[Amps] Voodoo Magnetic Fields
Ian White, G3SEK
G3SEK at ifwtech.co.uk
Wed Feb 12 22:09:58 EST 2003
> 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
'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
As a physical chemist by original training, I particularly like this
"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'
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