Damping resistances is a more general term for the more specific types
like parasitic suppressors (which are slightly tuned to damp only higher
frequency - VHF - spurious energy). Over the big pond they are sometimes
called Stoppers. In cavity circuits, we can use damping resistances
instead of parasitic suppressors, since the geometry of the resistor
placement and location alone might determine which frequencies or 'high
order modes' that the damper helps dampen. Sometimes they are not pure
resistors but instead are magnetic losses, which do the same thing for a
high H field area in a circuit. These are materials like ferrite,
Eccosorb. The whole point here is it de-Q some resonance, either in a
cavity circuit due to geometry, or in lumped or LC discrete circuit from
stray inductances and capacitances which set up their own resonances
around the active devices. In some MOSFET circuits, damping resistances
are just small series R in the gate leads to help stabilize the device
from taking off at higher frequencies. In tube circuits, sometimes you
see a low R like 10 ohms in series with a grid, does the same thing -
loading the circuit, at some range of frequencies. A tube tester can
apply DC to the elements to measure its parameters, or just apply beam
in the tube to help condition and clean it up. In this case, no tuned
circuits are there, but the tube might try to oscillate itself as TPTG
or other topology, just based on the stray inductance and interelectrode
capacitance. In this case, the solution is to add some R into the
circuits, damping this.
73
John
K5PRO
Subject: Re: [Amps] 3CX800A7 Spares - cook them periodically?
I understand the concept (although not the black magic) of parasitic
surpressors and chokes; but you raise the issue of damping resistances. What do
you mean by that? Where in a triode circuit would they go?
Steve Gilbert
K1SG
K1SG@AOL.com
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