>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.
>
** HF amplifiers invariably have two resonant circuits connected to the
anode. VHF amplifiers have one.
>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.
>
** So the g-g internal feedback-C from the anode to the cathode (or grid
in grounded-screen tetrodes) is nothing that a true amplifier expert
should be concerned with ?
>If you use the correct grounding techniques in a VHF amplifier, it will
>be stable with both the input and output disconnected.
>
** correct gounding has zero inductance ?
>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.
>
** VHF grounding also has no L ?
cheerz, Ian
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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