In article .com,
"K7ITM" wrote:

Soooo...if the modulator is transformer-coupled, how does the DC input
to the RF amplifier increase when modulation is applied? A transformer
can't couple DC...

You really don't know the answer to this? Did you actually pass the
Theory Exam? Inquiring minds want to know.........

It is so simple you can't believe it. It's only Ohm's Law.

The Class-C plate modulated final amplifier is driven hard on its
control grid such that it is driven into saturation even when its DC
plate volts is increased to twice its normal DC value.

When saturated, the peak RF plate output volts always swing between
twice the DC supply volts and some very low value.

The final amplifier therefore constitutes a fixed DC load resistance
across the DC supply, which depends on unmodulated RF power output and
amplifier power efficiency.

This fixed resistance is also the value of the audio-frequency load
impedance across the secondary of the modulation transformer.

For 100 percent modulation, the peak RF plate volts across the tank
circuit swing between twice the modulated plate supply DC volts and
zero. (Or some very low value.)

The harder the grid drives the plate circuit into saturation, the
greater the modulating linearity. The drive limit is reached when any
of the tube's electrodes approaches its rated power dissipation.

Eaxactly the same things happen with transistors.
----
Reg.

Thank you, Reg. My reply would have been much less gracious and not
nearly so complete. Probably something like, "Which part of 'constant
DC input' do you not understand?" I appreciate that you took the time
to write such a nice reply, one that the lurkers may well learn
something from.

Another reason, of course, that a tube can't handle modulation peaks is
that it's getting weak (or the initial design was inadequate). Seeing
the plate current meter wiggle during modulation that's below 100% is a
good sign it's time to check the RF output tubes, and as your reply
suggests, the drivers as well, though the grid current meter should
tell you enough about that. If the filament (cathode) no longer has
enough emission, the tube may handle the carrier OK, but not the
positive modulation peaks where the current must be about twice as high
as with just the carrier. Then the DC current will drop during
modulation.

Cheers,
Tom