Even if the grid did not draw current there would be an non-linear input
impedance on the input of a cathode driven tube amplifier.
That is because the plate current in not linear with grid-cathode voltage but
something like Vin(t)^3/2 during the positive swing of the grid-cathode voltage
or
negative swing of the input (cathode-ground) voltage. Since the cathode
current is the same as the plate current the V/I curve is non-linear.
True, grid current would make it worse but is not the only reason for the
non-linearity.
You could make a grounded grid AB1 amplifier with a 304TL which would
require a lot of negative grid bias. You could connect the grid to ground and
make the cathode positive by several hundred volts and call it grounded grid if
you like. Just don't allow the peak drive voltage to exceed the Bias Voltage.
Think about it.
The term common-grid is probably less debatable than grounded grid. Or call
it RF-Grounded Grid. But to me they are really all the same.
73
Bill wa4lav
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