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Building DIY Vacuum tubes

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Cool, cool, cool! I like the way your mind works. Give us more!

Post some photos of your shop and machines. I own a vintage Tool Room SB Heavy10.

Chris
 

Harald Kapp

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Awesome thread.
Even though I'm not into tubes, I just love to follow what's going on here.
Definitely not the usual EP stuff.
:):):)
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Awesome thread.
Even though I'm not into tubes, I just love to follow what's going on here.
Definitely not the usual EP stuff.
:):):)
Yeah, it's a visual feast! :)

Chris
 

George Schmermund

Jun 19, 2016
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The new fixture works very well for making grids to install into the current triode design. The yield and reproducibility of grids have gone way up. I plan to make a fixture for the next plates and filaments. It should be a reasonable task to build tubes with very similar characteristics when everything is in place.

The photos show the ladder grids before and after spot welding the two sections together. There is some variation in the support post wire bending.

The assembled tube is made from parts that were just spot welded together without the use of any fixtures. As long as it's welded up I might as well use it to practice fusing the envelope to the stem. That process alone is going to be a real test for the next iteration of glass lathe that I've been experimenting with. Doing this on a small budget is a challenge in itself. Fortunately I have plenty of beer to see me through this project.




1-P1040568.JPG 3-P1040572.JPG 2-P1040575.JPG
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
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George, your work is magnificent!

Chris
 

George Schmermund

Jun 19, 2016
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Chris - Thank you for the adulation. It may be premature depending on how well these tubes work once they're processed and in a circuit. The assembled tube in the photo is one that I just bent into shape with pliers and spot welded together. I expect the newer ones built using fixtures to be the best performers. we'll see.
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
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George, how well your tubes perform is a separate matter entirely. It can not change or diminish the beauty of your work. The fact that you're fabricating vacuum tubes is, in itself, worthy of all the accolades I can possibly shower you with!

Chris
 

Douglas Koo

Jun 18, 2021
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Hi , i know that you mentioned about the diffusion pump oil used in the setup.

DC704 diffusion pump oil. Polyphenyl-ether oil (oxidation gas resistant). hydrocarbon oil. I know these well known oil are best suited for diffusion pump oil depending on applications.
The vapour pressure of the oil (at 25 Celcius) will decide its ultimate achievable vacuum level.

I am wondering whether regular industrial Silicone oil ( like PDMS poly-dimethyl-siloxane oil) will work as well? Do you have any experience trying out such oil.
 

hevans1944

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I am wondering whether regular industrial Silicone oil ( like PDMS poly-dimethyl-siloxane oil) will work as well? Do you have any experience trying out such oil.
This thread has been idle since July 6, 2016 when @George Schmermund last posted:

A fellow named Trevor has started a forum aimed at the group of folks that might want to make vacuum tubes. I'll be posting most of my tube projects over there. If any one here is interested in following this here's the link: http://diyvacuumtubes.com/

I am retired now, but have worked with oil diffusion pumps and high-vacuum systems for more than thirty years, mainly with various small Varian models. My only experience with silicone oil was using it, with dry ice, to cool snow samples collected in a vacuum dewer (thermos bottle) to well below the freezing temperature of water. It is not important to this discussion why we did this. I personally have no experience with using silicone oil as a replacement for DC704 diffusion pump fluid.

Considering how small a quantity of pump fluid is required (usually on the order of a few dozen cubic centimeters for small pumps), why take a chance on using anything other than genuine diffusion pump fluid? It doesn't have to be the expensive Dow Corning DC704 fluid. Patents on diff pump fluid have long since expired, and there are now companies selling chemically identical products, as well as companies that reclaim diffusion pump fluid and resell it (after cleaning and fractional distillation) at a considerably reduced price versus the "high priced spread." Unless you are going for ultra-high vacuums in the 10-10 Torr realm, using top-grade diffusion pump fluid is not necessary. And if you do need such hard vacuums, a turbomolecular pump is surely necessary.

At what temperature does "industrial silicone oil" begin to break down? Does silicone oil have the high-molecular-weight hydrocarbon chains that provide momentum transfer necessary for efficient operation of "jet type" multi-stage diffusion pumps? Pumps used to evacuate electron tubes need to produce a clean vacuum, so a liquid nitrogen cold trap or a water-cooled baffle, located at the top of the diffusion pump, is a good idea to prevent pump fluid vapor from contaminating the vacuum.
 

Douglas Koo

Jun 18, 2021
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Dear hevans1944,
Thank you for helping out !

Yes, I would agree with you on the DC702, DC704, DC705 diffusion pump oil availability in the market and their price tags.

Anyway, I am going to try that out in my diffusion pump (I found from scrap yard), using PMX-200 (poly-dimethyl-siloxane) PDMS, I am going to use 50cSt and see it's performance. I know that this silicone oil comes with different viscosity (5 cSt, 10, cSt ... 1000cSt..)

I read from online research journal, that their molecular weight is highly dependant on their viscosity. (50 cSt) silicone oil probably has 2000-3000 g mol^-1

50cSt silicone oil by chance , it has 250 °C boiling point ( 0.5 mm Hg). 100 cSt silicone has even higher boiling point (0.5 mmHg), but I am not sure how much °C

50 cSt also has good surface tension (cohesion forces) properties (roughly 23 dynes/cm) , not too bad compared to dc-705.
I don't know whether such properties of 50 cSt silicone oil can be a good fit for diffusion pump oil.

Regarding the "break down" temperature of silicone oil, I am not sure. But I hope that it's boiling point at 0.5 mmHg would lower than it's cracking temperature, so that I can use it as diffusion pump oil.

I know that highly refraction mineral oil (those clear transparent vacuum pump oil) can be used as diffusion pump oil, but they can be oxidized. (Even though a bit cheaper than silicone oil PDMS, and dc-704)
 

hevans1944

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@Douglas Koo, it sounds like you know what you are doing. Please let us know if less expensive silicone oil works in a jet-type oil diffusion pump.

Meanwhile, @George Schmermund mentioned a design he did for a small diffusion pump that works by entraining gasses on the cooled inner wall of the pump using low-velocity heated pump fluid. The gasses trapped in the now-liqiud fluid fall with the fluid to the bottom of the pump where they somehow escape the pump fluid and are pumped out of the foreline by a mechanical roughing/foreline pump. I don't know how well this works, or any details of the design, but it could be that it works just fine with silicone oil. Maybe George will chime in and let us know, if he hasn't purposely left to use other sites.
 
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