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[Amps] more parasitic talk

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Subject: [Amps] more parasitic talk
From: (John T. M. Lyles)
Date: Mon, 5 Aug 2002 09:55:06 -0600
Tom, W8JI, did a nice job explaining the parasitic suppressors 
concept for power tubes. These things apply in tubes where the 
inductance, capacitance of leads and elements are quantified and 
measurable, or where we worry about these in terms of our circuit 
models as we measure the components.  What i mean is that they can be 
thought of as discrete or lumped elements in our amplifiers, which we 
must compenstate for with little coils and resistors or hairpins, or 
lossy materials in series with elements. I wish it were always so 
nice and explainable. While the following paragraph doesn't really 
apply with tubes that are about the diameter of a softball or less, 
they are real concerns with the big brothers to the tubes that we 
hams love.

Interesting things start to happen as the tubes grow in diameter for 
super high power at VHF. If you solve the equations for TEmn modes 
with m=1,2,3... and n = 1, with the screen grid to plate spacing (the 
largest gap in big tubes) as the boundary conditions, you find 
resonances where each mode can be supported. Then, use the transit 
time for electrons based on the plate voltage (accelerating voltage, 
so to speak) you find islands of operating voltage where the tube is 
damned to oscillate. Above the voltage, it will be stable again. And 
so on.

I am learning about these now (ouch), in working to socket the big 
Diacrode for 200 MHz.

In Internation Journal of Electronics, 1986, v. 61, no. 5, p 575-582, 
report by Egerszegi et. al. of Brown, Boveri and Company was 
published "A Model for the Parasitic Oscillations in Power Tetrodes". 
A former Eimac RF engineer, now retired, pointed this to me recently. 
Until now, I was always thinking about parasitic suppression as we 
think of it as hams, the coil around the resistor, etc. Using 
circular waveguide, or more accurately, coaxial geometries with 
exagerated dimensions (extremely low Z geometry) you find that there 
is a universe of parasitics to deal with up into the low GHz around L 

The magnetron people have their set of bad players too, when their 
tubes take off on pi modes and self destruct.

This is all what makes RF design with thermionic devices so much more 
interesting and fun!


>... with a tube that has no appreciable feedthrough way up into or near
>the GHz range, like the 8877 or other coaxial grid tubes, you can
>often not worry at all about any suppression or use a very minimal
>suppressor (like a length of brass strip) since normal transit time
>of electrons as well as external circuit loss reduces gain far below
>critical levels without any or with minimal suppression.


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