rlm wrote:
>
>Amplifiers that happen to have tubes with lower than average VHF
>amplification tend to be stable.
High-gain amplifiers for the VHF bands are perfectly stable too, if they
use tubes with short, well-grounded grid (or cathode) connections.
The key feature is the total length and inductance of that grounded
connection. The length outside the tube is controlled by the layout. The
length inside the tube is controlled by the choice of tube. If you also
prevent accidental feedback paths, you'll have a stable amplifier.
If you use the correct grounding techniques in a VHF amplifier, it will
be stable with both the input and output disconnected.
The gain of tubes does not increase dramatically at lower frequencies
like it does with transistors, so if you use "VHF" grounding techniques
in an HF amplifier, you'll have a very stable amplifier indeed.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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