Conserving our good CRT's
 

Just had a thought, some of the really good CRT's, new ones or low hours for example will usually test fine even at a reduced heater voltage, example is this 8DP4 test (image below) it checks this good at just a hair over 4 volts.

I wonder if something like a reverse brightener would extend the life of the CRT, if it has good emissions at 4 volts why not run it at 4 volts? then if it ever drops off you could put it back to 6.3 volts and get more life out of it.

Seems logical but I wonder if a CRT will actually last more hours at a reduced heater voltage or if the overall hours would still remain the same?

http://vintagetvsets.com/images/8dp4at4volts.jpg

I think its beam current, or cathode current (excessive brightness) that kills them....
Anyone know if old airline terminal screens had anything special about the crt's?
Those tubes had to run at higher than home living room brightness..... Were they special....?

I thinki it is all about how hard you work it. I have always run my sets at moderate brightness and contrast just because I thought it looked better even before I knew it conserved the CRT. Rarely have I had a picture tube go bad in a set. I only remember one time and that was a 1989 Zenith that I used for twelve years or so. Now that there is no rebuilding (at least for now) I am especially careful to not put excess hours on my sets. I generally collect sets that use common parts so that I can use them as much as I like without worrying about getting parts later. This applies to both radios and TV sets. This is why my collection will never have any monetary value but I don't care about that. I like to restore, use and enjoy my sets without worrying about unobtanium parts failing. That being said, we should preserve our vintage TVs by using them frequently, operating them at moderate control settings and not leaving them on when we are not actually watching them.

Eric, I have an example while not pertaining to running at lower voltage as you mentioned, might give a clue.

Back in about 1971, I was just married, had a new daughter, a newly purchased house and not much money. I had owned an all tube 1965 16 inch color Sears (rebranded Toshiba) since 1966 that had just developed the habit of cutting * out at random times and my wife was insisting on either fixing that set or preferably getting a larger one. I found a used Zenith with a booster on its almost dead CRT for practically nothing, at the same time as our local rebuilder was running a special on rebuilt color CRTs.

I got the old Zenith home, confirmed that the CRT was very VERY dead and it took about 20 minutes to get to viewable brightness with the booster and even then only bright enough to watch at night. Wrote down the CRT number and trotted over to the rebuilder, who promptly told me that he did not rebuild Zenith CRTs. Aaaarrrrrrgh! This means that I'm stuck with two bad sets!

OK, what next? I scrounged an old 6.3 volt transformer from my junk box and wired the CRT to be on 100% of the time, as that would eliminate the 20 minute wait and give me a little thinking time. Early the next day, when the set was turned on, I noticed the set was brighter than before and so I removed the booster, and it still was OK. We used this set operating the filament at its rated voltage and always on when the set was off for about 10 years! The filament pulled 6.3 volts at 3 amps, or about 19 watts. That was less than our security lights.

The CRT still put up a good picture to the very end, which happened when the horizontal output, damper and flyback cremated themselves in unison.

My theory is the constant CRT heat enabled the getter to do its job better.

The point of this is, if you use the set, keep it on long enough for the heat to warm up the getters to keep the CRT healthy.

James

* This is before cooling spray was available. The problem turned out to be a 5 cent intermittant resistor, but as the problem always fixed itself as soon as I touched a probe anywhere, almost no useful circuit tracing could be done and the voltages were within 10% when the set was operating and could not be measured while it was acting up. The trouble was finally pinpointed when I disconnected the AGC keying and used a battery to hold the AGC voltage constant.

If memory serves me right, General Electric did studies on operating tubes at different filament voltages. Too high killed the filaments, too low the cathodes. Somewhere I have data on this involving the GE "five star" tubes.
I remember reading that the cathode surface was not uniform in temperature at lower voltages and the hot spots died off earlier.
A friend of mine tried that idea on apower amplifier. He ran the filaments at 5 volts instead of 6.3 volts and it resulted in the rapid demise of four M & O valve company (Genalex for the audiophiles) KT88 output tubes.
CRT cathodes are different in construction but the principle is the same.

