions, or whether it should be audible without going to great lengths to make it happen.
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I love this forum! I knew someone would come up with an answer. Armed with the terminology, I looked in Seely's "Electron tube circuits" and found the following on page 7:
"...For response over the entire visible region, 4000 to 8000 Angstroms, the work function of the photosensitive surface must be less than 1.54 volts."
On page 4, he give typical work functions of 1.0 volt for oxide cathodes, 2.63 volt for thoriated tungsten, and 4.52 volt for pure tungsten.
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Veeeery interesting...
2.63 volts times a single electron, i.e., 2.63 electron-volts (or 2.63 eV) corresponds to a wavelength of 470 nm, or deep blue visible light.
1.0 eV, for the oxide types, is about 1250 nm, or near-infrared.
The formula in general is E = h c / L, where E is photon energy, h is Planck's constant, c is the speed of light, and L is wavelength.
Still sounding pretty plausible.
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I can try some experiments in the Laser Lab at work. Before anyone asks, I will wear my laser goggles.
It will be UV laser diodes (351 nm). We might have other wave lengths in stock.
I can change or modulate frequency and pulse width. Also, adjust the power (low mW range).
I can try different glass filters also, along with different focusing optics.
I'll try it using SEX and Quickie amps, because they are easier to hook up. Think the tubes in these are good candidates?
I could try an old foreplay 1 - just not as easy to hear in the lab. Probably would hook an o-scope up instead.
I do have lots of 12AU7. I'm thinking RCA clear tops are a good choice, easier to get the light to the cathode?
1. Does the sound vary with light intensity? Try holding the light closer and farther away.
2. Is there a "color" below which there suddenly is no sound? LEDs are pretty monochromatic but glass filters and such sometimes let weird frequencies through.
3. If you create a narrow beam of light, does the effect change if you shine it on different parts of the tube? Shining it on the anode should do nothing at all, shining it on the grid shouldn't have much effect, but shining on the cathode should have the greatest effect.
4. (Advanced) Does the effect change with heater current?
Probably won't do #4; but any other experiments to try while I have the stuff hooked up?
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If you've got actual lasers and know they're spectrally pure, don't waste time with color filters.
With a nice, tight beam I'd be particularly interested in the difference between illuminating the cathode versus the anode. You're likely to get some reflection either way so I doubt either will give you a zero reading, but you should see a significant difference. RCA clear tops sound like a good choice. However, the tube glass might attenuate or even block UV depending on exactly what sort of glass it is, so be ready for it.
Can you vary the laser peak power, or just total power (i.e., pulse width)?
I'd probably start with a 1 kHz square wave, and monitor the control signal and the output simultaneously using a storage oscilloscope. Should answer the basic question of cause very quickly.
If the effect is definitely there, the question would be what the efficiency of the system is -- how many volts out versus how much power in. Should be totally linear with respect to pulse width, more-or-less linear with peak intensity. If you have a strong enough laser you might even hit a saturation point where the cathode is fully illuminated and can't spit out any more electrons, but I'm guessing this would take a LOT of light.
Using a Foreplay I'm guessing the volume control will have no effect on the light-induced output signal, but you should probably check this too.
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Yes, real lasers. I won't bother with the filters.
I can vary both the peak power and total power.
I can set the current into the laser diode from 0 A to 4 A with an adjustable current source (based on a OPA549), and adjust the pulse width from 0 to 100%, continuous wave (CW). I have no plans to go above mW range. I do have 2 & 4 W laser diodes (but don't plan on using those $$$)
I'm guessing I can produce enough light to find the saturation point if I did turn it up.
With the optics I have, I can get a tight beam, but not a pin-point dot.
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Craig, if you don't mind me asking, where do you work that you get access to a lab full of lasers? I love the fact that you are interested in experimenting until you get to the bottom of this issue. I have the hardest time getting my students to adopt that sort of attitude.
Keep us posted on what you learn.
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Put the kite higher into the storm Egor!
They called me mad at the university, mad!
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Randall,
I work at a small company that designs instrumentation for measuring high speed train track and train wheels.
We use UV lasers and high speed cameras to digitize the image of the track while the train is moving at speeds up to 400 KPH, 300 KPH is more typical, not in the US of course.
This experiment is fun for me because I've been out of college for 32 years and it helps keep the rust off.
I'm an EE and generally just keep my head down doing circuit design and writing embedded code.
I do understand you wanting to get your students to just dig in and figure out. Just dare them to be wrong and learn from it.
Heck use a pencil and paper if have you to.
We should thank Bolivar for starting the experiments. If I could mail Bolivar the laser lab, I would.
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Craig,
Thanks, stay curious.
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Now this is what I call fun! :-)
Craig, that time since college puts us within a yer or two of each other, and I too used to do a lot of embedded design back in the day.
Looking forward to what you find out.
-- Jim