Antonio Vernucci wrote:
The advantage of class C isn't necessary greater efficiency. By
reducing the conduction angle the tube is drawing current for a short
period of time and therefor can run cooler. It also means that the
tube can be run at a bit higher power level than it could in class B
since the AVERAGE power dissipated is the same.

I cannot quite follow your reasoning. The tube temperature is bound to
the dissipated power. And the dissipated power simply is the difference
between the average DC power and the RF output power (neglecting losses
in the Pi network). So, if varying the signal duty cycles and tube
conduction angle, I anyway read the same output power and the same DC
power, the stage efficiency is the same.

Tubes don't self destruct in an instant when they are asked to dissipate
more than their max rated power. So long as the AVERAGE dissipated
power over time does not exceed the max rating things are safe. The
duty cycle will change the average power dissipation. Also the
temperature isn't bound instantly to the instantaneous power dissipation
due the the tubes' thermal mass.


HOWEVER the duty cycle of both the time transmitting vs not
transmitting and that of the signal also play a role. In other words
a class C CW transmitter in theory could be run at higher power than a
class C FM phone transmitter (even though both are usually run at the
same typical parameters) since the tube can cool between elements on
CW, while FM is key down forever. Also class B audio has a different
duty cycle than a class B RF linear amplifier running FM (don't need
to be linear for FM 'thou).

In the 30's there was an article in QST on how someone ran a 200 watt
tube at a KW CW. It worked because of CW's short duty cycle, but the
editor suspected 'short dashes'.

That's OK.

73

Tony I0JX