Introduction
The
rationale for this project was to try something unconventional
and to re-purpose some coomponents and modules that I had acquired from
various dumpster dives and hamfests over the years. The main use
will be for 2 Meter Band SSB on which I have limited capability
thanks to an old but good TS-700 all mode radio that I got
at a hamfest for only $100 that only puts out 5 watts.
Why do I like 2 meters sideband ? Since
1976 , most of my real activity has been on 2 meters. This was
largely due to having licence restrictions that limited activity to
50Mhz and above. Thankfully, that restriction is gone , however,
I still like 2 meters due its unique coverage and ubiquity. You
can always get a contact on FM. The perception of communication
range is set by the collective ham experience on this band, and that
experience of is defined by FM. How far can FM really go anyway ?
The answere is not very far. This lesson came home during
the last JohnMoyle field day. With my newly repaired device I was
able to hear SSB stations at ranges that are science fiction on
conventional FM. My lowely 5 watts was able to achieve two
way communication that are impossible on FM on high link loss paths.
So now I want more talk power on SSB. Linear amplifiers
are still expensive, even at hamfests, so the decision was reached to
do something about it and also make this a learning experience.
The
options are not attractive on my budget. Thermionic or solid
state ? Transistor power devices that are easy to use and high
powered are still very costly and require the use of hard to get
components like RF rated mica chip condensors. Then there is the design
learning curve with strip line reactances. Thermionic is the next
choice. I have a collection of good bottles. This needs really high
voltage and much metal work and much time.
At a long forgotten
hamfest I bought a handfull of these small power amplifiers for a few
dollars. Most of power transistors were dead and the images shows
the survivors. They are not even very good transistors, very low
gain and not even designed for linear service , they were designed for
VHF Hiband class C amplifiers and are about 30 years old or
worse. Could I do something with them other than construct yet another
FM classs C brick amplifier ? The modules actually came from a
very old Dick Smith Electronics (vale) DIY kit and use a
very old before stripline were invented design using
discrete LC networks to perform the impedance transformations.
Not being able to specify with any real precision the required
reactances, ceramic mica trimmers were used. Good, this is a
technology I am comfortable with, we have room for error . A stripline
design is better, but, if its wrong you need a new PCB. It must
be completely designed right the first time or else nothing usefull
will happen.
My modifications and repurposing efforts here are
challenging to say the least. I am using 2N5990 2N5991 Class C
transistors for linear service (bad) and I am putting two
amplifiers IN PARALLEL . What could possibly go wrong ? Unlike
reference designs I am not using a flat finned heatsink.
The baseplate is slab of 6mm aluminium scrap. The
heatsink is a repurposed computer heatsink. These little heatsinks can
easily remove 100Watts worth of power and my expect maximum dissipation
will not exceed 50 watts. There are base bias "diodes" in
very close thermal contact , so I am not expecting bias difficulties.
There are 2 willkinson power dividers on input and a combiner on the
output. I hope they provide sufficient isolation to prevent one
amplifier from killing its companion. The combiners are made from
75 ohm teflon coax and noninductive resistors.
All circuit
boards were cut out with a TCT rotary tool on a DREMEL. its faster than
etching and you can add afterthoughts as required.
My target
performance is to get 40 watts PEP of SSB from this arrangement.
This is a usefull amount of power on 2 meters. Have I powered it up yet ? Nope.
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