[Amps] Voodoo Magnetic Fields

[Ian White, G3SEK]

K8LV wrote:
>Ian:
>     I think this getter idea is fairly amazing (I have heard about it
>before but never really paid much attention.) As usual, I am not
>totally comfortable about it without being able to do some analysis of
>real numbers. Do you know of a good source of info that is publically
>available?

'Handbook of Materials and Techniques for Vacuum devices' by Walter H 
Kohl (AIP Press / Van Nostrand Reinhold, 1967, ISBN 1-56396-387-6) will 
tell you more than you ever wanted to know. It includes a whole chapter 
on 'Getter Materials' and another on 'Voltage Breakdown'. Unfortunately 
this reference is not entirely about radio tubes, so you have to sift 
through the information. There are lots of references, though.

Another source is 'Power Vacuum Tubes Handbook' by Jerry C Whitaker (CRC 
Press, ISBN 0-8493-1345-7). This book owes a lot to Eimac publications 
(and incidentally may explain why 'Care and Feeding ' has never 
reappeared - Whitaker has pretty much mined it out) but it somehow never 
tells you all you'd like to know.

For example, all it says about arcs and getters is:

"An electrode containing absorbed gases... will exhibit reduced 
breakdown voltage because the gas will form on the electrode surface., 
increasing the surface gas pressure and lowering the breakdown voltage 
in the vicinity of the gas pocket.

"To maintain a high vacuum during the life of the component, power tubes 
contain a getter device. The name comes from the function of the 
element, to 'get' or trap and hold gases that may exist inside the tube. 
Materials used for getters include zirconium, cerium barium and 
titanium."

As a physical chemist by original training, I particularly like this 
next one:

"The operation of a vacuum tube is an evolving chemical process."

A typical vacuum in a power tube is about 10^-8 torr (mm Hg), equivalent 
to the atmospheric pressure about 200 miles above the earth. For the 
typical volume of a small transmitting tube, try calculating the number 
of free gas atoms - the answer may be surprising!


If anyone can recommend a book about the construction and technology of 
transmitting tubes, that bridges the information gap between Kohl and 
Whitaker, I'd be very grateful to learn of it.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek