Glad to see you here Rich. I have learned a lot from you in your articles &
your great help on the phone with my 3-500 amps that I couldn't stop a
parasitic in. Your kit you sent me along with a lot of literature cured that
amp & the suppressors don't burn up . Amps runs full out on 160 thru 10
mtrs. Again thanks for your help.
"• R. L. Measures." wrote in message
...
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