Drop DC heater voltage by 1 volt, what resistor

[soundguruman]

Wombaticus Correcticus.
See, In the audio world, we don't notice if the tube is 25% shorter life. Or most people don't anyway.
In the broadcast world, tubes cost way more usually, and they pay way more attention to it.
That's the difference.
Still happens in both worlds.

The Heavy Metal Monk
Counts down the hours on the tubes.
And he can hear when the tube is deteriorating, and when the tube needs re-biasing.
he has very sensitive ears.
There really is such a person.
He can even hear it better than me.

[Wombaticus]

Here's the figure I was referring to. And Sound"guru"man, I'll caution that while it looks very interesting indeed, context is everything, and that presently is still missing. Posts elsewhere on the intertubes referring to this figure hint that the situation might be significantly more nuanced than suggested by the graph alone.

Wombaticus Correcticus.
See, In the audio world, we don't notice if the tube is 25% shorter life. Or most people don't anyway.
In the broadcast world, tubes cost way more usually, and they pay way more attention to it.
That's the difference.
Still happens in both worlds.

The Heavy Metal Monk
Counts down the hours on the tubes.
And he can hear when the tube is deteriorating, and when the tube needs re-biasing.
he has very sensitive ears.
There really is such a person.
He can even hear it better than me.

[R.G.]

RG, your thoughtful comments notwithstanding, I do seem to recall a figure reproduced in my old copy of "The Audio 'Cyclopedia" that purports to show just this detrimental effect of low heater voltage on tube life. If memory serves, low voltages were actually worse than high. On the other hand, the AC itself is something of a not-entirely-reliable second-source, but perhaps it's a place to begin digging for the primary reference.

Yep, I'm familiar with that. I have looked and cannot find the sources that Tremaine used to generate that.

I'm still looking for the article I saw. GE - who used to make a few vacuum tubes - published the study I was using as a starting point. They were heavily involved not only in broadcast tubes, but also small signal tubes in receiver and audio gear and computers and radar - remember those Eniac and DEWS line things that had people walking rows of equipment looking for dark tubes, and pushing shopping carts full of replacements?

Life for small, low current tubes was a BIG DEAL in this domain, and GE was worried about it. It was a fairly scholarly article showing results of test on numbers of tubes run at modest filament reduction. It is true that if you reduce emission too far, you poison the cathodes, and that's bad. However, the article went into how far you could go and what life extension you could expect.

I'm well aware of Tremaine's graph, as well as the advice in NAB tube maintenance guides and the stuff from Richardson on modern transmitting tube maintenance. They are, as I noted, worried about not putting out enough electrons to maintain a space charge cloud to shield the cathode.

The issues in direct-heated cathodes are simpler. Thoriated tungsten cathodes have to get hotter, and there is a simple tradeoff of emission versus temperature and life versus temperature for both the cathode surface and the burnout of the heater filament as a heater. The desired operating point is the intersection of three curves. An old Federated Broadcasting (?) guide had this shown explicitly.

However, none of the modern stuff I can find talks about low current tubes - like 12AX7s in audio use - except the GE article. I'll find it one day. I don't toss these things in the dustbin.

Wombaticus Correcticus.
See, In the audio world, we don't notice if the tube is 25% shorter life. Or most people don't anyway.
In the broadcast world, tubes cost way more usually, and they pay way more attention to it.
That's the difference.
Still happens in both worlds.

Wombaticus Correctus as far as he go-icus.
In the broadcast world, they use their tubes differently . Found that information yet? I'm still looking. I think I've found a lot of your info for you. But do keep looking.

[Wombaticus]

Here's a link that with some further digging might lead to the primary sources. I'm on the road with just a smartphone, so best I can do at the moment. The Glass Audio article sounds like a good starting place.

[R.G.]

That sounds very familiar. I think you've unearthed a thread to it. I dimly remember the GE stuff as a 5xxx tube, similar to our duotriodes.

I've found a bunch of other stuff, now including a book on making equipment reliable by the military in the 50s, that hits on tube characteristics and operations. I'll do more reading, but they point out that high glass temperature is the biggest predictor of short life, and high temperatures inside the glass are what cause the issues. The high internal temps cause increased gas evolution from glass, wire, and spacers, and this is what poisons cathodes. High heater temps flake off the heater insulation coating with repeated cycles, and also account for higher heater/cathode leakage, as well as faster cathode poisoning. The cathode temp is largely controlled by the heater temp, unlike the plate, and so higher heater temps make the aging process worse at the cathode. The caveat is always that the cathode has to be hot enough to emit enough.

