In a message dated 4/4/01 9:17:26 AM Eastern Daylight Time,
na9d@mindspring.com writes:
>
> I went and dug into my memory archives a little more and I also woke up a
> little more.
>
> It appears from the text I wrote below that my primary motivation for using
> the "L-coil" as a I call it or step down transformer in the anode output
was
> to force the B&W 850A to work. Actually, that wasn't it at all. It was a
> side benefit, but I was ready to roll my own inductor coil.
>
> The primary motivation behind stepping down the impedance was the tune Cap.
> The output capacitance of a 4-1000A is on the order of 7 pF. By the time
> you add the socket and capacitance of the amplifier housing that all add to
> it, you end up with a tube capacitance on the order of say 15 pF. Some
> people have commented to me it could be as high as 20 pF.
>
> With a plate load impedance of 5300 Ohms, at 28 MHz, the needed value of
> Tune C for a Q of 15 is 15 pF! For a lower Q value, you need even less
> capacitance.
>
> Since the output capacitance of the tube needs to be added to the real
value
> of tune C, that means that tune C would need to be zero pF or less which is
> not possible!
>
> This was my primary motivation for using the step down coil. A typical
> minimum value of C for a vacuum capacitor is say 10 pF. So that means, the
> actually total tune C in the circuit is about 25 pF. With matching 5300
> Ohms to 50, a value of 25 pF for tune C would give a Q at 10M of about 26.
> A little too high, IMHO.
=============================================================
Jon,
What is your estimate of the magnitude of the reduction of capacitance due to
the addition of the "step-down" coil, if that is what you infer in the first
& second paragraph above?
What is the purpose/function of the "step-down" transformer you used in your
design?
Would a step-down transformer enhance the efficiency of my Pi-network output
which has an impedance of 2000 ohms (3000V@ 750mA)? If so, what value would
you recommend?
Dave, WT8R
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