Cooling fan and tube life

[mikepukmel]

Hi All,
Cruising the endless blogistan, I found a thread a while ago, a young player bought a fairly new Marshall tube amp. The guy said he used it in his room, no A/C so it was hot as heck in there, and he had big trouble with tube life. Needless to say, no idea about bias or anything like that, but he said he had gone through tubes every few months until he put a cooling fan on the amp.
I don't remember this coming up, but if tubes are biased right, or wrong, why would a cooling fan help tube life? Maybe it was a red herring and he got several sets of s**t tubes, and just lucked into a good set when he happened to put the fan in.
Anyway, would one of those whisper quiet fans help with tube life, if so, why?
Thanks,
Sparky.

[Chuck H]

Well... There are two directions to go with this answer. One is that there is a vacuum in a tube, so any heating of the elements inside must cool exclusively by radiation. There is no convection. The relatively large space between the glass and the elements means that the temperature of the glass, as it relates to being a mass that can absorb and radiate heat away from the tubes internal elements, is almost moot. So, WRT tube life as it relates to tube temperature, a fan does very little.

But... Then there's the many amps that actually DO benefit from having a fan. I've read here that the fan is there to cool everything else in the amp that is heated by the tubes. While this is also relevant it doesn't account for reports of extended tube life.

So on the one hand we have physics telling us it doesn't work. On the other hand we have builders and users doing it that tell us it does. It wouldn't be the first time real world experience and scientific explanation are at odds.

[R.G.]

The guy with no AC and problems with tube life was probably a red herring, although one which was being dragged along the track in the right direction. This anecdote has the same problem as most anecdotes - it is a retelling of that person's recollection and evaluation of events that may or may not have happened as he remembers them or perceived them, and leaves out a lot of things that would be needed to figure out if it was useful or not. One biggie would be whether he was using a power soak/attenuator to run it at full blast in his room.

First, there is no question that keeping tubes cooler will lengthen their lives. The application where tube life really matters, the finals of radio station transmitters, are actively cooled with serious blower or even water cooling systems. The advice from everything I've read from the "Golden Age" says to keep them cool.

The problem is how to do this in a way that works with guitar amps. The quiet computer cooling fan is kind of ideal, excepting that you need to come up with 12Vdc at a modest current. The stuff I've read on using a fan to cool tubes says that you can make the glass outside of an output tube cool enough to touch even at full power by using even a little airflow across it. The standard naysaying response to this is "it's too loud!", which is nonsense with the fans you're thinking of. I have messed with the fans in my computers and I can't hear some of the fans from 3 feet even outside the cabinet. The second issue is dust. Fans will cause dust collection. A layer of dust on your tubes will insulate them, and make them even hotter inside. Sigh. Filters, cleaning, etc.

My best guess is - yes, they'll last longer if cooled, but that means you'll need to do some worrying about dust, and that the blog guy was possibly relating what actually did happen, but that may not mean much.

[Helmholtz]

One is that there is a vacuum in a tube, so any heating of the elements inside must cool exclusively by radiation. There is no convection.

This is not completely true. Equilibrium electrode temperatures are the result of several heat flows, mainly
1) Radiation from plate to bulb and ambience (cooling)
2) Radiation from hot bulb to plate (heating)
3) Heat conduction via pins and socket (cooling).

A higher ambient temperature will increase bulb and socket temperatures and can be expected to have some effect on electrode temperatures. I don't think though, that an increase of electrode temperature by say 20° will noticeable affect tube lifetime by itself.

But tubes also have a limit for bulb temperature (e.g. 250°C - who knows if modern tubes can stand that much). Exceeding this limit may release absorbed gas molecules from the getter and other parts. And it may even cause the glass seals around the pins to get leaky, permanently degrading the vacuum inside. A "gassy" tube is defective.

Apart from exceeding temperature limits, the failure rate of electronic (and other) components increases with temperature. Often the rule applies, that the failure rate doubles with any additional 10°C. This means that an increase of ambient temperature by 20°C is likely to reduce lifetime by a factor of 4.

My advice to the OP: Cooler tube envelope=longer tube life. But I am not sure if it is a good idea to direct a strong airflow directly at power tubes, as this might cause non-uniform bulb temperature and stress in the glass.

[J M Fahey]

The application where tube life really matters, the finals of radio station transmitters, are actively cooled with serious blower or even water cooling systems.

Sorry but you can´t compare them to regular tubes in Guitar amplifiers.

"Ours" have a basically cylindrical plate inside a glass bottle, with vacuum (perfect insulator) between them.

High power transmitter tubes are not built that way.

Rather the metallic plate *is* the vacuum bottle, you still have vacuum between it and screen/grid/cathode but the plate itself is freely accessible for cooling.

See the popular 4CX250 as an example: the plate (the metallic tube in the middle of the assembly) not only is visible and accessible, but it comes with built-in fins and an aerodynamic tube enveloping them, for maximum efficiency:

[R.G.]

Sorry but you can´t compare them to regular tubes in Guitar amplifiers.

Sure I can - just not completely accurately.
And strictly speaking, I didn't really say that they were the same as our glass-envelope tubes, only that they were actively cooled.

You are correct on the differences in structure, and that it's much more difficult to get heat out of a plate inside a vacuum envelope than out of a plate directly exposed to cooling fluids.

