Re: [TCML] Handmade Vacuum Tubes Video: Build A Better 811A Mousetrap?

[Sfxneon@xxxxxxx]

In a message dated 9/12/2008 8:13:58 A.M. Central Daylight Time,  
jimlux@xxxxxxxxxxxxx writes:
Sfxneon@xxxxxxx wrote:
>> Hi  Dave,
>>  
>> That's a mighty ambitious what if!  Certainly not impossible, but it would 
be 
>> more of a learning  experience than a new source for 811's. Not only is 
the 
>>  glassworking part fraught with a lot of not-so-obvious difficulties, but 
so is  
>> getting and keeping a good enough vacuum inside the tube during  
operation. 
>
>I don't think it's all that tricky..  Anyone  who's worked with high 
>vacuum systems has probably dealt with all the  elements needed at one 
>time or another, and, in fact, a lot of the  pieces are available from 
>places like Duniway or JK Lesker.
 
Hi Jim, all,
 
I just don't want people to think that reaching a high enough vacuum for  
electron tubes is as simple as just pumping the air out. It's doable, but as you  
mention it involves cold traps, diffusion, turbo-molecular or drag pumps, 
baking  out the envelope while under vacuum, and an induction heater for firing 
the  getter. You can build some of the equipment if you are familiar with 
vacuum  systems and very good with your hands, otherwise _high_ vacuum equipment is 
 _very_ expensive! 
 
>The art is in getting high performance...
>
>you WILL need some form of really high vacuum, but I suspect that some  
>form of simple sorption pump cooled with LN2(see, e.g., The Amateur  
>Scientist) followed by a suitable getter would probably work.   Fooling 
>with a diffusion pump is best left for folks running a factory,  and 
>turbomoleculars are for those with lots o'cash or very good  scrounging 
>luck.
 
Silicone oil diffusion pumps are actually the simplest and cheapest  
(relatively speaking) type of forepump, but of course they have to be backed by  a 
good mechanical pump. I use one of these on my neon pumping bench, and once  they 
are set up, they function well without much further attention. However,  
you'll still need to use LN2 traps or sputter pumping to get in the necessary  
vacuum. But for electron tube vacuum levels, a turbo or drag pump with LN2 traps  
would be a better choice, although they are very fragile.
 
Best Regards,
 
Tony Greer



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