>-----Original Message-----
>From: Ian White, G3SEK [mailto:G3SEK@ifwtech.co.uk]
>Sent: 6. november 2002 17:26
>To: amps@contesting.com
>Subject: Re: [Amps] Re: [Amps] various parasitic sources... chassis,
>tubes, etc...
>On the contrary, every VHF/UHF amp I can think of has at least one
>parasitic resonance. For example...
>But who cares? The existence of parasitic resonances doesn't matter.
>What matters is the loop gain through the tube at those frequencies.
>If the loss for reverse feedback is greater than the forward gain at all
>frequencies, the tube will not oscillate at the parasitic frequency, the
>operating frequency or any other. And that brings us right back to
>grounding the grid in a way that's effective from LF through VHF.
YES, YES, YES, this agrees with what I have seen in my amplifier projects.
Reverse feedback must be kept below forward gain at any frequency.
The grids (including g2) must be grounded effectiviely at all frequencies,
where the tube has gain.
Reverse gain is best measured with a network analyzer or a spectrum analyzed
with tracking generator. With some testgear it is possible to build
equipment with predictable performance.
73 de LA6MV
Helge
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