> To this amateur Amateur the design concept here looks
> really elegant: http://www.qsl.net/ve5ri/cyclops.htm
>
> What is the perspective of the assembled amplifier-gurus?
It's a simple swamped grid to stabilize the amplifier.
The 200 ohm grid resistor loads the control grid. Hopefully
the resistor loads the grid so much that the feedback
capacitance is insufficient to cause the PA to oscillate.
The flaw I see in that design is the tube lacks reactance
compensation in the grid for higher frequencies. The 4CX3000
has about 130pF of input capacitance plus whatever the
circuit adds. That's less than 44 ohms reactance shunting
the 200 ohms on 28MHz.
You can get away with a system like that on 3.5MHz where the
reactance is 8 times the value, or 352 ohms. Not on twenty
meters.
When I build grid driven PA's like that I use an L network
to drive the grid, or shunting inductance that switches in.
Otherwise you have a pretty high SWR or uneven gain on
different bands.
Once in a while someone gets lucky of course and has enough
stray unplanned reactances to compensate the bad design.
When unplanned mistakes like that occur you can tell someone
copied without knowing how it actually works. :-)
73 Tom
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