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Of course, all tubes come to an end one day. For lifetime, directly and indirectly heated tubes depend on the same mechanisms, but in another order, and so the most occurring "death cause" is often another. Indirectly heated pentodes, like KT88 when constructed very good, will end their life by loss of vacuum, resulting from out gassing the plates (Anode). For KT88 and similar, this is the natural way to come to an end, if no quality issue occurred before during use. The occurrence and absorbing of very small fractions of gas gets visible by "eaten away" edges of the getter flash. Though such tubes apparently perform good, they are near the end of life and should be replaced. Once the getters can not absorb any more gas, the tube likely will self destroy during further use. Possibly damaging the amplifier, or blow the amplifier fuse. Self destruction can start with a short circuit between cathode and first grid. The short circuit can be permanent, or go away and re-occur soon. Next is severe off-bias and the tube will be severe overheated. The extreme heat may cause a vacuum leak along the pins inside the base. The tube is found with white getters one day, though it was working before. Such a tube is completely out of function, and will also not short circuit any more. In a push pull amplifier, this will now damage the other tube of the pair as well since it will try to draw the bias current for both tubes together. It must be said that indirectly heated tubes with "eaten" getter edges need to be replaced. (So the previous text only applies for KT88 and similar). Good quality Directly Heated Tubes (DHT), like 300B normally have no out gassing problem. Their cathodes are more fragile, and such tubes must be run less hot by default. So a 40 Watt 300B is much larger size than an (appr.) 40 Watt KT88. Also cathode to grid distance is much larger with DHT tubes. So a short circuit will not take place at all. DHT with a gold grid (as we build them at EML since a few years now) will not have thermal drift away. So a severe overheating situation will probably never occur. With DHT, the filament's lower Emission and/or breakage normally ends the tube life. So great care should be taken not to underheat or overheat the filament, and switch on the tubes gently, possibly with pre-heated filaments. For this reason you can find sometimes indirectly heated tubes with +/-10% filament tolerance, whereas DHT have maximum 5% or no tolerance at all. Also for such triodes with 5% tolerance, the 100% value is the best. Since there is no considerable out gassing with EML tubes, the getter will stay good during the full lifetime. It means a DHT tube that is at the end of life, will not short circuit, and will not show a bad getter, but have a loss of bias current. However a loss of bias current is no reason to have the tubes checked, unless it is more than say 25%. Still such a tube can be "good" on a tube tester, and as long as the tube sounds good, you can use it. |
