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
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
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.