Jim,
## The tube cant source anything. It can only sink current.
That's right, of course, as long as you think in the conventional
current sense. People who think in the true electron motion sense will
certainly claim that a tube can _only_ source, and never sink, current!
But let's leave that issue aside.
The tube only sinking current doesn't really impact any of the things I
explained in my post. But indeed when thinking about the inner working
of amplifiers we do need to consider that tubes and transistors only
ever conduct current in one way, and that this has a lot of
consequences. The tube or transistor by itself isn't an AC signal
source. Even when combined with a feed choke, and DC blocking capacitor,
the combination has become an AC source but typically not a sinewave
source (except in class A). Only when combined with a resonant circuit
of sufficiently high Q, or when two devices are combined in push-pull in
class AB or B and with the porper use of a good coupling device between
them, do we get a sine wave source.
But in practical amplifiers some such combination of components is
always used, and as a result we do have at least the equivalent of a
real signal source connected to an impedance matching circuit. And as
long as low-loss components are used, the only component that can
exhibit significant resistance, and thus can dissipate significant
power, is the tube or transistor.
For those reasons it is valid to analize the possibility of absorption
of RF energy entering via the output port, and being dissipated by the
active device, in the way I did. Although it's correct that the detailed
happenings inside the circuit are more complex than "impedance matching
circuit connected to active device acting as resistor", and that a tube
only ever sinks (conventional) current.
Manfred
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