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| dc vs ac vacuum tube filament winding for reliability/lifetime/durability? |
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| coppercone2:
--- Quote from: amyk on October 30, 2018, 12:22:12 am --- --- Quote from: coppercone2 on October 30, 2018, 12:02:30 am ---Also you have gravity inside of the tube. AC reverses direction periodically so it might allow something to fall down to the bottom vs get directed into something else its attracted to? --- End quote --- :wtf: :palm: are you trolling? Yes, electrons have mass and can be affected by gravity. No, in practice the electrostatic and electromagnetic forces are many many orders of magnitude higher. --- End quote --- I mean if some how a small particle flakes off of something and gets charged. I thought I read about that before with flaking. I think the wear occurs kinda like chipping of concrete, you get some dust but occasionally you get something larger being thrown off. I think this can some how result in electrically charged 'dust'. I thought with DC if that kind of generation occurs it would possibly be thrown more to one particular direction. I don't think the filaments always evaporate ideally into a gas like you think. I noticed in high power equipment like (old) argon lasers you actually end up getting crap that basically rattles around the bottom of the vacuum tube. If you get those kind of particles it means you get everything in between from nanosized particles to what we would consider dust by our standards. As it breaks off I imagine there is small point wehre its not connected to the circuit and there is particles rubbing against each other. I don't know if in that instant you can consider them to be separated by a vacuum (at that point in time in that tiny area) either as its fracturing. |
| vk6zgo:
In the real world, as it operated for many decades, ac operated indirectly heated heaters in valves (tubes) was the overwhelming standard. They were used in low signal level ampliers with no ill effects. Occasionally, a piece of test equipment would use DC for the heaters, but, in that case for signals several orders of magnitude lower than any audio or RF equipment. Portable equipment usually used directly heated filament type cathodes, because they were more efficient emitters, & the designers were struggling to get the most performance possible for the longest battery life. One place where indirectly heated tubes were widely used with DC heaters was in car radios. The use of DC was obviously because it was available at a voltage useable with 6.3v heaters, indirectly heated tubes were used because they offered higher performance, & also because the "DC" from cars is quite noisy This latter was likely to be injected via the directly heated cathode & lift the noise in the signal passing through the amplifier stage. One of the standard tests for indirectly heated tubes was "heater to cathode leakage ". If this was out of spec, tubes were discarded, & replaced with new ones in Broadcast, Comms & other critical work. Some, if not many tubes available today are "pulls" from years ago, which have been tidied up, (maybe) tested for emission, put in new boxes & sold. If these have out of spec' heater/cathode leakage, (which many do), & are put into audio amplifiers, they will almost certainly cause hum. If the heaters are fed with nice regulated DC, such tubes are probably quite useable, so the urban myth has developed about running tubes with DC heaters. Lifetime/durability? With new, in spec tubes, the lifetime with ac heaters was normally many thousands of hours for the less stressed ones, rectifiers & power output tubes had shorter life expectancy. Car radio tubes seemed to have similar lifetimes to those in domestic radios, which was a bit worse to a lot worse than those used in Broadcasting/Comms. In conclusion, I would say there is no difference, between ac & DC, all the cathode sees is heat. Open circuit heaters are fairly rare, most cathode faults are when the cathode material loses emission, or the insulation between heater & cathode fails. |
| LaserSteve:
On the arc based lasers, sputtered material defies gravity and heads for the coldest, most neutral place it can find, the critical mirrors and windows on the end of the tube As lasers run horizontally , most of the crud ends up above the cathode condensed on the cathode end bell. It buries a considerable amount of gas when it lands. Subtle design steps are taken to prevent the windows from being fogged, the emission requied being too great to use an indirect cathode with a seperate heater. The arc current returns through a center tap on the secondary of the cathode transformer to balence the ion bombardment on the cathode. On dc excited broadcast tubes and thyratrons using direct cathodes, usually the most positive end of a hot cathode is wired as to be toward the plate to minimize ion bombardment. Steve |
| David Hess:
For common indirect heated cathodes there is some reliability advantage to biasing the heater positive with respect to the cathode which prevents electron emission from the heater from damaging the insulation between them. https://archive.org/details/GettingTheMostOutOfVacuumTubes |
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