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Well I guess the filaments of at least some tubes are tougher than I thought they would be. I was trying out a different driver for the 91A and I neglected to change the filament transfo. The old driver was a 310A that needs 10v, and the new driver is a 6C6 which needs 6v for the filament.
It must have been a full 5 minutes before it dawned on me what I had done. It sounded a tad strange, but I had not heard this tube before. It was when I noticed the filaments glowing brighter that I thought they should that I yelled "oh crap!" and jumped for the variac and shut it down.
I am sure I have shortened the life expectancy of those two tubes by quite a bit, but I was truly amazed that both of them still worked fine when I hooked them up to a 6v filament transfo.
This makes me curious as to how much abuse in filament voltage the average tube filament can take. I mean that was more than 50% for Pete's sake! Does anyone know for sure?
Also, this was AC on the filaments, not DC. Does that make any difference?
Edits: 12/12/14 12/12/14Follow Ups:
I do know that if you put a sneeze more voltage on the filament of a 26 tube the tube fries almost instantly.
I think some tube filaments are more delicate than others, especially the lower voltage filaments.
In the manufacturing process, heaters are subjected to 1.5 times normal, sometimes more, for short periods like 30 minutes.
If I recall correctly, raw tungsten (light bulbs for instance) have a lifetime inversely proportional to something like the 15th power of the voltage. (10/6)^15 times 5 minutes is about 175 hours of lifetime lost. No biggie.
Thanks Paul. That's very interesting and the first time I have heard of either fact. Do you know if AC or DC makes any difference? Not likely I suppose.
Edits: 12/13/14
Depending upon vintage, I am under the impression that filaments and cathodes have certain types of coatings on them. If you operate the tube as far off as you did, I can not imagine the coating will act the same as if the tube always was within 5% of its rated voltages. BTW, I usually LIKE -5% of rated on my filaments.
Don't feel bad, you can buy more tubes, and its all part of the learning process. You won't do it again I betcha.
Jeff Medwin
"I can not imagine the coating will act the same"
I would think that some VTs could have their coating degraded within a minute or two at 2X the voltage. At that point, it's no longer a question of how long the tube will last as much as whether it still functions properly. A test for peak current capability might indicate that significant degradation has occurred in the scenario being discussed.
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Buy Chinese. Bury freedom.
It will be educational to see how long they last.
You could probably get many hours on those over-voltaged heaters until they failed. Tubes are pretty rugged as a whole. You can screw-up and red plate a tube and if you catch it relatively soon the tube will survive just fine.
5 minutes likely did very little damage if any.
I will be curious to see if they do have a shorter life or not. I always thought that much over would have vaporized the filament. Still wondering if AC or DC matters.
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