NOT special, and usually used to death, with lettering permanently burned into the phosphors. Why not special? because CRTs by their nature are always in wear-out mode. Anything that could improve their life would be applied to home sets too (not necessarily to get longer life, but perhaps better spot size by using a smaller cathode, or some other trade-off).

Wear-out mode:
The life of components exhibits a "bathtub" curve with three parts: infant mortality (high rate of failure for defectives), normal use (low rate of uniform random failures), and end of life increasing rate of failure (wear-out failure). CRTs, however, are in the wear-out mode from the start as the cathode is continuously degrading, as well as the lesser problem of the phosphor burn continually accumulating.

Electrolytic capacitors can be similar if subject to high temperatures.

My understanding is that the getter does nothing after its first firing. I think what you were seeing is the cathodes being kept cleaner. In the early Saticon tube HDTV cameras, we were advised to run the camera for a few hours at least once every 6 months to counter the effects of residual gas in the tubes. Whatever reaction was happening I believe was at the cathodes themselves.

perhaps you could reduce a crt to 6 volts or 5.9 or 5.8 and it will work ok but i wouldn't try going lower then 6 volts , the marginal 6 vs. 6.3 may be enough to extend it a little longer but unless your running the set for hours opon hours a day it seems useless to worry about it , it should be around for a long time , on the opposite i mentioned this in a different post that i have a crt that is on the weak side and put a brightner on it and with a resistor it runs 7 volts , thats enough to give a bright picture and it gets used 2 to 4 hours a week , no point in blasting it with more then 7 volts if its showing a good picture , other crts that i have like a 16ep4 i will eventualy experiment with reducing it to 7 volts and it it looks ok i will do that.
i do have an interesting 16GP4 that reads way high on the emissions and cutoff is good but to get a decent image it needs a brightner and the ion trap has to be pushed as far forward as possible , its touching the focus magnet , my only guess here is that it was rebuilt but not properly.

mike

No, the barium surface deposited by the getter on the neck or funnel is a long term active pump, that will maintain the tube at a low pressure until it is depleted. All tubes leak, but the barium surface can insure many years of decent vacuum.

jr

I have never heard this before. Now it is common knowledge that some of the early metal/glass tubes (19AP4) developed leaks over the course of many years, but I am compelled to ask the basis behind all vacuum tubes leaking when there are so many around from the 40s-60s that are as strong and void of air or gas as they were when new.

If they do leak, then it must be immeasurably slow. Too slow for our lifetime. My 1920s radio tubes (O1A and the like) are holding up well too. This might be an interesting topic.

Thanks - I learned something!

My major point was that the barium deposited by the getter flash continues to function to remove gas in a sealed off CRT for many years after it is flashed. There are several sources of this gas.
1. Incomplete processing... some surfaces may still be outgassing.
2. Virtual leak... gas trapped in very fine porosity of the CRT materials that may come out.
3. Real leaks... such as around base pins, tip off seal, frit joints.

Indeed, with decent techniques these are very small sources of gas...but get back to me in 100 years or so about the condition of your tubes. :)

jr

Barium is not exactly a great getter. Its great for oxygen and water,
but that's about all. Its totally useless for nitrogen, hydrogen argon, helium,
and hydrocarbons. I'm not sure about carbon monoxide.

The gasses that ooze out of hot nickel, iron, molybdenum or tungsten are mostly hydrogen and carbon monoxide. I've seen this over and over
again in vacuum systems with mass specs in them. I had one that
stayed sealed for over ten years and after initial bakeout and
pumpdown that's what I saw.

For vacuum tubes with soft glass envelopes you will always see
Argon developing due to radioactive decay of potassium in the glass.
For tubes with thoriated tungsten filaments of course you will see
Helium developing.

The great getter is Titanium. It will getter anything except rare
gases and methane. That sealed system used a titanium based
ion pump. I believe that large transmitter klystrons still use such pumps.

It might have been rebuilt with a straight gun. Remove the ion trap and see how it works.

I'm pretty sure i removed the ion trap and no picture but could always double check.

i guess the best way to keep a crt is to bring it up to a space station and keep it in a room exposed to the vacuum of space , this way if it leaked or cracked it will still work :)

mike