Class C is a single ended topology in which the output tube conducts for substantially less than 50% of the cycle. The LC tank circuit at the output of the tube "rings" at the RF frequency of operation, creating a sine wave. Efficiency is primarily determined by the length of time the tube conducts and the Q of the tank. The latter also affects signal purity (harmonic content). The RF wave can be modulated by an audio signal, but this topology is otherwise unsuited to baseband audio use.
This behavior isn't evident to me, looking at the curves. Starting at the bias position, the spacing of curves leading down toward cutoff is much more radical than the portion leading up toward increased conduction.
Can you expand on this a little? Maybe a few examples of tube types that are designed for this VS a few that aren't? I haven't heard this before.
Which curves are you looking at?
The ripple waveform increases and decreases simultaneously at both ends of the transformer. Tom is talking about a signal that's moving positive at one end and negative at the other because the tubes are driven in push-pull.
Seems to me the triode charts are better for analyzing UL static operating points. The anode and screen idle at almost the same voltage. Unfortunately, the Mullard chart for triode mode stops at 300V. In any event, I think there might be more distance to cutoff than is apparent due to the dynamic increase in screen voltage that takes place as the signal moves negative.
The signals at the transformer ends must be out of phase to be additive.
It's the differential between the two ends that creates additive flux in the core. If the two ends move positive and negative simultaneously (in phase), the flux created by each half of the winding cancels. This is why a phase inverter is required.
Many years ago, the classic definition of a Class A1 power amplifier - namely that the tube never enters cutoff - generally included the statement (or at least an assumption) that it would be biased in the center of the current swing. There would be little reason to do otherwise, because that's the operating point that permits maximum symmetrical swing and output power. Referring to the small signal characteristics of an AB1 amplifier as Class A is incorrect, not only because the intended use is to be excited with a signal that pushes the tubes into cutoff, but also because even small signal operation does not occur in the most linear area of the curves. It is never truly Class A.
There's no such thing. Class designations apply to amplifier types, not to selected areas of a waveform. The RDH definition is incomplete.
