Re: tube coils

To: tesla-at-pupman-dot-com
Subject: Re: tube coils
From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
Date: Fri, 6 Sep 1996 14:29:31 -0500
>Date:          Thu, 5 Sep 1996 22:25:35 -0600
>From:          Tesla List <tesla-at-poodle.pupman-dot-com>
>To:            Tesla-list-subscribers-at-poodle.pupman-dot-com
>Subject:       tube coils
>Reply-to:      tesla-at-pupman-dot-com

>>From major-at-vicksburg-dot-comThu Sep  5 22:18:48 1996
>Date: Thu, 05 Sep 1996 18:26:27 -0500
>From: RODERICK MAXWELL <major-at-vicksburg-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: tube coils

>I have not even finished my first capacitor discharge tesla coil and already I,m 
>looking forward to building a tube driven coil! I've odered a couple of books from 
>I.T.S. __Vacuum Tubes In Wireless Communication__ by Elmer E. Bucher ,and __Vacuum Tube 
>Tesla Coils__by J.F. Corum and K.L. Corum. I have built several high voltage projects 
>using induction coils and a solid state Mosfet driver, but I have never built 
>__anything__ that uses tubes. 
>  In __Vacuum Tubes In Wireless Comunication__ it describes the vacuum tube as a 
>rectifier. It also shows the direction of electron flow from the filament to the plate.
>This part I comprehend and understand well. What I have a hard time visualizing is 
>current flow from the plate to the filament! If the flow fom the filament to the plate 
>is composed of electrons, what is current flow from the plate to the filament composed 
>of and what is the mechanism that allows this to happen? Is it simular to hole flow in 
>semiconductor material???? Could someone that has experience with tube electronics 
>please answer these questions for me so I can sleep nights?

Roderick,

Ideally there should be NO current flow backwards in the one way 
device known as a vacuum tube. However, perfect devices almost never exist in 
the physical world and man-made  vacuum tubes are no exception.  Although
there is a pretty good vacuum inside the tube envelope, and efforts are 
gone through to make the vacuum as high as is practicable, there are 
small quantities of loose gas molecules floating around.  When 
electrons which are being emitted from the filament and travel at 
relatively high speed to the positive plate happen to collide with 
these loose molecules they can knock an electron out of these gas 
molecules thus ionizing them.  Since these ionized gas particles are 
positively charged, they will be repelled from the positive plate and 
be driven into the cathode.  Here you have a mechanism of reverse 
current flow.  The cathode can actually be damaged by being bombarded 
by these ions, especially in tubes operated at high voltages.  

Some tubes (especially rectifiers) have actually been manufactured slightly gassy
to deliberately allow reverse ion flow, and this ion bombardment is employed to
heat the cathode to incandescence so that it will emit electrons 
without using the traditional resistive heating element and 
associated fillament power supply.

You may have hear the term used saying an old tube has gone gassy.  
During the lifetime, gas molecules trapped in the glass walls and 
metal structures within the tube which were not released in the bake 
out and pumpdown at the factory, will over years of use free 
themselves into the vacuum space in the tube.  When this situation is 
extreme, the tube will be seen to glow blue, like the aurora borielis, simply 
from the light of released ionized rarefied gasses.

Hope you sleep better now.  :)


Regards, rwstephens