It can not be repeated often enough, that longest tube life results in the first place from the heater voltage being correct, and plate dissipation being not at the maximum limit. Though slow heat up has some theoretical benefit, a really large benefit will result from the filament voltage being exactly right.
Direct or Indirect heated?
We need to distinguish first between Direct Heated Tubes (called DHT here), and indirect heated tubes. The last have a heater, which is a heater spiral only, and not the emissive part. This heater spiral is not under mechanical tension, and can be made of Tungsten, which is very rugged. So with indirectly heated tubes, break risk is low.
Directly Heated Tubes do not have a heater, but rather what we call a filament. Though the words heater and filament are often used together. The filaments of a DHT have an additional requirement, compared to heaters of Indirect heated tubes. DHT tube filaments are under mechanical tension, with a spring, and they may not break or extend length over time. The breakage will end the tube life of course, and extended length may cause a short circuit when "specialists" are tapping on working tubes. These requirements can only be fulfilled, simply by making the filament not hotter than needed. So any heater tolerance of more than 5% is not wise to do. If you exceed this 5%, tubes may seem to work normal for a long time, but maximum lifetime will not be reached. So this targeting of the lowest temperature, has an additional result when we under heat the tubes. The filaments will become not warm enough when the voltage is more than 5% too low. This explains in a nutshell, why tube filament voltage may have so little deviation.
Historical reality says: 5%
Throughout history of making Barium coated electron tubes, we see always the same specifications for the heater voltage: +/- 5%. In some rare cases tubes are specified at +/-10%, but this does not mean these manufacturers have invented something spectacular. This was just for special applications where it had to be specified this way, and a huge compromise was made with lifetime and reliability. Allowing for -10% means the filament must be designed a bit hotter by default, so at -10% there will be enough heat. Then, if that tube is used at +10%, end of life will occur at 500 or 1000 hours. Meaning any +/- 10% tube has a low lifetime specification. Also a higher number of tubes will fail unexpected, so not just lifetime is lower also, but reliability is lower also. It appears high lifetime and high reliability is only possible when the filament voltage is specified +/-5%. We take this as reality.
Tubes have a balance inside, to maintain emission.
- The emissive Barium surface layer evaporates simply by heat of the filament. This layer is just a few microns thin, and evaporation accelerates very much at higher temperature. So just 5% more filament voltage has a much larger effect on this as just 5%.
- Inside the filament coating is the Barium Oxide depot. This migrates slowly to the surface, and forms new metallic Barium at the surface, filling up the missing parts of the metallic, emissive layer.
- Both processes (1 + 2 ) go a lot faster at greater heat, like all chemical processes. So this simply wears out the tube much too fast, and problems are non reversible when the depot is empty.
- If the tube filament is not warm enough, there is not enough migration of new Barium to the surface. Of course there is also less evaporation, but the balance will fall into the direction of not enough migration. So the metallic Barium layer will disappear. However now the depot is not used up. So such tubes may be recovered by just normal use.
- If the tube filament is really much too cold, it may begin to suffer from cathode poisoning, as some specific unwanted chemical traces do not evaporate also, and this can develop into an emission killing surface. Such a tube may be recovered by the classical process, of over heating the filament, but with EML tubes we found this not needed. Besides a tube that was first under heated, and then over heated to "repair", it is easy to understand it will not be like a new one any more.
- The ideal situation comes, when evaporation and regeneration are nicely in balance. You can trust, we designed the filaments exactly in such a way, that this happens at exactly the specified heater voltage. (meaning +/- 0%).
Highest possible lifetime
Admitted, there are cases where lifetime is not the first concern, for instance when companies specify an amplifier with exceptional wide range of mains voltage, and still use AC heating for the tubes. Any change in the mains voltage will also mean a change in the heater voltage. In such cases it will come down to stressing the 5% limit as much as possible. Or worse, they do not respect that this 5%, and we have a debate why they can not exceed this by just a few % extra. However when tube life is your first concern, deviation of the heater voltage should be 0%. Saying this in other words: The maximum deviation of heater voltage (+/-5%) is not a safe limit you can choose as you like. At 1% deviation a tube will last longer than at 2% deviation, etc, and 5 % is the hard limit. Positioning yourself at this limit will result in reduced tube life.
So after reading this, one may think, are EML tubes so sensitive against heater voltage deviation? The answer is: No, they are not more sensitive than other tubes. It is just every time when we get reported very high lifetime of our tubes, we found many times this was a result of good amplifier design. It must be said like this: At maximum deviation of the heater voltage, so just within the specs, it should not come as a surprise to you, that lifetime will be also just within the specs and no more than that. In other words: The less deviation of heater voltage, the longer the tubes will last, and this is so for any DHT, any brand, new made, or NOS.
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