Combining some of Helmholtz' observations, the main ways that plates are cooled in guitar amp outputs is by radiation from the plate to the glass envelope, with some minor conduction down the supporting pin structure. But cooling the glass envelope does help cool the plates, as glass re-radiates heat if it's hot, so there is a back radiation from the glass into the hot plates that reduces the net heat the plates get rid of. Taken to the extreme, you could heat an unpowered tube plate by just heating the exterior glass enough to have it re-radiate to the plate structure. So keeping the glass cool helps keep the plates cool.

And you can do that by blowing just a little air across the glass. Glass isn't too effective a heat conductor, so removing even a little heat per unit area from the outer surface cools the outer surface a lot. Probably more importantly, it increases the heat flow through the glass by forcing a big thermal gradient from inside the bulb to outside the bulb, so the glass does conduct more heat out. Yeah, glass isn't a good heat sink material, I know, but it's all we have to work with.

How much better things get are tricky to evaluate. The glass and plate exchange heat back and forth according to the Stefan-Boltzman Law as modified for gray bodies, emissivity, incidence angles, and strings of things that are why I took my one thermo course and walked away slowly...

But from a boundary-conditions viewpoint, every watt-second carried off the glass started out on the plate, screen, and cathode, less only the minor pin-conduction transfers.

I mess with this a bit back in my amp design days, and concluded that although fans help, they're a PITA. Better to design for lower tube dissipation or better natural convection. What any air movement at all is good for is safety. A hard-working 6L6 or EL34 will have envelope temps as high as 200C. Touch that and you get burned. Minimal airflow means you don't get flash burns.

The >>safety<< testers pointed this out to me when they noted that although I had put a perforated steel plate over the tube access at the rear, children could poke their hands through the speaker cone from the front and still get blistered that way. Sigh.

[R.G.]

I went off to look up some numbers. Turns out that glass is dang near a perfect radiator in the IR. A perfect black body has a radiation constant of 5.7 (times a bunch of units and constants), lamp black is 5.16, and glass is 5.13. So glass is a lot better than many other materials. A high radiation constant also implies a lot of absorption as well. So glass both sucks in the radiated heat from the innards and radiates it away well.

What it sucks at is transferring the heat from the inside of the glass to the outside, and at transferring it to the atmosphere gasses outside. The thermal constant of glass is about 1 W/m*K, or one watt per meter (thickness) times the temperature difference. If I knew the thickness of a typical output tube glass envelope, I'd have a go at calculating the temperature difference across the glass in normal versus cooled situations.

[Helmholtz]

I know for a fact that conduction cooling via the socket is considered essential for the lifetime of (low and high pressure) discharge lamps. There is a close relationship between lamp and tube construction and technology. No wonder that all major lamp manufacturers also made tubes in the past: GE, Sylvania, Philips, Osram, Tungsram, Mazda, Matsushita, Toshiba etc.
It's not about plate temperature. Its bulb and pin temperatures that matter.

[R.G.]

I suspect that the pins are a significant cooling mechanism for discharge lamps. Different setup, dramatically higher temperatures, and (I think) much higher dissipated power.

I do know that all the tube data books I've seen talk about maximum bulb temperature.

Electron tubes die by several means. A big one is loss of emission from the oxide coated cathode. Emission testers were the commonest drugstore tube testers. Over time, the oxide layers get poisoned through several paths, but they all lead to the tube getting "weak". If the tube gets too hot, it can bake the trapped residual contaminant gasses that were adsorbed onto the interior surfaces off, and this poisons the cathodes quickly. Overheating can kill a tube by letting the fine grid wires sag and touch other elements, or by the repeated heat/cool cycles mechanically stressing a grid or heater wire until it opens or shorts to something else. Plates can simply melt. I've seen this.

One of the most unusual ones is where the heating from the plate was so concentrated on the area of glass nearest the plate that the glass softened and air pressure forced the glass into an inward-pointing cone toward the plate.

The picture that JM showed us shows how to deal with this: rearrange the elements in the tube so that the part that generates the most heat - the plate - gets the most cooling.

[soundmasterg]

I can testify to tube glass getting hot and radiating heat away! I know that in one amp I built out of an old Bogen, I used the basic Bogen layout, but I added a backlit panel with LED's, and I located those too close to the power tubes. I can get the whole string working, and then at some point a couple will get too hot and the whole string goes out, so I'll have to add a fan to this amp, but I think I'll blow it right on the LED's. Should solve the problem.

Greg

[Jazz P Bass]

"One of the most unusual ones is where the heating from the plate was so concentrated on the area of glass nearest the plate that the glass softened and air pressure forced the glass into an inward-pointing cone toward the plate. "

I have seen this more than once on 6L6 tubes.

It's pretty neat.

[Enzo]

I have an EL34 on my shelf with the glass melted and pushed into a tiny cone towards the plate.

[nosaj]

I have an EL34 on my shelf with the glass melted and pushed into a tiny cone towards the plate.

Pics please

nosaj

[Chuck H]

In this image I found you can actually see the glass flattened against the plate on the left tube.

[Helmholtz]

To summarize my thoughts on this:

1) (Glass envelope) power tube plates dissipate thermal power mainly via radiation.
2) Forced air cooling has no direct and only little indirect effect on plate temperature.
3) When saying "It's not about plate temperature" I meant that raising the ambient temperature by say 50°C will hardly affect the plate under normal operating conditions, but
4) It makes sense to decrease bulb temperature, as this reduces heating by re-radiation from the bulb, but most of all reduces the risk of outgassing and maybe leaking seals.
5) Once there are enough free gas molecules inside, the risk of catastrophic runaway dramatically increases.

For more detailed information:
http://www.tubebooks.org/Books/Atwoo...um%20Tubes.pdf

See especially pages 24 to 29.