In Eimac Amateur Service Newsletter AS-33, also in QST in June of 1967, Bill
Orr and Bill Sayer (both at Eimac) state this:
"Strictly speaking, the extra output power is not 'free', as r.f. power is
expensive compared to d.c. plate power and may only be
'free' if it is unavoidably available. It is generally referred to as
feed-through power, but the implication in this term may be
misleading, as this portion of the drive power does not appear in the load
circuit of the cathode-driven stage until after it is converted to a varying
d.c. plate potential effectively in series with the main amplifier power
supply. This converted drive power performs a useful function in class AB2
and class B linear service by swamping out the undesirable effects of nonlinear
grid loading and presenting a reasonably constant load to the exciter."
(reference is made here to "SSB Principles and Practice", 1964 edition, by
Pappenfus, Bruene. and Schoenike)
Its a pretty good explanation of how it works. I think he means that the
cathode to grid voltage swing from the driver is in series with the plate to
grid voltage swing in the output. Since
the cathode current and plate current are in the same path, with the slight
difference being the grid current, it is essentially an increase in the plate
voltage swing, with the same current, so that
it appears as if the driver power is added to the output power. But there is no
mechanism inside of the vacuum tube for RF to flow through and get from the
input to the output, besides the
effect of the electron beam which converts DC into RF at the fundamental
frequency.
Fun reading.
73
John
K5PRO
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