WSQT wrote:
There are a few other factors here. The big one ne is that no real
tube or transistor swings ALL THE WAY to zero volts at full current!

It does niot have to be at full cuttrent, and it actually cannt be at
full current or the modulation will not be linear.

The power has to follow a square law relationship with voltage change.
meaning current is twice when voltage is twice, current is zero when
voltage is zero.

This is why the anode impedance is stable over a wide range of anode
voltage, and why PEP power is four times the carrier power in a good
clean plate modulated transmitter.

Another is that for the final to be truly ohmic would require that a
near-Class E or F condition, with the grid swinging from cutoff to
saturation almost instantly(as in a square-wave drive, sometimes used
in broadcast AM Class F setups).

Class F is an invention to describe certain circuits. It technically
fits the definition of class C.

RCA and other manufacturers used "square wave" drive in class C
amplifiers in the 40's and 50's. The method of obtaining the square
plate current waveshape was the addition of third harmonic resonators
or traps in the grid and anode circuits of low mu triodes. Allowing the
3rd harmonic inherent in the tube switching from cutoff to positive
grid voltage by not "grounding" the grid or anode at the third harmonic
caused the PA to switch into and out of conduction rapidly. Typical
efficiencies were in the mid 90% range.

It was called class C back then, and it techincally fits the
description of class C.

When the tube switches very hard, the linearity of anode power input
(and RF output) follows the desired square law change very well.


The next condition is that the current drawn through the load at 2X
supply voltage must not cause the tube or transistor's bottoming
voltage to more than double! In the real world, this means that the
current(loading) must be backied off from CW conditions for any
particular device, just as the voltage must be. If you load a final
for maximum output at carrier, guess what-you will be lucky to see 30%
upward modulation with MOSFETS or somewhat better with tubes!

What does that have to do with plate modulated stages?

The anode operating impedance is nearly constant throughout the full
audio cycle, and the ratio of E/I tracks very well regardless of load
setting in low-mu triode class C modulated stages.

The tetrode is a problem only because the anode does not follow a
square law power change as voltage is changed. This is because, as you
pointed out, the anode current is controlled by the screen voltage more
than anode voltage.

In a tetrode or any other screen grided tube, some audio has to be
applied either to drive, control grid, or screen voltage. This is to
ensure anode current tracks a square law relationship with modulation
voltage, plate operating impedance is reasonably constant, and peak
power is four times carrier power.

No matter how I load a class C plate modulated triode, modulation
remains reasonably constant. It is only in multigrided plate modulated
tubes that modulation can be seriously affected, since screen current
and the effects of screen voltage and current change can vary
drastically with load setting.

The issue the orignal poster missed was how the class C PA behaves as
voltage is changed by the modulation transformer. Power output should
square as voltage is doubled, but that's tough to do in a tetrode
unless screen operating conditions are controlled and the circuit
applies audio voltage in the proper relationship to anode voltage in a
grid.

Most of the AM pages I see don't really understand the importance of
that, and think just throwing a high inductance choke in series with
the screen makes the tube follow square law rules as modulation voltage
is changed.


73 Tom