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I'm thinking of building one and trying it out. Do you have any experience doing this? Is it effective?
Thank you,
Adam
Follow Ups:
I think the realizable aim is to heat the getter flash on the inner glass surface, such that some percentage of the getter flash becomes active.
For a vintage tube, the getter flash may well have become passivated due to capturing gas. The plausible situation is that the getter flash may only get to some moderate temperature during normal operation (especially for preamp tubes) and that raising the getter flash temperature somewhat higher may achieve a net benefit (given competing outgassing process from the glass). However such a heating situation would most likely be best achieved by a controlled local heating of the glass behind the getter flash, such as by adding thermal insulation to the region.
Perhaps the DUT could have its grid leakage current measured, as an indicator of any change/benefit. Even though grid-current is the sum of many processes, a gassier tube may well show up some measurable change.
Do you even know what 'Getter' IS?
Getter is a way to remove residual oxygen from an evacuated system.
You basically BURN something and the oxygen gets tied up and both reduces the overall presure but the Partial Pressure of Oxygen. O2 of course will oxidize hot filaments and may 'embrittle' them.
We had a vac system backfilled with HYDROGEN.....Generally a BAD idea, right? A spark and the residual O2 in the system caused an explosion putting the Bell jar 8 feet in the air and crashing down on the tooling.
When doing repairs, I found I COULD NOT FILE aluminum busbars bas to a sembelence of shape. I had to take 'em to the machine shop who had a heck of a time with Hydrogen Embrittled ALUMINUM.
from google::
Hydrogen Embrittlement occurs when metals become brittle as a result of the introduction and diffusion of hydrogen into the material. The degree of embrittlement is influenced both by the amount of hydrogen absorbed and the microstructure of the material.
Oxidizing metals MAY produce similar effects. I don't offhand know what the filament materials in vac tubes are made of? Thoriated Irridium, maybe?
I KNOW Titanium needs an inert atmosphere to weld.....Thus 'TIG'........The 'I' being inert.....
Too much is never enough
Pictureguy, strangely I do have a pretty good understanding of valve getter operation, and sadly you have not done your homework on valve getter operation. There are some excellent technical references for you to make an effort to read through and appreciate before dissing others' comments. Can I suggest the getter section in RCA's 1962 book is a good start for newbs.
Edits: 12/29/20
Link takes me to an EXTENSIVE table of contents. Those guys really did their research.
Takes me back to looking thru papers on topics of interest to my line of work where research was done by the ream.....
My end of the pie was in vacuum. Pumping down for various semiconductor processes. One machine had the ability to burn a filament to remove the last remaining Oxygen. We'd measure using an RGA....residual gas analyzer.
So I'll have to defer to the real pros on gettering. I just wonder how you 'reactivate' it.....And how much more bad stuff is emitted by the hot parts of a tube that such a getter can address....
I also don't know how good the original vacuum of tubes is.
Too much is never enough
Morgan Jones has experimented with "baking tubes" and reports improvements. Especially worthwhile if tubes have been stored for a long time. YMMV.
AM
I'd be curious....How hot? How Long? And in what atmosphere.....I'd suspect something fairly inert, like perhaps Argon.
Too much is never enough
Excerpt: (the document can be found using your friend google)There are some 'fixes' for low-emission and poor heater-to-cathode resistance that will be covered later in this
article, but ten valves with the same symptoms as the faulty D3a could now be sacrificed to science with impunity.
Since chemical reactions double their rate for every 10°C rise in temperature, perhaps the getter could be provoked
into restoring the vacuum by heating the valves in a kitchen oven? Maximum envelope temperature is typically
specified as 200°C, but kitchen ovens are hardly precision devices, so the author's oven was set to 'warm' (which
turned out to be roughly 100?C or 212?F). Three hours later, the valves were removed and re-tested. Gratifyingly,
there was a noticeable improvement, with all ten valves registering approximately half their previous gas current.
Clearly, the hypothesis was plausible, but the process was rather slow and it was a hot summer, so leaving the oven
on for a long time was not attractive. A back-of-an-envelope calculation suggested that reducing the residual gas
and gas current to <1% of its original value would require more than 7 x 3 = 21 hours (27 = 128). A cookery book
suggested that gas Mark 2 is around 130°C (266°F), and as the oven temperature had previously been measured
with a thermocouple probe as being approximately 100°C (212°F), this should give an eight-fold improvement and
reduce the time required to 7 hours.
