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

[Sfxneon@xxxxxxx]

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. 
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. 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. 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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