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.
I know this for a fact cause my 4-1000A has NO resistive suppressors in the
anode and it is stable and has been stable under all sorts of loads and
conditions for a year now. With a traditional suppressor network, one could
easily find a resonance at the typical 4-1K frequency of 80 MHz or so.
However, with my inductor on the output of the tube, the resonance at 80 MHz
is no longer there. It has moved somewhere, but I've not been able to find
it anywhere in the range of 30 MHz to 120 MHz. I did this for multiple
reasons. First of all, I did it to improve the matching of the tube at 28
MHz as the inductor acts as an impedance transformer and makes it much
easier to match to the Pi-Net with any reasonable Q. Secondly, due to the
high amount of circulating current with the 4-1K on 10 meters, I was burning
up resistors like crazy (AG6K's nichrome wires were a fire hazard as they
glowed like toaster elements and his resistors would thermally burn up as
well). I'd gotten the suggestion of removing the resistors from an old
timer who successfully built 4-1Ks without them. It worked well.
So what I have coming right off the anode is a single turn inductor of about
an inch in diameter. Then after the DC blocking cap, I have my step-down
transformer inductor which is about 5 turns (if I remember right) of copper
tubing with an inch in diameter.
Pictures of my setup are on my webpage: www.qsl.net/ke9na
It works and it works well. I get full legal limit from my tube with about
100 Watts of drive. And I've used it on CW, SSB, RTTY, AMTOR, etc with nary
a problem of any kind.
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|