Still digging. 8-)

[R.G.]

Now that I think about it, I *have* that issue of Glass Audio somewhere in the boxes. I got it as a freeby for being a subscriber to Audio Amateur at the time.

Now... where was that box? 8-0

[Wombaticus]

Now that I think about it, I *have* that issue of Glass Audio somewhere in the boxes. I got it as a freeby for being a subscriber to Audio Amateur at the time.

Now... where was that box? 8-0

I'm having some fun digging through some of this material too. The ultimate high reliability application for vacuum tubes were in repeaters for deep-sea submarine transmission cables. This article is an especially fascinating review of the unique design considerations.

[R.G.]

I found a bit more. It wasn't the GE report I'm still foraging for, but it backed up my understanding.

This is from "Electron Tube Life and Reliability" by Marcus Acheson, 1954.

Heater Voltage
Since the user does not, in general, know how much the cathode temperature varies as he varies applied heater voltage, we can briefly restate the example this way: a 1 percent increase in heater voltage increases emission about 4.8%, but decreases life by about 7 percent. Consider what what an unregulated heater power supply - where voltage varies by +/- 20 percent - can do to life. Even the conservative 5 percent, often recommended as limits for some classes of reliabilized tubes, will cut life to about 71 percent of the normal to he expected at rated heater voltage. (Note: A reversal of this process, i.e., a decrease of 1 percent in heater voltage, would, accordingly, increase life by about 7.0 percent. But this is out a means of obtaining conservative tube operation, since emission simultaneously decreases by 4.8 percent. As we further decrease heater voltage, the emission will rapidly become inadequate, and this will also decrease life, but by a different mechanism.) Those figures for variation of life with heater voltage are theorectical. They are, however, confirmed quite closely by several collections of unpublished observed data.

As I thought, the issue is that life drops pretty severely with increases of heater voltage, but increases (at least as regards the heating mechanisms) substantially with lower heater voltage. The price you pay is that emission drops fairly quickly too, and unless you have an application where you don't need much emission, you can kill off the tube through it being starved for electrons to conduct.

It seems that for small perturbations around nominal heater voltage, raising heater voltage kills the tube by one mechanism and lowering it extends its life, but only in small doses.

Still looking for the GE report. That would be one of those "unpublished data" things that Acheson refers to that finally got published.

Anyone here a Hathi Trust member or Hathi Trust partner member?

As an aside, "Reliabilty Factors In Ground Electronic Equipment" (Henney 1956) is available for download free, and is a fascinating look at reliability in vacuum tube electronics, with a few fun surprises. If you're not a book reader, don't bother. You have to read a lot to glean a few nuggets.

One of the nuggets was that the poorest reliability component the military found in trying to figure out why its electronics kept failing was the carbon composition resistor. Apparently, they were terrible. And remember, this is 1956, the early Golden Age.

[Chuck H]

I don't have the hidden stash of reports and data to reference but I think it's fair to say that SGM's scenario proposed for large transmitter tubes is real for large transmitter tubes. The same low voltage mechanism that would be detrimental in that case wouldn't seem to apply as strictly with typical audio tubes, as R.G. mentioned in post #41. Not until a much more profound starvation occurs. So, taken to a reasonable point lower voltages in audio tubes would seem to extend their life at the expense of lower emissions, which may or may not be undesirable also from a tonal perspective.

Taking this in, and relative to R.G.'s last post where regulation is reported on, it seems to me that the typically high filament voltages in guitar amps offers an opportunity for regulation!!!

I've read that preamp tubes can last for YEARS! I usually get a year or a little less from tubes in the first gain stage before they either sound poor, become noisy or do some random weird thing that makes them unusable. Later, less sensitive stages do a bit better. And, as I mentioned, I build a lot with off the shelf Hammonds that have 115V primaries. My filaments are typically 7.0V to 7.2V. Hmmm.?.

Based on this thread my next move will be to regulate the amps I have on hand to 6.2V and see if anything changes tonally for the worse. If not I may do a recall on some of my customers amps and make what would seem to be a prudent correction to my previously cavalier thinking on this matter. Good sounding, non microphonic NOS 12ax7's aren't cheap!!! No good sense in burning them up prematurely.

[Wombaticus]

Anyone here a Hathi Trust member or Hathi Trust partner member?

RG, my institution isn't a Hathi partner, but I believe that Hathi will work with University of Michigan "Friend" accounts. You can get one of these here.

