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#11
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Randy or Sherry Guttery wrote:
Depends on the type of tube -and the service it's in. For instance - many large transmitter tubes are run at 100% filament voltage for their 1st 100 hours - then reduced to (usually) 92% and run until their emission starts falling off The reason for dropping a high power transmitter tube's heater voltage when running under power is to compensate for the heat added to the cathode by the anode current, and the reflected heat from a toasty hot plate. The hot anode acts like an oven surrounding the heater, and as such forces its temperature up. If you don't reduce the heater voltage from the nominal value, the heater will overheat, and cathode life will be reduced. -Chuck |
#12
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Whether operating with reduced filament voltage is detrimental seems to
depend on whether the tube is an oxide-cathode type. Pure tungsten or thoriated tungsten filaments do not suffer damage from filament under-voltage, while oxide cathode types do. Thanks to W8JI for the info: http://www.w8ji.com/vacuum_tubes_and...e_failures.htm Vacuum power tubes, using old valves, and vacuum tube failures Chuck |
#13
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Randy or Sherry Guttery wrote:
Running tubes such high power tubes (like the 3CX2500, 4CX3000, etc.) at 92% for as long as possible adds years to their life. snippage You haven't posted anything contrary to the +/- 10% rule given by RCA. |
#14
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Chuck Harris wrote:
wrote: NOT. The resulting reduced emission shortens, not lengthens, tube life in most cases. I've heard that, and it makes me wonder if the reduced life is only as viewed with the tube running at the reduced voltage. If you take a tube that has reached end-of-life with a reduced heater voltage; I wonder if it would still have a bunch of time left at the correct heater voltage. I think this also depends on the tube.... as I recall it is much less of an issue for power tubes than small signal tubes.... --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#16
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chuck wrote:
Pure tungsten or thoriated tungsten filaments do not suffer damage from filament under-voltage, while oxide cathode types do. If you reduce B+ and screen voltages proportional to the fil. voltage reduction, you negate the problem of depleting the space charge and stress-cracking the oxide coating (assuming there is a decent amount of emission left, of course). Yes, output is reduced. But I think most of us know the difference between 100 watts and 70 watts isn't worth mentioning. 73 Dave S. |
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