>As for broadband tuning, 10 to 20% bandwidth "should" be achievable using
>lumped/discrete L and C low pass networks but it should be calculated for a
>real tube case, to see if it is really possible. I know it has been done
>for several FM broadcast amplifiers on the input network, to cover 88-108
>MHz without tuning or with one less knob. Transformers certainly play a
>part, with quarter-wave lines doing transformation. Broadbanding the input
>to a cathode driven triode is common.
Anything greater than 10% bandwidth is typically considered wideband.
However, at 14 Mhz, this is 1.4 Mhz. Not wide enough to cover anything
more than the 20 meter band. I'd dare say that it would be pretty easy
to do a no-tune output for just 20 meters since it is just 350 KHz wide.
At 3.5 MHz, 10% bandwidth is 350 KHz. This is not even enough to cover
all of 80 meters. Furthermore, covering 1.8 to 30 MHz is extremely large
bandwidth.
In the FM broadcast band 88-108 MHz is about 20% bandwidth (98 MHz
midpoint and cover 20 MHz of BW). So conceivably you could get a
wideband design here. Note that the FM bcast band is two-thirds the size
of the entire spectrum from DC through HF!
You can broadband the input to the tube because it's possible to get
ferrites that don't saturate with 100 W of drive. However, as I said in
an earlier note, it may be possible theoretically to tune the output with
a transformer, but I doubt there are ferrites that wouldn't saturate.
Also, the input impedance to a cathode driven tube is on the order of 50
to 150 Ohms. Much close to 50 Ohms than the 2500 Ohms on a Plate and
therefore much easier to match.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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