Tom said:
>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.<
You can sometimes work around this with a pi network input, arranged so that
the output C of the network is the tube input capacity, and the network acts as
a low pass filter. But you wouldn't be able to step up the input voltage by 4
times with that much capacity - it would have to be a straight 50 ohms swamping
resistor.
Another minor point is that a straight pi network on the output and Class AB1
is highly unlikely to meet the regulatory requirements for harmonic suppression
- that's why most people go for a pi-L. Still, that amp is obviously capable of
running far more power than is legeal, so why worry about harmonics?
73
Peter G3RZP
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