Life expectancy of tubes vs heater voltage.

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MaxDM

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I’ve heard a couple of u47’s with old VF14 and my feeling was that the tubes were shot

That was my first thought when looking at this graph.

When is underheating considered safe?
 

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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...
 
On a few tube related power supplies with silicon ht rectification I permantently connected the heater supply and wired the mains switch to HT on/off instead , I find it convenient especially with higher anode current gear .
 
Morgan Jones had a relay kick in a series resistor in the heater chain on standby so it was undervolted /current limited on power up .
 
Most Fender amps have a thermistor in series with the primary. It reduces the initial surge by a factor of 10. It protects both the xfmr, the rectifiers, the reservoir caps, and the heaters.
It also protects the fuse, which, as we know, is the most protected element. ;) Transistors are known to protect fuses very well...
 
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. :confused:
 
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. :confused:

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 should know this, but don't.
 
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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...

By safe, I mean that it doesn't shorten tube life.

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 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.
 
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?

The graph is Tremaine's 'Audio 'Cyclopedia' yes?
Anyway, I was just going off what I gathered from the article I linked but maybe I read it incorrectly?

I don't generally sweat it too much, heaters have been at 6V DC for the last 20+ years so, slightly under spec., but not anything
that's given me, or past customers, issues.

I'm all about protecting those fuses though! 😛
 
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 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...
"At switch-on, the large inrush currents and the mechanical spiral construction of the heater always cause a very small mechanical movement of the heater structure inside the cathode. With a DC supply, these movements are always in the same direction and of similar magnitude, causing a kind of scraping."
I'm not sure about this, since the mechanism of said very small mechanical movement is not formally identified. I believe it's the sheer magnetic effect of close turns that attract them.
IINM the force is always attraction because it's the product of two vectors of the same sign, since the current in one turn is the same sign as in the other. So I don't see how the mechanical effect could be polarity dependant.
I should know this, but don't.
I don't know either, just speculating. That would really matter if we didn't have semiconductors.
 
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.
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.
The only case I know in audio applications of drifting from recommanded value is in low noise designs, in particular with very high grid leak resistance.
 
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.

Sounds good to me. Your speculation will do just fine until another comes along. Thanks.
 
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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.
 
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.

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.

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.
 
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.
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.
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.
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.
 
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.

I believe I am aware of that process, which, I remeber correctly, loosely speaking, creates a 'new' surface on the cathode, by overheating and thus 'burning out' the surface and exposing fresh emission-ready material, so to speak.

What I was referring to was emission characteristics due to the pulsing of AC, which although brief, affects emission, under strain. The reversing of polarity might also have something to do with it, although these are very slight effects, compared to voltage variations, I imagine.

...I need to find the source, though, so a pinch of salt
 
I read something along time ago about flipping the polarity of the heater supply and mechanical/magnetic effects in tube heaters but I cant recall the reference .

I generally go with 2x100ohm 2w to ground on the output of the heater regulator , so in effect either end of heaters are at +/-3 or so volts relative to ground . Im not a huge fan of single ended heater wiring , even when supplying quiet DC , AC style lead dress, balancing resistors and helical filiment also helps cuts down induced hum . Sometimes balancing the heater to ground is unfeasable or unwanted due to circuit opperation(like in a U47) , the grounded side of the heater is often arranged to sheild or otherwise minimise interaction between adjacent pins in the tube base .

There was the old thing of re-generating cathode ray tube years ago as well , it involved running the heater overvolted for a while on a reduced HT . I forget the name of the box .

Ive also heard for small signal valves with low HT voltages(and possibly reduced heater supply) like used in tube mics sometimes benefit from cathode regeneration . The circuit diagram for the U67 quotes a heater voltage of between 5.7 and 6.3 volts , might be to allow for the variation in cable lenght from the powersupply .
I wonder what voltages are valve heaters formed at , presumably they must undergo some kind of burn in at the factory , maybe theres a clue there. Might be worth a try at the regeneration on an VF-14 or EF86 thats otherwise beyond use because of noise in a mic circuit .
 
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