ORIGINAL MESSAGE:
On Sun, 14 Mar 2010 00:16:41 +0100, Dave White
<mausoptik@btinternet.com> wrote:
>
>Isn't the real cause that the tube and/or circuit has Q at VHF? So isn't the
>'cure' that we try to lower the Q at VHF whilst trying to keep Q at the
>fundametal that we're interested in? It may be a moot point as to whether
>we're treating cause or effect. Search me, mate. That's too deep a question
>at this time on a Saturday night after a few glasses.
REPLY:
For a tube to oscillate, the circuit Q has to be high enough on both
the anode and grid circuits, enough so the gain is one or more and the
phase of the fed-back power is correct or nearly so.
In other words, the problem can be attacked in either the anode or
grid circuit, or both. I have built several amplifiers with no
parasitic suppressor at all, but where the grid circuit was designed
to have a very low impedance at VHF, and the amps were all perfectly
stable.
Basic oscillator theory.
73, Bill W6WRT
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