ORIGINAL MESSAGE:
On Wed, 28 Jul 2010 09:22:44 -0500, donroden@hiwaay.net wrote:
>
>I believe a lot of us would like to know your better way.
REPLY:
Any oscillator, parasitic or not, requires some of the energy in the
anode circuit be fed back to the input circuit in the proper phase and
amplitude to sustain oscillation. You can't do much about the phase,
but you can reduce the amplitude.
The traditional way has been to "de-Q" the anode circuit with an L-R
parasitic suppressor. This reduces the tube gain below the point
needed to sustain oscillation. This method works but can be tricky to
get the proper balance between L and R to prevent overheating the R
while still suppressing the oscillation.
Another way is to swamp out the fed-back energy in the input circuit.
This is especially easy in a grounded grid amp where the input
impedance is low to begin with. My approach is to use a relatively
high Q input circuit, with a Q or five or more. Conventional wisdom on
in put circuit Q says only two or three. By going to a higher Q, the
impedance off resonance is reduced. The higher the Q, the greater the
reduction. The tradeoff is less bandwidth so you may have to add a
tuning cap or use a tuner between the transceiver and amp. Always keep
lead lengths short - excessive lead length can inadvertently create a
"transmission line" VHF tank circuit.
Another approach I have used is to place a 10 ohm resistor in series
with a 33 pF cap from cathode to ground. This presents a relatively
low impedance at VHF while having little effect at HF. The values can
be tweaked for each particular case.
A third approach, which I have not actually tested, is to place a
series resonant circuit from cathode to ground, resonant at the VHF
parasitic frequency. It might be desirable to "de-Q" this one with a
low value resistor to make tuning less critical. As I say, this is
just an idea which needs some testing. In theory, it should be highly
effective, but there could be a "gotcha" in there. Worth a try if
nothing else works.
Questions?
73, Bill W6WRT
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