[R.G.]

I don't have the hidden stash of reports and data to reference but I think it's fair to say that SGM's scenario proposed for large transmitter tubes is real for large transmitter tubes. The same low voltage mechanism that would be detrimental in that case wouldn't seem to apply as strictly with typical audio tubes, as R.G. mentioned in post #41. Not until a much more profound starvation occurs. So, taken to a reasonable point lower voltages in audio tubes would seem to extend their life at the expense of lower emissions, which may or may not be undesirable also from a tonal perspective.

Yes. You're absolutely correct. Big, high current/power tubes need as much emission as they can get. To that end, broadcasters typically have techs that monitor the emission/power output of the one or two transmitter tubes in the final and diddle with the heater voltage to keep them at an optimum specified emission. The tube's life is over when the absolute-maximum heater voltage/current/power can't get the specified emission from it; the spare tube is swapped in and adjusted to run correctly, and the old tube is sent off (often to Richardson Electronics) to be taken apart and rebuilt with a new heater and cathode.

Taking this in, and relative to R.G.'s last post where regulation is reported on, it seems to me that the typically high filament voltages in guitar amps offers an opportunity for regulation!!!

Now you're on it.

I've read that preamp tubes can last for YEARS! I usually get a year or a little less from tubes in the first gain stage before they either sound poor, become noisy or do some random weird thing that makes them unusable. Later, less sensitive stages do a bit better. And, as I mentioned, I build a lot with off the shelf Hammonds that have 115V primaries. My filaments are typically 7.0V to 7.2V. Hmmm.?.

A 12AX7 and its ilk can run for 20,000-50,000 power on hours before failing either by not enough emission, too low gain, or outright failure. Eight hours a day, 270 days a year is 9.2 years for 20k power on hours. If Acheson was on it, you're running them ((7.1/6.3)-1.0)*100 = 12.7% hot. If you lose 7% of life for each % hot, you're losing 89% of their possible life. I'm sure that Acheson was quoting a "median" number from his experience, but it's pretty gruesome.

For incandescent bulbs, which are kind of vacuum tubes without the cathode, grid and plate, and with a gas fill that forces heater metal back onto the heater, life varies inversely with the FOURTEENTH POWER of the applied voltage if I remember right. That's a truly ugly life reduction, and one reason that "130V" incandescent bulbs last so much longer than "120V" bulbs. (Not that this matters at all any more since its' now illegal to manufacture incandescent house light bulbs in the USA because they're "too inefficient to trust citizens to use".)

Based on this thread my next move will be to regulate the amps I have on hand to 6.2V and see if anything changes tonally for the worse. If not I may do a recall on some of my customers amps and make what would seem to be a prudent correction to my previously cavalier thinking on this matter. Good sounding, non microphonic NOS 12ax7's aren't cheap!!! No good sense in burning them up prematurely.

One tenet of the "Immortal Amplifier" idea of mine is to spend cheap, cheerful silicon to make the world safe for the Elder Technology of vacuum tubes. I think you're on the path.

[Chuck H]

Ok, regulation would be nice, but... It needs to be super simple and cheap too. As has been discussed it's easy enough to just reduce the unregulated voltage, and that's what I'll do if regulation is complicated. Would simple regulation with an in line resistor and diode clipper of two 5.6V zeners (cathode to cathode) across the winding work? I figured 5.6V + .7v forward drop for 6.3V with this sort of regulation, right? 5.6V is a standard zener value and the largest rating I can get would be 5W. I think that would do because the series resistor will be doing most of the dissipation so the zeners wouldn't be conducting much, right? What about the square wave on the filaments, could this cause a noise problem?

Continuing to evaluate the proposed circuit I realize that it isn't a regulator as much as a limiter since the series resistor would dissipate the voltage below 6.3VAC should the winding voltage drop much. If I used the zeners without the series resistor they would need to do more work and a square wave on the filaments is certain. Should I assume the current across the zeners will be equal to the filament current? In which case I'm still safe with the 5W rating but I want to be sure.

[Chuck H]

Ok, I think I see another rub here. Would the simple zener circuit cause problems with a hum reducing center tap (or false center tap)? Almost certainly a DC elevation circuit, right?

[NateS]

I'm assuming you went with DC heaters for some sort of hum related reason. If you have to knock the edge off a bit, instead of just throwing it away with a diode or a resistor, you could put that diode drop to good use with a capacitance multiplier.

[Chuck H]

AC heaters.