I wonder whether someone can help me to clear an issue regarding AM signals
produced by a (class-C) tube transmitter.

It is well known that the peak-envelope power of a carrier 100%-modulated by a
pure sine wave is four times the unmodulated carrier power. This is simply
because, according to the Ohm's law, doubling voltage power gets four times
higher.

I would have no problem if a tube would behave like a normal resistor. But a
tube instead behaves like a differential resistor, and this makes a lot of
difference.

If you consider the plate characteristic of a typical pentode, you see that
plate current does not change much increasing plate voltage. So, doubling the
plate voltage, power would not get four times higher, but just a little more
that two times.

The triode plate current is instead much more dependent on plate voltage so
that, doubling plate voltage, power would grow probably at least three times.
But definitely not four times.

Can someone pinpoint what I am missing?

73

Tony I0JX

On Feb 16, 8:47*pm, "Antonio Vernucci" wrote:
I wonder whether someone can help me to clear an issue regarding AM signals
produced by a (class-C) tube transmitter.

It is well known that the peak-envelope power of a carrier 100%-modulated by a
pure sine wave is four times the unmodulated carrier power. This is simply
because, according to the Ohm's law, doubling voltage power gets four times
higher.

I would have no problem if a tube would behave like a normal resistor. But a
tube instead behaves like a differential resistor, and this makes a lot of
difference.

If you consider the plate characteristic of a typical pentode, you see that
plate current does not change much increasing plate voltage. So, doubling the
plate voltage, power would not get four times higher, but just a little more
that two times.

The triode plate current is instead much more dependent on plate voltage so
that, doubling plate voltage, power would grow probably at least three times.
But definitely not four times.

Can someone pinpoint what I am missing?

73

Tony I0JX

You could be a bit mixed up ... the final is acting as a high
level mixer ... you end up with a spectrum .. not a single
carier .. in fact the carrier amplitude is not changed by the
process (should not be ...) that is , the additional power is in
the sidebands

G ..

"Antonio Vernucci"
I wonder whether someone can help me to clear an issue regarding AM signals
produced by a (class-C) tube transmitter.

It is well known that the peak-envelope power of a carrier 100%-modulated
by a pure sine wave is four times the unmodulated carrier power. This is
simply because, according to the Ohm's law, doubling voltage power gets
four times higher.

I would have no problem if a tube would behave like a normal resistor. But
a tube instead behaves like a differential resistor, and this makes a lot
of difference.

If you consider the plate characteristic of a typical pentode, you see
that plate current does not change much increasing plate voltage. So,
doubling the plate voltage, power would not get four times higher, but
just a little more that two times.

The triode plate current is instead much more dependent on plate voltage
so that, doubling plate voltage, power would grow probably at least three
times. But definitely not four times.

Can someone pinpoint what I am missing?


** I'll give it a go.

In a class C stage, the grid is overdriven with RF voltage so the output
tube is biased either fully conducting or off - as a result, the plate
voltage stays fairly low whenever the tube is conducting, which is less than
50 % of each RF cycle.

However, the actual current flow through the tube depends on the ratio of
the external circuit impedance ( ie a tuned load) to the B+ voltage. Since
the tuned load does not vary in impedance, the B+ voltage controls how much
RF current flow there is.

IOW, the output voltage from a well designed class C stage follows the B+
voltage linearly over a wide range cos to a large degree, the plate voltage
/ plate current characteristics of the tube have been rendered irrelevant.



...... Phil