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

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.

I would suggest folks not use a glass envelope, but, rather, use metal 
or ceramic in some form. One less thing to have to do.  In fact, you 
could mount all the leads on a Conflat or similar flange.

One can get the feedthroughs (braze in) as an off the shelf part.


Tungsten wire is readily available (it's used to make invisible 
suspensions for magicians, commercials, and store displays)

What you're talking about is not much more difficult than, say, building 
a fusor or almost any other sort of ion beam apparatus.

I suggest taking a look at "the BellJar" (a newsletter devoted to vacuum 
experimenters) and at the book "building scientific apparatus" by Moore, 
et al.

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.

On the other hand, if you want to return to the roots and use a Sprengel 
 pump filled with pounds of mercury, make your own graded seals, smelt 
your own tungsten.





There's a lot of metelurgy that goes into the selection of the materials for the 
elements, heater and support wires too, along with inserting and firing the 
getter material. 


You can buy the getters.

It looks like the guy in the video built a lot of specialized

tools and equipment that he uses and has a really well equiped machine shop at 
his disposal, and knows how to use it. 

Definitely something where you need a clean machine shop.  For building 
all the electrodes, something like a spot welder or jeweler's oxy 
hydrogen torch might be useful.

Take a look at the fusor people, and see what they're using to make 
their grids.


The video actually left out a lot of

important steps that were just too tedious and involved to be included in the 
movie clip.
 
That said, I'd still be interested in helping with the glasswork and the 
evacuating part if you are really interested in building the innards. I'm always 
up to a good "it can't be done" challenge!
 
Tony Greer

*************

 
In a message dated 9/11/2008 9:04:10 PM Central Daylight Time, 
Davetracer@xxxxxxx writes:

You're right, that's an absolutely astonishing video! Making your own tubes  
... !

And it makes me wonder... 

... What if we made a power triode with high power ratings for  ourselves? 
I'm actually not kidding after seeing that video.

I know from direct vacuum tube experience that 811A-based Tesla Coils are a  
great deal of fun.

I'm a little concerned that the 811A's available now, especially from  
Russia, suffer from quality control, because I've seen their plates go red 
far  
faster than US-built equivalents, and I've seen plates melt unexpectedly  
quickly.

But the French HAM radio operator in the video building his own tubes is  
obviously running off qty 20 at a time. (You see him building inner parts 20 
at  
a time)

If he can do that for tubes which are small and complex, and look something  
like 12AU7's rough size (and please, I'm using the word "rough" here; I  know 
a 12AU7 is pretty complex), and he gets some use out of them in  his HAM rig 
... well ...

Why can't we make some power triodes? Let me draw an analogy here on  what 
they might be like.

Do some early tries the Soviet Way: Bigger And Clunkier.


Like this: Heck, when you started up the older model Soviet tanks, you  
expected to gather a full kilogram of engine shavings in the engine oil 
during  its 
first run, because the tolerances were so sloppy. You have to change that  
oil before moving on.

The 811A's were a design from the 1930's. (My old datasheet was copyright  in 
the 1930's). We don't have to match their "form factor" (their size and  
shape). They have 4 pins -- 2 for filament, 1 for grid, 1 for plate.  
Filament is 
6.3 volts @ 4 amps. We don't have to match that, either. We can make  
something far clunkier that isn't designed to have all the smooth  
characteristics of 
a transmitter tube made for Class A, B, and C operations.  We're not going to 
be broadcasting voice through this.

What we need is something that will accept an input at the grid around 90  
volts (100 V max) at 50 ma, max, to modulate a plate voltage of around 1200  
to 
1500 volts max, switching on and off a current of 130 ma to 160 ma max. This  
is around a 160 Watt tube (max). 

[A lot of the 811A specs are specifically for two-tube systems; pretty  much 
double the above figures, except plate voltage. Many 811A circuits are  
push-pull and so forth.]

There are also better metals and alloys than there were in the 1930's  
(obviously). 

Most of the rest appears to be professional grade glassblowing, and I know  
very little about that, so I'm smart enough to assemble the tube innards and 
let  pros encase it in glass and evacuate it. (And possibly not glass if a 
stronger  material would work.)

My question for the List is this:  What characteristics of the 811A  should 
be improved upon? For example,maybe we should put a physically  stronger 
plate 
in there that won't fail so quickly if it is heated to red. Do we  need more 
current carrying capability? Then it needs a better anode and grid. I  might 
have to blow dust off the books, but vacuum tube design is very well  
documented.

This might be really interesting.

Thanks,

Dave Small




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