It's best to just forget about the whole Class A thing. It's become used as a marketing tool and is essentially meaningless for guitar amps - firstly because almost none qualify (especially not the one that gave rise to the whole hype about it in the first place probably, the Vox AC30), secondly because the definition applies strictly to *un*distorted sounds, and thirdly because the whole point of Class A is to *reduce* distortion ('harmonic' distortion, not clipping - basically any alteration of the waveform). Apart from the fact the "Class A" sound like it means "best", there is no reason to even use it in anything related to a guitar amp.
The definition of Class A (if you still want to know) is that the amplified waveform must be conducted by all the amplifying devices throughout the whole 360º cycle of the wave cycle at the maximum undistorted output signal. This means that by some parts of the definition all single-ended amps are Class A, because up to the point one side or the other of the waveform clips, that is true and the maximum undistorted signal stops there. The problem is that most SE guitar amps are biased so far off the correct operating point to achieve an equal swing between clipping at each end of the waveform that this can't really be considered the maximum signal, and most SE amps don't achieve anything like their rated power output at this point. In fact, the waveform is much more like that of an inverted Class AB amp, cut off for some (less than half) of the wave cycle. Class AB amps cut off at the bottom end (zero current) end, whereas most SE guitar amps are biased far too hot and cut off at the forward end, but the result is roughly the same - the current draw changes drastically as the amp goes into clipping on one side of the waveform... although in a push-pull Class AB amp the current increases, whereas in a hot-biased SE it actually *decreases*.
One reason Class A is useful for hi-fi is that by having all the devices conducting all the time, the total current draw is therefore constant - this means that there is no change in the current supplied by the power supply, so even though it has internal resistance, no sag is caused, and no harmonic distortion of the waveform due to that. But one of the desirable characteristics of guitar amps is the sag and dynamic response when the power supply is heavily loaded! Which is why tube rectifiers are still popular... but in a true Class A amp, there is no difference between tube and solid state rectifiers (as long as the final output voltage is the same). Hence Class A would actually be counterproductive in a guitar amp. In fact, the characteristic compression and "bloom" of so-called Class A amps is very specifically because they are Class AB, and *not* Class A - this is what causes the current to vary. And usually because they are cathode-biased, hence the bias voltage is generated by the signal current and self-modulates - cathode-bias is also often confused with Class A, whereas in fact the two are not related... you can have a fixed-bias Class A amp too, as well as cathode-biased Class AB, and in fact it's *easier* to make a true Class A amp using fixed bias (because the operating point can be accurately fixed!).
Really, basing any conclusions about a guitar amp on its operating class will be at best irrelevant and at worst will give directly opposite results to what is desirable. What really matters is how the current changes and affects the power supply voltage and/or bias voltage as the power output goes up and especially as it clips.
So - if you're not totally lost!
- is a 5F1 Class A? When running strictly clean, yes (although not well-designed for it, the bias is completely wrong). When running distorted, no - not by any part of the definition. And why would you want a 5F1 only to play clean?!
Class B (where the waveform is cut off for half the cycle) can't really be used for audio amps because it always generates fairly heavy crossover distortion, but some high-power guitar amps running at the 'other end' of Class AB from Class A do come close - old MusicMan amps for example.
