Even though it does not appear on the schematic diagram, every tube-type
HF amplifier has a resonant circuit in its anode circuitry that resonates
somewhere in the VHF region. This called a parasitic resonance. Whenever
the DC anode current changes, the parasitic circuit rings - much like a
struck bell and generates a smallish damped wave signal at the VHF
resonance point. - note - this is the same principle that enabled spark
transmtters to produce RF from a DC source.
Since all tubes have feedback C between the output (anode) and the input
(cathode for cathode-driven and grid for grid driven), the damped-wave VHF
signal is amplified - whereupon some of the amplified signal can be
fedback again and re-amplified -- resulting in oscillation. Because
tube gain is pettty much tube-transconductance x the resistive load (RL)
on the anode, one way to reduce the chance of VHF oscillation is to
artifically reduce the VHF gain of the tube by lowering the VHF-RL
presented to the anode by the parasitic resonance. This is done by
decreasing the VHF-Q of the parasitic resonance circuit. In other words,
to decrease Q, increase R. Traditionally this has been done by winding a
Cu wire coil around a carbon-comp resistor and soldering the coil in
parallel with the resistor. In a typical 2. 3-500Z amplifier this
configuration produces a Q of c. 5 at 100MHz. Misfortunately a Q of 5 is
not quite low enough to reduce VHF gain enough so that oscillation can not
be sustained It apparently takes a Q of 2 at 100MHz to achieve
acceptable VHF stability. One way to decrease Q involves exchanging the
highly-conductive Cu wire for highly-resistive Ni-Cr wire This simple
change results in a Q of c. 2 at 100MHz as measured on a HP 4191A
Z-analyzer. By using two Ni-Cr VHF suppressors per 2-500z, Q can be
further reduced to c. 1.5.
- end

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Richard L. Measures. 805-386-3734,AG6K, www.somis.org