OT: Looking for folks interested in 3l01i CRT discussion

[gregebert]

I just bought a dozen 3l01i tubes from Yuriy and if there is anyone in this group who wants to start a side discussion to share knowledge, send me a private message and I will start a new google group.

[petehand]

I've got something to say about them. It's a good thing you bought a dozen, because that means if you do a project using one, you'll be able to keep it going for a whole year.

[Tomasz Kowalczyk]

Should we understand that as "these tubes have very short expected operating time"?

By the way, this clock is gorgeous. I feel like this picture will start a new obsession of mine - to make a CRT clock...

[petehand]

After a couple of weeks the phosphor gets too dim to see in daylight. I know it's the phosphor because when I adjusted it to move the ticks to new locations where it had never been scanned, they were bright again. I put a new tube in and the same thing happened. The phosphor coating is really really thin, almost transparent.

[petehand]

Here's the schematic, if anyone's interested.

[Terry S]

Pete, what would explain the low lifespan? Surely the tube wasn't designed to last a month. 

Is there some biasing change that would extend the life? Was it maybe designed only for dark room application where the beam current would be much lower? 

I just have a hard time understanding such a short lifespan.

Terry

[Nicholas Stock]

I’ve run one of these for 2 months with no discernible loss in brightness.....maybe a bad batch?

Sent from my iPhone

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[chuck richards]

Could it be that these CRTs were used in night-vision
apparatus? If so, they would have been designed to operate
very dimly.

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[petehand]

I don't know - it's a mystery. I checked everything very carefully, including the heater voltage (with a true-RMS voltmeter, since the driving waveform isn't a sine wave), in case I was overheating the cathode. I have a few more tubes, but when the second one failed like the first I put the clock away. Maybe I should get it out again and sacrifice another tube just in case.

I've seen a lot of tubes, but I've never seen any with such a thin phosphor coat as these. Usually they're white and opaque. These are gray and translucent.

[John Rehwinkel]

> I just bought a dozen 3l01i tubes from Yuriy and if there is anyone in this group who wants to start a side discussion to share knowledge, send me a private message and I will start a new google group.

I'd send you a private message, but I'm not sure how...

In any case, I'm interested. I have a few of these CRTs (of various vintages, the electron gun has been redesigned over time). I'm also curious whether the lifetime can be improved by changing the operating point (modifying the ratio of accelerating voltage versus beam current, while keeping similar brightness and spot size) like with magic eye tubes (detailed in the paper "Degradation of ZnS:Cu,Au,Al phosphor powder and thin films under prolonged electron bombardment"). I suspect it's not a surface layer phenomenon like in the eye tubes (since the phosphor is viewed from the opposite side that the electrons strike, and it's so thin), but it's worth a try.

- John

[gregebert]

@Pete - Does the phosphor show any signs of burning when the tube is off ? 

Maybe the static pattern of the clock-face is the problem.

I dont know much about phosphor degradation, but I cant help wondering if it's not 100% cumulative. For example, if one section of the tube is illuminated at 10% duty-cycle and another section at 100%, perhaps the 10% duty-cycle section might last more than 10X longer than the 100% due to heating or other effects.

[GastonP]

A technique that could extend tube life would be to shift the image lightly in a random way from time to time to even out.
As the deflection is 100% digitally controlled, it should not take much effort to implement this effect. Just code in a random variable displacement of the whole image every, say, 10 minutes or so...

[petehand]

That would not prolong its life for more than a few days. The screen is too small, there are no dead areas except between the ticks around the edge. The hands pattern visits the entire screen 24 times in a day. It gets more action at the center, near the hands roots, and that's the section that goes out first, leaving just the tips showing. There is no sign of burning - you can't tell a used tube from a new one by inspection.

[gregebert]

@Chuck - Nightvision uses a much different tube construction; basically it's a photon amplifier that receives photons at one end (converts them to electrons), and the phosphor coating at the other end (electron to multiple photons). It's truly an ingenious device, and the second-generation tubes (microchannel electron amplifier) are even more interesting (and of course, more expensive). There are no deflection plates (or yoke) in (around) the tube. They are also much smaller than a traditional CRT.

Sadly, nightvision tubes can suffer permanent phosphor damage from bright light (eg, laser), as I found out from a night-game of paintball [another story, but what an adrenaline rush!!] .

[gregebert]

Received my tubes yesterday; on close inspection the phosphor coating appears thin (it's definitely not opaque). My guess is that the low anode voltage (only 500 volts, versus typical of a few kilovolts for scopes) results in relatively low-energy electrons, so if the phosphor is too thick it might make the trace less-visible. Most likely, I will try NOT to focus the beam too sharply, and paint single-digits on the tube, like a NIMO, rather than a clock-face, or multiple digits, with sharp features, The price of a 3L01I is certainly much lower; 15 USD for a tube compared to a recent auction for 2 untested NIMOs that sold for 650 USD.

I know that older color-TV picture tubes (eg, 21FJP22) had a rather thick phosphor coating because a friend of mine lit a fire inside the tube and we could barely see the flames, or the several gallons of water we poured into it to put the fire out....Not to mention how thick the glass is across the face (we had to use an axe to break it up on his front lawn).

I also think the writing-speed of TV tubes is significantly faster than the 3L01I, so it likely requires higher current.

BTW, does anyone know what the anode-current spec is for a 3L01I ? I did not see it mentioned on the datasheet.