Hi Paul, the single tube design has less capacitance at the tube than the
dual tube design. The lower the capacitance at the tube anode, the more
inductance required to resonate. More inductance equates to a longer stripline.
If you need more inductance, a narrower stripline will have a higher
impedance and that will cause more inductance for a given length.
Added C at the tuning flapper will also lower the resonant frequency just
as increased inductance does. But, that comes with a price of higher loaded
Q that translates to higher losses. More tuning C will involve closer
spacing between the stripline and tuning flapper. Closer spacing means more
likely to flashover. You can increase the width of the flapper to increase the
C and retain the wider spacing.
Lot's of design trade offs here!
73,
Gerald K5GW
In a message dated 1/5/2012 9:51:16 A.M. Central Standard Time,
kg7hf@comcast.net writes:
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_10296933
54_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 wi
ll 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
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