3-500ZG shelf life

[KT0DD]

I am planning to buy an Ameritron AL-80B and was thinking of getting an extra 3-500 tube for it as a backup. Do these tubes last a long time in storage, or would I be wasting my money? Thank You.

Todd - KT0DD

[G3RZP]

FWIW, I would buy the spare, but swap them over every 18 months or so, and get the plate red.

[W8JX]

I am planning to buy an Ameritron AL-80B and was thinking of getting an extra 3-500 tube for it as a backup. Do these tubes last a long time in storage, or would I be wasting my money? Thank You.

Todd - KT0DD

The problem with them is the integrity of the seals in it. Some are sealed better than others. What will kill tube over time in gas/air leaking into them. If they were stored in a vacuum that would last a lifetime and then some but such is not reality. As suggested above I would use tube at least every 12 to 18 month for a few days and get it good and hot and then store it again. This will help degas tube.

[G3RZP]

The other point is that at the moment, you can get spare tubes. That cannot be guaranteed in the future....except maybe at a crazy price.

[N8CBX]

Is this a good idea, in general, for most all power tubes? I have some spare 6146s & S2002 tubes that haven't been installed in years.
Jan

FWIW, I would buy the spare, but swap them over every 18 months or so, and get the plate red.

[KD0REQ]

depends on the tube as to the extent of degassing in the getters and leakage through seals.

"hotwire" heaters, thoriated tungsten, have the getter where it gets hot enough to work, but not so hot it loses the captured gas.  that would be on the plates.  so instant-on tubes need to be operated with color to keep them hollow-state.

cathode tubes usually have oxide coatings that reach the correct dark red to maybe orange temp.  so in the case of "wat two minutes" tubes, you can getter them by leaving them in normal cooling air flow, but with only the heaters lit, no HV.  a tube tester works fine overnight in the case of 100-watt bottles like the 807 and 6146 and so on.

the amount and material in the glass/metal seals also appears to make a difference.  Eimac has written that the 3-500 and similar tubes used tungsten alloy rods to pass voltages out of the bottle.  since these can go to air in a year or so, this seal may be more prone to leakage.  smaller commercial/industrial tubes used a Kovar steel section where the pin meets the glass.  I have a bunch of tubes from the 20s through the 40s in the collection that tested perfectly from day one.

there could also be a size of seal issue on larger pins.   if it's a big costly tube, it's worth bringing it up with a hard current-limiting resistance so voltage goes on the plate with only a milliamp or so.  if it arcs or gets a plasma glow, you have to use unusual methods to get the plate red hot without doing so at fflashing voltages, and cook for a while, to have a chance to save the tube.

[W5JO]

Almost all seal issues in glass tubes are the result of rocking the tube back and forth in the socket to install or remove it.  That can cause minute cracks which will expand when the tube reaches operating temp.  In my numerous years dealing with power tubes I have never experienced a failure due to problems unless the manufacturer made big mistakes. 

I have seen ceramic tubes suffer the same fate.  Do a search on gettering tubes and you will see manufacturers take extensive steps to prevent problems.  There is a lot of misinformation concerning tube life and operation.  Follow the tube manufacturers recommendations, not wives tales.

[W8GP]

I rotate the tubes in my SB-220 every fall. One set is the original tubes and they still put out 1100+

[K4RVN]

Hi Todd,
I bought a spare for my AL 80 A a few years ago and think it was a waste of time and money. I rotated my Amperex
for about two years with the spare, then got tired of all the screws to take out and chicken stick discharge. I put in the newest tube  made in china a 3-500c and left it in for the past two years and plan no more rotations. Both are graphite tubes, but the amperex is 16 years old now. If I were you I would not buy one . Just my opinion from my experience. I think the 3-500 will last a very long time and will be made many more years. I forgot to mention that the AL 80B was provided with glitch protection to help prevent damage from such an event like a tube failure. I recently added it to my old AL80A, the B model's predecessor. Some older amps do not  have glitch protection and may benefit
more from tube rotation.

Frank

[K9COX]

It depends, how old are you?

[W7VO]

I have a slightly different philosophy with the tube rotation problems and possibly breaking seals, etc.  I rotate out amplifiers every few contests. This ensures the tubes get gettered, the electrolytics get reformed, and the relay contacts get cleaned..... 

[W4VR]

A word of advice.  Buy your spare tube now.  The price will only go up if you wait until you need a second tube.  Shelf life is generally not a problem, but you should burn them in a bit before you apply HV as gases may build up over time.  I had spare 3-500Z's for as much as 15 years before I used them.

[W8JX]

A word of advice.  Buy your spare tube now.  The price will only go up if you wait until you need a second tube.  Shelf life is generally not a problem, but you should burn them in a bit before you apply HV as gases may build up over time.  I had spare 3-500Z's for as much as 15 years before I used them.

Burning them in will not degas them you need to get anode hot. Quality of newer tubes seals does not seem as good as old ones too.

[W4VR]

A word of advice.  Buy your spare tube now.  The price will only go up if you wait until you need a second tube.  Shelf life is generally not a problem, but you should burn them in a bit before you apply HV as gases may build up over time.  I had spare 3-500Z's for as much as 15 years before I used them.

Burning them in will not degas them you need to get anode hot. Quality of newer tubes seals does not seem as good as old ones too.

So, you're saying that my tube was not being degassed after all because there are no getters in a 3-500Z.  Apparently you have to run the tube, as you say with plate voltage applied and driven, until the plate shows color to get rid of the gases.  I do believe, however, that in the case of a ceramic metal tube...there are getters inside the tube and leaving only the filament on for a few hours will degas the tube.  Therefore, shelf life of a ceramic metal tube is probably much longer than that of a 3-500Z.

[N2EY]

A word of advice.  Buy your spare tube now.  The price will only go up if you wait until you need a second tube.  Shelf life is generally not a problem, but you should burn them in a bit before you apply HV as gases may build up over time.  I had spare 3-500Z's for as much as 15 years before I used them.

Burning them in will not degas them you need to get anode hot. Quality of newer tubes seals does not seem as good as old ones too.

So, you're saying that my tube was not being degassed after all because there are no getters in a 3-500Z.  Apparently you have to run the tube, as you say with plate voltage applied and driven, until the plate shows color to get rid of the gases.  I do believe, however, that in the case of a ceramic metal tube...there are getters inside the tube and leaving only the filament on for a few hours will degas the tube.  Therefore, shelf life of a ceramic metal tube is probably much longer than that of a 3-500Z.

There's a getter in a 3-500Z. It's the anode coating. But it only works if the anode gets hot enough; it does nothing at room temperature or with just filament lit.

Most ceramic-metal tubes I know of have a getter arrangement heated by the heater, so they getter just from being lit.

Shelf life depends on construction quality (mostly the seals) which I am told is quite variable.

----

What I would do to maximize life is to get a spare or two and then rotate them every so often. Say every two years. And I'd keep records of which tube was in the amp on which dates, and the operating characteristics.

73 de Jim, N2EY