3-500ZG shelf life

[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.

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

Never rotated my 3-500Z's, but I did rotate my 3CX800A7s when I had the Alpha 89.  Those tubes were so darn expensive I felt that rotating them would help longevity.  Good point about the 3-500Z and the anode behaving as a getter...I guess I never thought of the anode as a getter.  Ron, W4VR

[N2EY]

Never rotated my 3-500Z's, but I did rotate my 3CX800A7s when I had the Alpha 89.  Those tubes were so darn expensive I felt that rotating them would help longevity.  Good point about the 3-500Z and the anode behaving as a getter...I guess I never thought of the anode as a getter. 

I never did, either, until a few years ago when I saw info on it. Makes sense, really. Goes way back; big glass tx tubes often had tantalum plates for the same reason.

How much did 3CX800A7s used to cost?

73 de Jim, N2EY

[WX7G]

http://www.getnitrogen.org/pdf/graham.pdf

Might storing 3-500Z tubes in a dry nitrogen atmosphere increase shelf life?

[W8JX]

http://www.getnitrogen.org/pdf/graham.pdf

Might storing 3-500Z tubes in a dry nitrogen atmosphere increase shelf life?

Yes but not for reasons you think. Nitrogen molecules are larger than oxygen ones so there is no smaller molecules to slip past seals easier. Air you breath is about 78% nitrogen.

[G3RZP]

Best thing to store them in must be a very high vacuum!

[W4VR]

Never rotated my 3-500Z's, but I did rotate my 3CX800A7s when I had the Alpha 89.  Those tubes were so darn expensive I felt that rotating them would help longevity.  Good point about the 3-500Z and the anode behaving as a getter...I guess I never thought of the anode as a getter. 

I never did, either, until a few years ago when I saw info on it. Makes sense, really. Goes way back; big glass tx tubes often had tantalum plates for the same reason.

How much did 3CX800A7s used to cost?

73 de Jim, N2EY

When I bought them in the 90's they were $700 for a pair.  As you know they are at least twice that expensive now.

[WA7PRC]

The used-to-me 3-500Zs in my Heathkit SB-220 lasted 30 years of daily use.  Only cockpit error caused them (and the sockets) to self-destruct.  Sure, the cost has risen but (given how long the last in service) I'd still rather have the distributor deal with storage.  About the only valid argument for keeping spare(s) on hand is to minimize down time to a few minutes.  To me, minimizing down time is just not that important.  

[WX7G]

3-500Z prices at RF Parts:

China      3-500ZG  $160
Amperex 3-500Z    $330

[K1ZJH]

I pulled my spare 3-500Z out of storage a few months back, and it went right to work after being in storage for over 30 years. Ditto for several PL-175s and 4-400A tubes that I picked up in the late 1960s!

I'd suspect one of the main issues is outgassing from the metal used in the plates, which should be correctable by running the plates with color showing to activate the getter material.

Pete

[G3RZP]

My suspicion is that the older tubes had better seals and better vacuums than the current Chinese production.

[W8JX]

I'd suspect one of the main issues is outgassing from the metal used in the plates, which should be correctable by running the plates with color showing to activate the getter material.

When they use graphite anodes oxygen is trapped in graphite when anode is made and needs to be baked out properly which requires anode to be heated while a vacuum is being drawn on tube for a period of time. It this is not done properly the tubes will gas over time unless they are pushed hard from time to time to heat anodes and getter them.

[WX7G]

http://www.rfparts.com/pdf_docs/572B%20Svetlana.pdf

http://www.thevalvepage.com/valvetek/getter/getter.htm

It's not the graphite that is the getter, it is the titanium coating on it. See the links above.

I suspect that a tube that is not properly processed from the beginning will not getter itself with use.

[W8JX]

It's not the graphite that is the getter, it is the titanium coating on it. See the links above.

I know that. The coating is actually a mix/alloy of a few metals.

I suspect that a tube that is not properly processed from the beginning will not getter itself with use.

I think it is just a question of how much oxygen the coating can absorb and convert. In case of 572's when used in 811 amps they likely never get hot enough to properly getter because the power supply is not up to task to properly feed a 572 especially in 4 tube version. 

[N2EY]

3-500Z prices at RF Parts:

China      3-500ZG  $160
Amperex 3-500Z    $330

IIRC, back in 1964 or so a 3-400Z was about $34. Which inflates to about $270 today.

73 de Jim, N2EY

[AA4HA]

My suspicion is that the older tubes had better seals and better vacuums than the current Chinese production.

And isn't that ironic? Semiconductor manufacturing requires vacuum levels many times better than an electron tube. We understand more about materials compatibility, how to make metals and welds without oxides or contaminants. It is not as if it is a "lost art", more of a case of the people who make them just do not put as much emphasis on tubes.

Look at televisions, I have two Sony units that still have picture tubes. Right now one is unhappy about turning on due to an intermittent high voltage problem that the CPU does not like. Back in the day there were even ways to fix that stuff too. Sadly this television is soon for the junk pile and will end up being replaced with some LED flat screen device that costs 1/2 as much.

I grew up in an age where a red box with white lettering meant RCA, yellow and black meant someone else and so on. It was weird, when the pharmacist was also the guy with the key to the bottom of the tube tester I would be anxiously hoping for a red box to replace the bad tube I had. Eimac, those were magical words, said in a quiet voice filled with reverence.

Then again, I bought the very last two 67 1/2 volt and 22.5 volt batteries from the local electrical supply house. They had kept them around for curiosities sake, figuring that "someone " would come buy them.