On 5 January 2012 01:38, Paul Decker <kg7hf@comcast.net> wrote:
>
> Hi group,
>
> NHY to all! I am working on an amplifier mod and need some advice. I have
> a currently working 70cm stripline amp based on the 3cx400A7 tube. As
> everyone knows, that tube is getting rare and expensive. Since I have a few
> 3cpx800A7's around here, I thought it might be a good fit to change to that
> tube, especially for the eme contest at the end of the month.
>
> Besides the obvious physical size and heater voltage, there is a slight
> difference in cin and cpk, I'm not sure what that will do. I've never found
> any design plans, but this stripline is 3.5" x 8.625" long. and the tube is
> 0.25" (tube center is 1.05" from end) from one end.
>
> To fit the 3cpx800, I would have to extend the stripline 0.375" so the tube
> would still be connected making it 9.0" long. You can see what I mean here:
> http://a4.sphotos.ak.fbcdn.net/hphotos-ak-ash4/388232_2684319340428_1029693354_32301613_317232212_n.jpg
>
> Does extending the "null" end of the stripline make any difference? What
> effect would this have?
>
> I dug up the K1FO single 3cx800A7 70cm stipline documentation and it shows
> the stripline as being 3.5 x 9.6875", unfortunately I don't think that will
> fit in my deck without extra work. On the other hand though, I looked at
> K1FO's 2x3cx800 stripline and it is 9.0" long, just like mine would be. Any
> speculation on why K1FO single has a much longer stripline than I do and why
> his two hole has the shorter one? My guess is the tune flappers might be in
> different locations and that makes the difference.
>
> Thanks for the insight on this and 73,
> Paul, kg7hf
I'm feeling rather tired now, but will do my best to have a go at this.
The idea of the transmissioln line is to present an inductive
reactance that's equal in magnitude to the output capacitance and
strays of the tube. If the inductive reacance of the transmissiion
line is the same as the capacitive reactance of the tube and strays,
then resonacne occurs.
The capaitance reactance of the tube and strays is given by
Xc=1/(2 Pi f C)
f is in Hz, C in Farads
The inductive reactance of the transmission line is more complex to
understand, but basically follows a tan function assuming it is
short-circuits (for RF) at the far end.
Xl=Zo tan(2 Pi l lambda).
http://en.wikipedia.org/wiki/Transmission_line#Short
To get this to resonate, you need to make the inductive reactiance the
same magnitude as the capacitive reactance. i.e. resonance occurs when
Xc=Xl
So if you
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