Vacuum tube burn-in

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Tom Rauch:
Typo. Phosphorescent materials in the ***glass*** make a glow.

James Miccolis:
Great stuff, Tom! Couple of small points:

- The use of the anode for gettering in glass power tx tubes goes back a very long time, to at least the late 1930s and probably farther. Before the zirconium-coated anodes were developed, tantalum anodes were used, with the same principle. One of the differences between a glass power tx tube designed after about 1930 with an A suffix and one without (833A vs. 833) is that the A versions usually have zirconium coated anodes and the non-A have tantalum anodes.

- "Small" glass tx tubes such as the 6146, 807, etc. usually have flashed getters (like receiving tubes) and do not use the anode for gettering.

- The idea that "the easier you run a tube, the longer it lasts" is generally but not always true. Tubes with anode gettering are the exception, but they're a very important exception!

- The "blue glow" in tubes requires some care and experience to interpret. Usually it's not gas but flourescence from electrons hitting the glass. This happens in receiving tubes as well as transmitting types, and is absolutely harmless. VR tubes, mercury-vapor and gas rectifiers, and various thyratrons and control tubes all glow in normal operation. Only when there's a blue glow down inside the tube structure, between the elements, is it gas. And as W8JI says, in a tube like the 3-500Z you'll have arcing in that case.

- A lot of this sort of thing is explained in the Eimac publication "Care And Feeding of Power Grid Tubes" which can be found on the web at various sites.

73 de Jim, N2EY

Hello all

I have a NOS Eimac 3cx1500A7 that has been sitting in my drawer for the past 20 years.

Should the cooling fan be running during the 12 - 24 hours filaments burn-in period or should I leave it off in order to reach a higher cathode temperature?

Will it harm the seals if it is left off?

Thank you for your expert advice.

73, Al VE2AMT

Let's clear up some misinformation. Glass-metal seals leak. Glass-ceramic seals do not leak significantly. Therefore ceramic tubes do not leak. The 4CX250B is an example of a ceramic tube.

Getters are not placed on filaments or cathodes. Oxide cathodes (filament or cathode-type) are getters. They will getter to some extent. And they can become poisoned by long periods of operation without drawing cathode current.

Internal anode tubes commonly have zirconium on the anode for gettering. This must be heated to a dull red or hotter to getter. This getter is supplemented in some tubes by a barium getter that flashes barium onto the glass. This is the silver material you see on the inside of the envelope of receiving tubes. Each getter material absorbs different gasses and operates in a different temperature range. A getter is not a substitute for good tube manufacturing. A tube needs to be pumped down while heated for some time for the adsorbed gas to be released from the tube elements and the envelope. A clean process must be used. A clean room in effect.

MFJ recommends 10 minutes of on time plugged in the amp with HV applied before running RF. This seems reasonable and harmless. The book The Care and Feeding of Power Grid Tubes by Eimac makes NO mention of burning in tubes or rotating tubes in and out of service.

The advice to run a tube without HV for 10 hours seems harmless but useless. As far as helping the cathode to complete the 'cathode activation' cathode current must be drawn. No HV means no further cathode activation.


Here is a link to a good paper on Getters. I believe that thoriated tungsten filaments operate too hot to be effective getters. Reducing the filament voltage to where the filament is less than 800 deg C might make for good gettering. However, when heated to normal operating temperature the adsorbed gas will be liberated, defeating the gettering process.

What one is trying to do through gettering an old tube is to avoid HV arcing. This is done by (hopefully) lowering the gas presure in the tube. The way to do this is to run the tube at rated element dissipations and drawing cathode current. The anode will then getter while the grids outgass.

If one could reduce the anode voltage to lets say less than 1/2 of operating voltage, gettering could be accomplished with a greatly reduced the risk of arcing. Control grid and screen grid dissipation would be pushed to maximum specified to outgas these structures. This is then followed by or preceeded by a HIPOT operation, sometimes called debarnalcing.

I lue of this what can one do? Plugging a new tube in and immediately running it at rated anode dissipation is likely to liberate gas and could lead to arcing. I think that running the tube at reduced anode temperature and bringing it up to full anode dissipation over a period of several minutes to hours could help and not hurt.

Sounds like a business someone can start. Send a tube to them and they will condition it as described.


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