It sounds like you are trying to describe over modulation?
Otherwise, if the tube is operated within the linear portion of the
plate curve, the current will double when the plate voltage doubles as
Tom described.

Decreasing the loading on the tube means that you must also decrease
the modulation applied.

73
Gary K4FMX


On 2 May 2006 12:05:50 -0700, "WSQT" wrote:


If the LOAD on teh device is too high, it is possible for the
"bottoming voltage" or anode voltage necessary to pull two times the
current through it to be more than doubled for double the current.
High plate resistance devices operated with too high a load will NOT
have a constant dynamic anode impedance with varying plate voltage.
This is because the instantanious voltage across the tank circuit
during conduction is the supply voltage minus the tube voltage drop.

If the tube drop is more than doubled when the current(and supply)
voltage are doubled, it means the voltage across the load is LESS THAN
doubled. The load being ohmic, this causes less than double the current
to flow, and cuases downward carrier shift.

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