If someone would get retentive about it, they would include delayed B+, constant-current heaters, and a mechanical bistable relay that would invert heaters polarity at every turn-on.Sadly, one of the best ways to prolong tube life seems to be to use AC heaters so, I say, damn the torpedos, just use DC and replace your tubes when they reach end of life.
Edit: running heaters in constant current rather than constant voltage can stop in-rush current surges if you feel like going that route...
If someone would get retentive about it, they would include delayed B+, constant-current heaters, and a mechanical bistable relay that would invert heaters polarity at every turn-on.
I'm not sure the actual MTBF would improve, though.
What do you mean by safe? Underheating will result in a lower cathode temperature which will affect the emission which in turn will affect the the parameters of the tube. Which way they will be affected is unknown and may or may not be beneficial. So the question is why would you wish to under heat?
Cheers
Ian
Data suggests that a 5% decrease can prolong tube life by up to 50% So 6V rather than 6V3.
Underheating is sometimes used for noise reasons but, as Ian says, will reduce electron emission if taken too far.
Sadly, one of the best ways to prolong tube life seems to be to use AC heaters so, I say, damn the torpedos, just use DC and replace your tubes when they reach end of life.
Pretty good article here:
https://audioxpress.com/article/the-internal-life-of-vacuum-tubes
Edit: running heaters in constant current rather than constant voltage can stop in-rush current surges if you feel like going that route...
If a 5% decrease indeed makes tube life longer, it seems to contradict the above graph.
It would seem to me that manufacturers would find it in their best interest to specify the ideal voltage, not an overrated one, but who knows?
I was basing that on the premises that a failure mechanism is caused by assymetric mechanical stress. Not for electrostatic coupling or assymetric insulation, or whatever...Assuming for a second someone is retentive - on the inverting heaters every turn on thing, would a bipolar +/- 3V supply negate the need for that?
Centre is grounded or elevated as need be.
I don't know either, just speculating. That would really matter if we didn't have semiconductors.I should know this, but don't.
Tube mfgrs have devoted thousands of hours making sure their customers were happy with the life expectancy of their products, including in their research fluctuating line voltage, ambient temperature, marginal design.I have always been told (by old guys) to stick with the specified heater voltage for best lifespan...
Also, that pre amp stage tubes can last well over 20 years in a properly designed circuit.
That's what I've been told, so I'd like to know more, interesting subject.
...I was basing that on the premises that a failure mechanism is caused by assymetric mechanical stress. Not for electrostatic coupling or assymetric insulation, or whatever...
I don't know either, just speculating. That would really matter if we didn't have semiconductors.
By many years of experimenting in this issue with probably more than 2500+ tubes in mine Fairman production I've experienced repeatedly, STAY for the specified values.
The correct factory specified heather voltage defines the correct cathode TEMPERATUR, optimized for keeping all tubes specifications correct, and for the longest possible life time. Often people conclude idel the tube softly will extend life and less wear. But, like a badly adjusted gasoline engine it will build up a faster wearing slag to the cathode emission layer, making the tube cathode earlier weaker and noisy and worn out earlier.
That's the "rejuvenation" process. It was used to prolong life, but couldn't be repeated more than a couple of times. Also it did cure some problems, not all.I've also read that the pulsing of AC current, at least on some tubes, has a positive effect for keeping the cathode in the best possible working condition, although right now I don't have a source to quote for that.
Elevating heaters does not suppress hum as efficiently as running them at a very clean DC, particularly if the cathode is un-bypassed.It's not too much of a laborious process to add a couple of resistors and a cap and float the AC supply above the potential of the cathode, to reduce hum, as an alternative to DC.
That's the "rejuvenation" process. It was used to prolong life, but couldn't be repeated more than a couple of times. Also it did cure some problems, not all.
Elevating heaters does not suppress hum as efficiently as running them at a very clean DC, particularly if the cathode is un-bypassed.
AC heaters have provided very usable performance, though. Depends on expectations and pratical issues.
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