>
>on 2/8/00 01:17, Ian White, G3SEK at G3SEK@ifwtech.demon.co.uk wrote:
>
>>> "In order to get a good effiency on 28 MHz why not lower the anode
impedance
>>> by inserting an inductance beetween the anode and filter? I see an example
>>> in my 1991 ARRL HANDBOOK an example:
>>>
>>> A tube with 3500 ohms anode impedance and 10 pF output capacitance is
>>> transformed into 1481 ohms and 16 pF by inserting a 1 uH inductance
beetween
>>> anode and output filter. Will this inductance act as a parasite choke as
>>> well?"
>>>
>>> I did not get any answer on my very last question from you pi/piL
>>> specialists.
>>
>> The quick answer is no. It will move the VHF resonance, but a simple
>> inductor does nothing to add extra damping at that frequency compared to
>> HF. To do that, there has to be some loss mechanism in there too - a
>> resistor and/or a lossy conductor.
>
>Ian,
>
>I don't disagree with you, but an inductor of significant size can affect
>the resonance enough to move it out of an area where the tube will not want
>to parasitically oscillate.
>
The fly in the ointment is that the significant inductor can act as a 1/4
wave line as well as a 3/4 (or 5/4) wave line. If the 1/4 wave resonance
is below the grid-resonance, oscillation is unlikely. However the 3/4
wave resonance is virtually certain to be above the grid-resonance -- so
oscillation is possible.
cheers, Jon
- Rich..., 805.386.3734, www.vcnet.com/measures.
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