>Subject: [AMPS] Broadband input circuit
>Sent: 3/28/1997 5:39 AM
>Received: 3/28/1997 6:39 AM
>From: Carl, km1h@juno.com
>To: amps@contesting.com
>
>I remember somewhere back in the 80's an article by Ulrich Rhode that
>described a 1.8 to 30 MHz network.
>Has anyone tried such a circuit? It would appear to offer advantages over
>the cumbersome switched networks commonly in use.
>
>Tnx...Carl KM1H
I have doubts that such a contraption would work, Carl. Since the
cathode in g-g service behaves more like a diode than a resistor, a
tuned-circuit/flywheel keeps the input Z on an even keel. Eimac
recommends a Pi-network with a Q of 5. However, for hamband amplifiers,
a Q of 2 will do.
A easy solution is Class AB1. A small roller inductor is used to tune
the grid to the desired operating frequency. At this point, the tube is
neutralized, the radio happily sees a quite low SWR. The power gain can
be over 100 instead of the paltry 15 or so from grounded grid operation.
Getting there is easier than most builders realize. My Web site has
details on Class AB1 operation. OTOH, changing frequency is not as
speedy as with an SB-220.
FS------Anyone who is building an 8171 or 8281 Class AB1 amplifier, who
still needs a bias supply transformer with HV insulation, I have a
suitable unit for sale. It is new, potted to mil. specs, and comes with
a lifetime unconditional money-back guarantee. The price is $25 plus
shipping. Telephone me if you are interested. ..... A 4% discount is
available to Novice Class licensees.
R. L. Measures,ag6k,805-386-3734
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