Paul, adding some C at the short end of the stripline will also lower the
resonant frequency by simulating a higher tube anode capacitance.
This can be done by adding a small piece of stripline material at right
angles to the end of the main stripline. The closer the added piece is to the
end wall of the cavity the more C and lowered frequency.
The added piece can be any size from a couple of inches X one inch to the
full width of the stripline by one half to one inch in height. More height
and width = more C = lower frequency.
Another way to lower the resonant frequency is to remove some of the
fingerstock fingers. Fewer fingers = more inductance = lower frequency. But,
each remaining finger has to carry more rf current and that increases losses.
There are lot's of ways to make this work!
73,
Gerald K5GW
In a message dated 1/5/2012 8:53:31 P.M. Central Standard Time,
kg7hf@comcast.net writes:
Hi Gerald,
I could use K1FO's design, but I already have a "nice" working unit. The
K1FO won't physically fit in my RF chamber ( well it might with a hammer),
and I just wanted to do something quick to get perhaps a few extra watts
and a bit more headroom for JT65B, and know that I have a replacement tube.
I've got a 2x3cpx800 started, but if I could get a new stripline
shoehorned into the existing amp, that would be great. It does sound like I
have
lots of options.
Maybe I should just build a new strip and fire it up to see what happens?
Paul
----- Original Message -----
From: TexasRF@aol.com
To: kg7hf@comcast.net, amps@contesting.com
Sent: Fri, 06 Jan 2012 02:41:11 -0000 (UTC)
Subject: Re: [Amps] Stripline design
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 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
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