Roger, I have built my share of VHF, UHF and Microwave amplifiers through
the years. In the process, I have learned a lot about how they work and will
share a few "secrets" with you.
The book you are looking for does not exist. You are left with three
options: 1>copy a known good design (such as the W6PO 8877 2m amp), 2>wing it
on
your own and be prepared for some cut and try (that is how the known good
designs were developed) or 3> buy a new or used unit.
You ask: why can't I just design the amplifier? The reason is that the
internal inductive reactance of every tube model is different and get this:
the inductive reactance changes with frequency. As the inductive reactance
rises with frequency, the capacitive reactance between tube elements lowers
with frequency. There is even a frequency that the two reactances are equal
and look like a short circuit. Above that frequency the reactance between
the tube elements turn inductive instead of capacitive. This is at the tube
connection points, the capacitance is still there internally.
The tube manufacturers for some reason have never characterized the
internals of the tubes to allow us to accurately predict the inductance at
various amateur frequencies. One of the reasons may be because the tube
internal
connections become part of the resonant circuits. If your plate resonator is
a stripline the Zo of the line is very different compared to a coaxial or
waveguide design.
At HF, the variation of this reactance is not as severe and we can use the
input and output capacitance and be close enough for matching network
designs. Not so at VHF and UHF. Even a small 15 pF capacitor has the same
problem. For example, one with .25" leads has enough series inductance to make
the 15 pF work as if it were 1000 pF or so. With longer leads, it is no
longer a capacitor for the frequency of interest but looks like an inductor.
At these frequencies, you do not get to choose loaded Q as one might wish
to deal with various plate load impedance variations caused by changing the
plate voltage or current. You have take what you get as the loaded Q can be
60 to 100 or more at 70cm. Even on 2m typical loaded Q can be 20 to 30.
There is little you can do to lower these numbers other than using good
common sense: minimize any shunt capacitance, use the highest quality
resonator
design (strip lines are not the best), Use the lowest plate load impedance
possible with the tube you have (maximum plate current, less than maximum
voltage). And finally, pay attention to rf current flow in the plate
resonator. If a strip line is used, nearly all of the current flows on one
side of
the tube anode. There is that inductance mentioned above and every
inductor has a given Qu and resulting loss resistance. If most of the current
is
flowing on 1/3 of the tube plate connection, there is 3X current compared to
an optimum design. Losses being related to current squared, they are 9X-2
or 7 times an optimum design. These losses are subtracted from your output
power and can damage the tube or fingerstock. Also, in some tubes, such as
GS23B, the material being heated by the losses will change dimensions
enough that the resonant frequency drops. With the loaded Q being so high, it
takes a few seconds for the power output to drop to half or less. Of coarse
you can keep one hand on the plate tuning control and bring it back but who
wants to do that? These problems go away with a circular resonator design.
Did I mention that I am down on stripline designs?
By now, you have found the ND2X website with all of the various Russian
tube designs. There is a wealth of information there and most of it is good.
A few of the published designs by YU1AW are from computer models and do not
work in practice. Beware of any designs designated "lazy builder". I
recently helped W9IIX with a GS35 222 MHz amplifier project started from the
YU1AW info and it was a ball of flames at turn on.
I will be happy to critique any design you might wish to try. In fact, I
will offer unending moral support for your VHF or UHF amplifier projects.
73,
Gerald K5GW
eme capable on 6m, 2m, 135cm, 70cm, 23cm, 13cm, 9cm, 6cm and nearly ready
on 3cm
In a message dated 2/12/2010 3:36:42 A.M. Central Standard Time,
sub1@rogerhalstead.com writes:
I've brought this up before on the Amps reflector, so far I've not come
up with any information.
I'm looking for information (book or software) that doesn't cost a
fortune, for designing a legal limit 2-meter amp with a fair amount of
overhead so it can just loaf along using either one or two tubes in a
strip line or parallel lines.
I've spent hours searching, but most just lead me in a circle.
I know absolutely zip about 1/2 wave and 1/4 wave strip lines and how to
calculate their dimensions.
The same is true of the parallel lines. How does the construction and
dimensions change with power.
How about a pair of GS-23B Parallel lines, or strip line, or a single
GU-84B/4CX2500, or a pair of GU-74Bs/4CX800's. On HF or even six meters
I can come up with a tank circuit, on two about the only thing I can do
so far is to copy the dimensions of the strip line out of my Henry 2002A
which uses a single 3CX-800. If I went to a pair of them, or the
4CX-800s how would that affect the dimensions of the strip line, or if I
replaced the one or two 3CX-800's with a pair of GS-23Bs/4CX1600U (IOW
it's a microwave version of the 4CX1600B which was so popular until it
disappeared although I see Alpha has a few of them yet.
At any rate, this is the kind of information I'm looking for. The
problem is being able to find it, or construction articles with a bit of
theory, books, or design softtware that doesn't cost a fortune.
73
Roger (K8RI)
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