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## [Amps] more parasitic talk

 To: [Amps] more parasitic talk jtml@lanl.gov (John T. M. Lyles) 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 band. 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! 73 John K5PRO >... 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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