>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.
** Good point, however, for instance, the 3/4-wave resonance of a
1/4-wave line for a 432MHz amplifier is pretty far up there. I have
never built a 144, 432, or 1296MHz amplifier that proved to capricious.
>
>You can't avoid resonances, but you *can* prevent oscillation, by making
>sure that feedback at all resonant frequencies is kept very low.
>
** How do You keep internal tube feedback 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.
** If it made perfect sense, I would have not have asked you the dodged
question.
>
>>>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...
** So why should internal feed-back capacitance be of no concern ?
>
>>>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...
** Say what ?
>
>>
>>>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.
>
** ?
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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