Although the AVO valve tester was capable of indicating the gas current and its improvement, it could not make
an accurate measurement, so it was modified by breaking its grid bias supply and connecting a Fluke 89 IV -
ammeter in series to enable a better reading.
The D3a valves were added to the EF184, the oven was set to gas Mark 2 - later measured to be 120°C (248°F) -
and the valves were left overnight for 13 hours. On removal, the valves were tested for gas current, and the results
after baking were added to Table 1.
Although the gas current measurement before baking was necessarily somewhat inaccurate - 1µA on a 100µA FSD
movement - the average improvement in gas current due to baking was a factor of five. Testing the baked valves
showed that although anode current and gm were noticeably lower for all valves, they were far more stable.
Further, they now agreed closely with a known good valve, suggesting that the previously high and unstable
characteristics were a direct consequence of the grid gas current.
Although the baked valves showed an improvement in gas current, the very soft valves remained low emission and
were set aside.
Baking tips
Valves that have no manufacturing defects but have been in storage for many years may accumulate a little gas. If
there is any suspicion that this may have occurred to any significant degree, it is better to bake the valves before
testing rather than risk damaging fragile oxide-coated cathodes.
Grid ionization current can easily be the dominant form of noise in high resistance circuits, such as condenser
microphone head amplifiers. As a result, it makes sense to routinely bake valves intended for this type of use
before selecting for low noise.
Although valves with glass button bases may be safely baked at 120°C (248°F) for 12 hours without damage,
baking valves with phenolic bases at this temperature causes the surface of the base to erupt.
This was not a welcome discovery...
Edits: 02/15/21
Great information. And has enough convincing data.
I learned how to run a vacuum system, quite similar to the one shown at the link. Later systems had automatic valve sequencing so the rough value and hi-vac value would not be open at the same time.
Most systems also includ a Cold Trap on TOP of the diffusion pump to trap any volitiles going either way. The pumping media of a diffusion pump is a VERY high grade of Silicon Oil. The cold trap is typically full of Liquid Nitrogen which condenses LOTS of bad stuff.
When doing system service, You'd close the hi-vac value and let the system pump. Then 'blow out' the cold trap using nitrogen. That warms it up. And you'd see a huge pressure burst in the foreline as the stuff trapped on the cold trap evaporated and was pumped out. Forelline pressures over 100 microns were common at that point.
The best systems would be to the mid to bottom of the '7 range' while a PERFECTLY clean system which had been baked (internal heaters) THAN having the cold trap filled could JUST bump into the 8 range.
Too much is never enough
In my younger years I did built a few reflector telescopes and made my own vacuum coater for the telescope mirrors. Had to get some deep vacuum otherwise the reflective coating would just come off. Learned the value of slowly heating the tungsten filament. The things we figured out before the days of the internet or satelites, far more satisfying and a slower pace of life to boot. But one could not get the quality parts to build a tube amplifier these days without the web so not all is bad.
AM
How good a vacuum could you achieve and how did you measure it?
What kind of pump? 'Mechinical' only, either piston or rotary vane will probably not get you a good enough vacuum for metalization of a mirror.
I coated LOTS of Silicon wafers. Front AND back. Frontside coatings are or Were a 6000 series Aluminum alloy. That would be Silicon at a maximum %age to prevent precipitation and other bad stuff when doing the sinter / alloy step near the end of hte process.
Backside coatings were either a proprietary layering ending with Silver or simplly Gold.
Some metals dont stick well to others and surface preparation is critical. Some films will 'shrink' or 'stretch' after depostion resulting in FILM STRESS measured by the bow of the product. Such films will be either Tensile or Compressive....In extreme cases stuff will simply 'peel' off. Control of depostion parameters helps. Temperature? Rate of deposit? Ultimate thickness? And a bunch of other stuff.
I'll skip the gorey details, but the 2 main techniques of applying such a film are either Sputter or Evaportiaon. Sputter is done in an Inert 'backfill' atmosphere, usually Argon. But Still and all? Depostion presssures in teh 5-range are necessary.
Too much is never enough
I used a two stage mechanical rotary pump and that followed by a diffusion pump. Largest mirror I could do was 10".I had left Philips by that stage but I knew the purchasing officer and I knew that sometimes perfect new tools were scrapped. So I just rang up the hthe guy in charge of destroying stuff in the scrapyard and mentioned that I had heard from the head of purchase that there was a diffusion pump floating around to be scrapped but that I needed it. I picked it up for scrap metal price. Same with the rotary pump. Brother was an engineer and he made a 20mm thick stainless steel bottom plate holding the diffusion pump with some feedthroughs for holding the tungsten filament.
