Re: Are deflection plates separate from the tube? Do Tek tubes use wehnelt caps?


Don Black <donald_black@...>
 

I think you will find the SEM cathode is a tungsten filament and can stand being exposed to air. Its drawback is much lower electron emission than oxide coated cathodes which are destroyed by exposure to air. The first tubes used tungsten filaments, then thoriated tungsten which had higher emission but less than oxide coated types. Both pure tungsten and thoriated filaments are still used for gas filled tubes and transmitter tubes but they use a lot of filament power, kilowatts in some big tubes.
Don Black.

On 16-May-13 5:27 PM, cheater00 . wrote:
 

Hi Guys,
Bryce, yeah, I'll work in high vacuum.
Don, thanks for the book suggestion, I'll try to locate a copy. It
should improve my understanding quite a bit.
Bob, I am not entirely sure that has to be right. I've seen Ben
Krasnow make an SEM from scratch - plates and all - and he used an SEM
filament bought off ebay. He handled it in normal air and he
re-pressurizes the chamber every time he places a new object to scan.
Maybe the filament he's using is coated with something else though.
Regarding whether people will know what an electron gun is: vacuum
technology and old scopes got so cheap by now that one is now
seemingly able to build complex stuff, such as SEMs, and other
electron gun based devices at home. I'd say once you can start DIY'ing
stuff the interest rises sharply.
Fred, I wouldn't like to hijack this thread :^) but let's not make
this a trip down memory lane (just yet). But to placate you, in
Germany you can walk into a Conrad shop and buy a new CRT scope. It
lacks a lot of features compared to a 7104 with the best time bases
and amps, but it's got a good old CRT.

Cheers,
D.

On Thu, May 16, 2013 at 9:14 AM, magnustoelle <magnustoelle@...> wrote:
>
>
> Hi Steve,
>
>> They are still used in X-Ray tubes and electron microscopes.
>>
>> Another question: Will anybody know why analog scopes needed a delay line?
>>
>> - Steve
>>
>
> A delay line allows the time base to start sweeping before the +/- VERT signals reach the CRT. I always considered it as a helpful trick for good viewing performance when probing faster signals, so that one path would be allowed more time. The delay line has some disadvantages regarding high speed signals. Not seen more than 100..150 ns delay or similar. Pre-trigger circuits made delay lines obsolete as it seems.
>
> Kurt has a nice, related description on his fine webpage here: http://w140.com/tekwiki/wiki/111
>
> Cheers,
>
> Magnus
>
>
>
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