What causes long tube life?
During normal use, the cathode must be so hot that electron emission takes place at a high level. Unfortunately this temperature is so high, that a small fraction of the Barium layer will evaporate by this. This gives small openings in the layer, and at these openings, the Barium Oxide layer underneath gets exposed to electrolysis. The result of the electrolysis is Barium Oxide (white powder) gets reduced to metallic Barium, and Oxygen gas. The Oxygen finds it's way to the getters, and the Metallic Barium closes the hole. This process is like a self repair. However after some time , the Barium Oxide layer gets depleted. Long tube life will only appear when everything is in careful balance. So the cathode must be hot enough to give the emission, and also more heat gives more electrolysis. However the heat will also cause evaporation. So one way or another, wear out is unavoidable.
Once this process is understood, it is clear we want two things, which are no friends of each other. :
- Keep Barium evaporation at a minimum level. (Reduce heat).
- Create a self-repair process. (Increase heat).
Over heating. In fact, this is a very delicate balance. So we must take care NEVER to overheat the cathode, also not just a little bit. Any questions to what possible tolerance is heater voltage is allowed, we answer always like this: Zero tolerance is the best! Any overheating will must faster evaporate the Barium metal. Furthermore, over heating will speed up the migration of Barium from within the cathode. So from electrical testing, it seems as if over heating doesn't matter much. The tube seems to take it, and tests normally good afterwards. Yet this impairs lifetime very much. Overheating will empty now the Barium depot very fast, and when it is used up, migration will stop, the Barium layer will evaporate finally, and this time without repair. The tube life is over suddenly and unexpected.
Under heating. If the cathode is not warm enough, like when heating a 5V tube with 4.5V only, this is very good to reduce the Barium evaporation, and initially it seems like a good idea, since the tube works still normally, even below 4.5V. So users do not understand why exactly 5.0V is important. However the balance is disturbed, the migration in Barium to the surface is now a LOT lower. After weeks of use, the Barium layer gets too thin, and holes begin to appear. Now, soon emission will go down, the tube will sound distorted, and test "defective" on a tube tester. If corrected timely, this damage will repair by normal use, at exactly the right heater voltage again. However if the loss of emission was only corrected by adjusting the bias, this will cathode damage becomes too large, and the tube damages permanently.
Balance. This the key and the secret of long tube life. So use just the right heater voltage, do not overheat or under heat the cathode. Do not overheat the anode, as this will effectively overheat the cathode as well by back radiation to the cathode. Do no draw excessive current peaks from rectifier diodes by using too large capacitors. Do not overdrive Push Pull amplifiers just for "fun" to hear what this sounds like. All of these things will blow holes in the Barium layer, which need continuous repair. This uses up the depot faster.
Warm up. Make sure you have no circuits that draw peak current while the heater is not fully warm. Very infamous are rectifier circuits which by default have the full AC voltage from the transformer, but are loaded by an empty capacitor at switch on. So oversized capacitors do extra harm here.