What is tube activation?

The electron tubes as we produce at EML, have so called "Oxide" cathodes. In such tubes, there is a layer of metallic (non-oxide) Barium present on the cathode surface, which metal is the source of electron emission. Metallic Barium can not exist in air, because it would oxide immediately at oxygen contact.

It is technically interesting how this metallic Barium is applied on the cathode, since we have to produce the tubes in a normal (air) environment. So after the glass bulb is around the tube, and the air is pumped out, there is no metallic Barium present on the cathode yet. Still we can get in on there now, with a chemical / electrical process, under vacuum.

The tube cathode can create metallic Barium by itself. By a process, called activation, the original hand applied cathode paste which active part is Barium Carbonate , will chemically change into a Barium Oxide, within the first seconds after the heater is switched on the first time. It is CO2 and H2O gas molecules that are chemically released, due to the heat. After the CO2 gas and water (gas) is gone, what is left is Barium Oxide. The operator will notice after a short while, the vacuum improves again, meaning the cathode is not out gassing any more.

Now, the tube can be used to generate electric current, as a diode. By a process called electrolysis, metallic Barium will start to build at the surface. This is nothing but a chemical reaction, which takes place very quickly, since chemical reactions go very fast at high temperature, and the cathode is very hot! In fact it is red-glowing hot, so you can wait for this happening while you do it.

Electrolysis may seem a difficult word, but it is taking place from the birth of the tube, until the end of life. Most electrons that leave the cathode, are by the well known thermionic (electron) emission, and they flow from Cathode to the Anode. Yet the electrical current, flows the opposite direction, from Anode to Cathode, and this energy causes the Barium-Oxide molecules on the cathode, to separate into Barium (metal) and Oxygen (Gas). It is this Barium (pure) metal we want, and the Oxygen we do not want. Though for the Oxygen during normal tube life, we have the getter to absorb it, during the manufacturing process now, we just vacuum pump the tube all of the time. The getter is not even flashed yet. The dirt level is too high for the getter flash, it would be ruined. The getter stays inside the getter ring, and stay inactive, as long as not flashed.

The cathode comes now into a condition, in which continuously Barium is generated at the surface. At first we have only little spots of metal layer, but after these spots have grown, they grow together, and after some hours the whole surface becomes covered with a metal layer. Once the layer is complete, the electrolysis process reduces to a very low level, by itself, and the tube is now born and alive, but not mature yet.

It is biased now for a very long time, under great heat, while still vacuum pumping the tube. We orange glow the anodes for a very long time. Apart from electrical load, we also heat the anodes externally with a magnetron. This overload will outgas the materials, and the longer we do so, the better the lifetime of the tube is going to be. Machine hours of the vacuum pump, and magnetron tube, and electricity costs for this are very high. An oil change of the vacuum pump costs 3000 Euro, and a new magnetron tube costs even more. So imagine what it costs to run these for days or a weeks. That makes a huge difference. Where as a 15 Euro EL34 can only use the machines very short at this small price. At EML we have better options with tubes that cost 350 Euro one. Though such long run hours are nothing for mass production, on a small scale it can be done. So here we have a hidden quality aspect of the tubes, hopefully you understand it.

After this is done, the tube gets sealed, the glass pipe removed from the pumps. Now, the gas pressure can not be measured any more. After this, the getter is flashed, and the vacuum quality jumps to it's final level since the getter itself is the only factor responsible for this.

Now comes burn in, which formats the cathode layer, and the tubes become thermally stabile and long-term stabile. After this is observed, the tubes are ready for electrical selection, and matching. A pre burned in tube, the user can regonise as it performs right out of the box exactly by the test values as on the box. It needs no long warm up time to become stabile, and is not changing the bias point very much.

 

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