OK, I guess it's time to go through this one more
The so called "W7IUV preamp" is documented on my web
page under the "rotatable flag" topic. The only place
you can be sure of getting the straight scoop is on my
The schematic is incorrect as it appears in many other
places. ON4UN's "Low Band DX'ing" has it wrong. If you
build it from that schematic, it won't work like it is
documented on my web page. At least a half dozen web
sites I have seen this schematic copied on are wrong.
I don't know if these are typos or somebody trying
"improve" the design. If you build according to these
erroneous schematics, it won't work as I claimed it
would. If you substitute parts because you are too
cheap to go buy the right stuff, it probably won't
If you build it according to the documentation on my
web page it will work exactly like it is specified to
work on my web page. If it doesn't work like I said it
should, you screwed it up somehow. Judging by the
emails I get, there are several hundred of these world
wide that have been built and work like I said they
I don't claim to have invented this circuit. I have
however optimized it for performance on the low bands
over a period of more than 20 years. The design has
been stable for more than 5 years now and I think no
more can be easily squeezed out of it. Before I
retired, I was able to verify the claimed performance
using a few million bux worth of HP test equipment on
The last modification was to increase the IMD
performance by changing the bias to provide more
current through the device. With the original 10 ohm
resistor the current through the device is about 60
ma. with a 13 volt supply depending on the *exact*
value of the resistors used. Changing to 4.7 ohms
increased the current to about 114 ma., again with 13
volts and depending on the *exact* value of resistors
used. This improved the output third order intercept
point by more than enough to make the change
worthwhile. If you do something stupid like change the
resistor in question to 20 or more ohms, you will
indeed cause the transistor to run cooler and you will
also cause the IMD performance to go into the toilet.
There ain't no free lunch.
Since heat is a major contributor to early device
failure I did not make this change lightly. Having
been employed for more than 30 years in the aerospace
industry on various government financed projects where
we had to *demonstrate* MTTF's of greater than 10,000
hours on a routine basis, I am well aware of the
consequences of operating a component at it's limits.
I believe I am capable of determining when a device is
operated "at it's limits" or beyond.
If the 2N5109 is operated at my recommended current,
without a heat sink, it will get very hot. In
non-engineering terms, if you put your finger on it,
you will get burned! However even with a small
heatsink, the operating temperature is low enough to
be able to put your finger on the thing and hold it
there. The heat sink I use is a snap-on thing with 10
small fins and an outside diameter of about 3/4 inch.
Don't ask for a part number, I don't have one, they
came in a box of junk I picked up 30 years ago!
Since I run these preamps 24/7 most of the time, I
have way over 10,000 hours of total operating time on
the devices with no failures or degradation of
performance. Now we can debate reliability as applied
to manned space flight or weapon systems, but we are
hams and this is a hobby. Like they used to say out on
the manufacturing floor, "There comes a time on every
project where it's necessary to shoot the engineers".
In summary, build the thing like it's documented on my
web page and it will work great and last a long time.
Anything else and you're on you're own! This preamp
circuit was provided to the topband community free of
charge simply because I think it's part of the amateur
radio spirit to share technology. I suggest that
instead of picking my design apart, those that think
they know better might publish their own designs on
their own web pages and we could all benefit from it.
Larry - W7IUV
DN07dg- central WA
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