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In Reply to: RE: Idea for Prolonging Tube Life posted by Lee of Omaha on September 01, 2011 at 08:37:05
With a vacuum in the outer envelop? Instead, pressurize it with a heavy inert gas like argon. That'll keep the air out and the vacuum in: seems much easier than employing a pump.Also the getter flash is installed within a tube to "pump" out the residual gas in a vacuum tube. One of the many issues is that the materials used in fabrication often have molecular gas dissolved into the glass and metals. A pure hydrogen bake out helps alleviate this molecular gas. This involves a bake out in a pure hydrogen atmosphere at about 1K degrees. The lighter hydrogen tends to pull out the heavier gasses like oxygen and nitrogen, the principle constituents of the atmosphere, and then the lighter hydrogen is easier to evacuate when the vacuum pumps are hooked up.
Obviously this can be rather dangerous, but is still employed in developing mu metal, and it was used on critical tube elements in the US and Britain during the cold war era. I doubt if the procedure is employed in the Communist manufactured tubes, which is why they tend to become very gassy fairly quickly. Older American and Western tubes have significantly better vacuums and treated subassemblies.Glass often develops a layer of oxidation on the surface: not visible, but present. An acid wash just prior to assembly helps eliminate this oxidation layer. These procedures can be found on Pete Millet's excellent website, BTW, where he has scanned a large number of documents related to tube manufacture.
Nothing you an really do about osmosis through the leads though. That primarily due to the glass not adhering to the metal. The oxidation on the tube pins, curiously, is a necessary step in order to insure the glass adheres to the metal
For tubes which develop residual gas after use, the easiest way to get a better vacuum is to reactivate the getter. Theoretically the getter flash is activated whenever the tube is heated (as in use). However, I find that the residual gases often ionize and then cling to the cathode or anode, never reaching the getter flash. Activating the getter flash by placing the tube in a simple oven activates the flash without ionizing the internal gas contaminants. This uses the existing barium getter to work on the developed contaminants.
I've been using this procedure for years now. Tubes which start glowing blue after usage, have the blue glow disappear, after a prolonged bake out. In addition the initial top extension returns.
Stu
Edits: 09/07/11 09/09/11Follow Ups:
They should use semiconductor grade silicon glass/ Si02 for the envelope.
Hydrgen meeting oxygen at 1000c will instantly form steam.
Semiconductor oxidation is most rapidly done in steam. The steam is made by flowing oxygen and hydrogen into a special 'torch' chamber where it is flame on. Keep the mix 'oxygen rich' to avoid possible left over hydrgen, which is a no-no.
Hydrogen is WAY reactive and if I were doing the process would flow a TCA mix into the glass tube to do a chemical getter. Works in semiconductor industry.
Too much is never enough
a 2nd envelop is not a great idea. You can't force the vacuum 'in' with external pressure.
As for an acid wash of the glass prior to pumpdown / seal, maybe. I don't know what is used for envelops of tubes, but in the silicon industry, a Silicon wafer will develop 20u of 'native oxide' nearly instantly after immersion to hydrophobic in an HF solution. Severe time limits are imposed from clean to the next process. Usually an hour or so.
Now, vacuum is a funny condition. Mechanical pumps like piston or the more common rotary vane and in some cases a roots 'blower' can only go down so far in vacuum. 10e -5 militorr is common for these. Most systems will cross over to what is called 'hi-vac' at about 25 to 50 mt where the NEXT stage of vacuum is used.
The real hi-vacuum pumps are of several different types.
1. Oil diffusion. Don't make me explain this one. Very clean when used with a LN2 'cold trap' to capture backstreaming oil, the will go to 10e-7 vacuum range easily.
2. Turbo-molecular. A VERY hi-speed driven turbine. Any atmosphere molecules which come in contact with the spinning blades are driven down, into the pump and a higher pressure region .
3. Cryo Pump. Using an external compressor of helium, the coldest part of these pumps is about 10degrees kelvin. Other parts of the array are warmed, but still everything freezes out except hydrogen and helium.
4. Gettering is used as the last stage.
Vacuum doesn't 'pump' in the water or pressurized air sense. Once below a certain pressure molecules don't 'flow'. They will bump into walls more often than one another. So, the flow regimes of gas in a vacuum system start with a viscous flow....sort of like water.
As higher and higher vacuums are reached, flow is called molecular. This is pretty random and pumps rely on entrapment or entrainment...see above.
TUBES? Well, that is a good question. It would be very difficult to reduce the atmosphere in one of them to very low levels. The small hole and odd shape work against it. The getter is that last best chance. And as Unc points out, the seal between glass and metal pins is problematic. Helium will diffuse thru glass. Our leak standards were a helium bottle with a valve and inset piece of thin glass. Leak rates were on the order of 10e-6cc per second. The bottles, smaller than a coffee thermos last for decades. I never knew one to run out....in over 30 years in the industry.
Too much is never enough
Stu,
How do you do this bake-out in a regular oven? If so, at what temp and for how long?
Thetubeguy1954 (Tom Scata)
SETriodes Group --- Central Florida Audio Society
Space Coast Audio Society --- Fullrange Drivers --- Front & Back Loaded Horns
======================================================================
"The man that hath no music in himself nor is not moved with concord of
sweet sounds is fit for treasons, stratagems and spoils." - William Shakespeare
I've written about this before.I use an old toaster over and stuff it full of tubes, If you have inquisitive household members, cover the tube with some fiberglass cloth to avoid thermal shock from curious eyes.
Once loaded I turn the oven on at the lowest setting and let the load sit for 3o minutes, I raise the temperature every 3o mites another 5o degrees till I reach about 300 degrees, and then I just let the tubes bake at that temperature for as long as I feel comfortable ( I've let the tubes bake over night). Once the baking period is over, simply unplug the oven and let it cool slowly to ambient room temperature, about3 or 4 hours.
I often do this to even new tubes as some residual gas seems to leach out after sitting for long periods ( NOS types). The result is a more dynamic sounding tube with a better top end.
Incidentally polishing tube pins also gives a much improved top end ( at least for 7 and 9 pin tubes where the pins enter directly through the glass). You also can effect big sonic changes in octal type tubes by changing the solder in the pins!
StuPS I should add that you could go higher in the bake out temperatures, but I do not recommend going past 350 degrees, primarily because the printing will yellow. Makes it difficult to resell. Ive gone up to 450 degrees with no issues to the tube however.
Edits: 09/19/11
You know that vacuum is a heat insulator, of course?
The tube envelop will get hot and the pins will conduct heat to the interior structures, but inefficiently.
I don't have a clear idea how hot you could get the innerds of a tube by your method.
Nothing is going to 'leach out' of a tube.....ever...while it is under vacuum.
Too much is never enough
Can I use a fan-forced oven?
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Yeah, you know us metrically deprived Americans. Convection ovens are fine, the key is to heat and cool the tubes slowly, or at least slow enough to insure that the differential between expansion rates of the metal and glass do not cause any issues.
Stu
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