rlm wrote:
>
>
>>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.
>
Not true. VHF/UHF amps all have multiple resonances, both above and
below the operating frequency.
You can't avoid resonances, but you *can* prevent oscillation, by making
sure that feedback at all resonant frequencies is kept very low.
Stability in a VHF amp has very little to do with choosing a tube with
low gain - it's all about preventing feedback. This is just as true
about VHF stability in an HF amp.
I hope that what I *did* say below makes sense to you.
>>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 ?
>
To anyone else reading this: I feel sure you understand why I'm not
willing to let Rich to lead me by the nose with exaggerated questions
like the one above...
>>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 ?
...or like that one...
>
>>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 ?
...or that one.
--
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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