I did not have any measuring equipement but a friend of mine told me to wait for the air to get cloudy and then wait so long before switching on the diffusion pump (forgot how long) and then pump for another whatever time he gave me. I did have a meter on the rotorary pump but have by now forgotten all the details. (That friend also ground an 1 meter experimental mirror for NASA.)
In the end I realised that I was extreemly lucky that nothing untowards happened as I did not have any protection around the glass. It would have been fatal if it had imploded. I only coated pure aluminum for reflection.
(Originally I used a chemical process for depositing silver but never found it satisfactory).
Edits: 02/25/21 02/25/21
You could have improved your vacuum by about an order of magnitude AND made it cleaner inside had you a Cold Trap OVER the diffusion pump. You 'd have needed a source of Liquid Nitrogen and a 'thimble'....
That would prevent backstreaming of the really nasty Silicon Oil (DC 702, for example), or maybe Fomblin? But that might increase the complexity of the valving.
My prize system, a CHA had a Roots blower on top of a mechanical pump. This thing would go to 50 to 100 microns in about 1 minute and cross over where it would bury the pressure meter and immediately drop into the 6 range. This was a fairly open chamber design so the mean free path was large. The High Vac valve was maybe 18" in diameter. A real monster, as such things go. After a service once where I cleaned it thoroughly I got it to the bottom of the 7 range before flashing with aluminum to 'passivate' the interior.
I keep wanting to get involved with Palomar which re-metzlizes the 200" mirror every couple years. I know they don't have a good setup. I've seen their work and plugging leakss with Apezion L is not the way to go.
The problem doing it your way was outgassing of the DP as it warmed than waiting a LONG time for it to cool.
More elaborate valving would have helped at substantial additional expense.
I finished my metalization days with sputter systems. Varian made systems for than common 5" and 6" silicon wafers. No DP on those guys since they'd gone Cryo by that point.
Too much is never enough
The object at the time was to be able to maintain without having to rely on someone else who might clean the mirror of the old aluminium reflective layer and in the process damage the surface. As such it was not important to me how long the process would take and the trick was to invest as little as possible: all up I spend less than 10% of buying through normal channels all the parts. Most of the costs were for buying a new glass dome, seals, oils and a variac for controlling the filament.
The mirror was made from zerodur which came from Zeiss in Jena. Darn hard stuff.
AM
When I worked making low frequency quartz oscillators, we had an optical flat which I think was Zerodur. 1/4 wqve? 1/8 wave? We used it as a reference to maintain grinding tables for making quartz (harder than the law allows) wafers for the manufacture of tiny tuning forks....
Aluminum is prone to oxidation. In a more sophisticated process, you might want to, and without breaking vacuum, apply a very thin coat of Silicon as an anti-reflective coating. Or something else to passivate the surface. I don't know common practice in telescope mirrors.
The OLD mirror can be taken off the substrate using something like Phosphoric Acid or another aluminum etch. Maybe Potasium Hydroxide? NO abrasives or anything else touching the surface.
the layer shouldn't be more than a micron or so thick, anyway.
If you are within driving range of me, I'd love to come watch.....
Than if possible, heat the substrate in a decent vacuum (DP not needed in this case) in order to outgas all the moisture.
Too much is never enough
besides having to travel to New Zealand (where I live) you would need to do also time travel to 1981 as this stuff has long agao disappeared....
Just when I ran out of time machine frequent flier miles!
BTW? Another way to get hi vacuum is to use a Turbo molecular pump. Basically a VERY High Speed vaned turbine. An air molecule 'wanders in' (at very low pressures, random or molecular flow rules) and gets 'batted' towards the exit.
VERY clean but low tolerance for injestion of larger particles. A littel $$, too, and needs an external controller / PS.
Too much is never enough
More web reading and feedback from others indicate that it's not a solution to dealing with gassy tubes.
Thank you,
Adam
I doubt that few enough have tried it to be statistically significant. But you with an induction heater, what is to lose, even if wrong?
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Most likely, as 6bq5 stated, all the raw material, that should be vaporized, is already gone from the cup. In that case you will end up heating the metal structure, which will outgas like crazy. So your net effect might be lower vacuum.
In the unlikely case that you still manage the vaporize the remaining material, the abovementioned outgassing is probably going to swamp that additional effect.
The best advice is to leave the tube alone, if its getter is still there.
Typically once the getter is 'fired off' all the raw material for the getter has been consumed.
Not sure that there would ne much to gain from 're-firing' the getter.
Happy Listening
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