[Elecraft] HW-40 based amp, Class C PAs in general.

[Ferguson, Kevin]

Class C amplifiers can be used as the final stage in a CW rig, 
but should NOT be used as "boots" for a CW rig.   When 
designed into the rig, provisions are made to control the
rise and fall times of the keying waveform of the PA, eliminating 
key clicks.  If used after, say, a K1/K2, the non-linearity of the 
PA will alter the Kn's carefully shaped keying waveform, and
generation of key clicks is extremely likely.  The station you are
QSOing with will not report a problem, but your clicks will
be heard up and down the band for many KHz.  This is IMD,
not harmonics. It is to CW what splatter is to SSB, and the cause
is exactly the same. 

When used in CW service, a class C PA stage needs
to be keyed as well as the exciter. Such is possible, but would 
be extremly difficult to synchronize this with the Kn's keying 
and not seriously "weight" the keying as a result.

It has been sugested that class-C amplifiers are a "tube thing"....
Not so.

Filtering harmonics is not any easier at high impedance, than at
low impedance.  Tubes DO require a high impedance load 
if they are to produce significant power, and feedlines are
fairly low impedance. The only practical way to transform a high 
impedance to a low impedance is with a high-Q
tuned circuit, which kills the harmonic generation bird with the
same stone.  High-Q tuned circuits are used for feedline
coupling to ALL classes of hollow state RF amplifiers, 
not just class C.

At low (transistor) impedance levels, broad-band
RF transformers are practical, which do not suppress harmonics, 
so additional low-Q (no tune) harmonic filters are employed. You
could use a high-Q circuit, but then you would lose the no-tune
advantage of a transistor amplifier. All this works just fine if the
transistors are biased and driven for class C operation.

The demise of class-C coincided with the rise of SSB, NOT with
the invention of the transistor.

Class C power amplifiers went out of vogue for HF amateur 
service because they are not compatable with SSB. NOT because
they are incompatable with transistors. 

The final nail in the coffin was when the FCC regs changed
from limiting DC input power, to limiting RF output power.
With this change, Class C amplifiers lost thier 1 dB or so
edge when operated at the leagal limit. It was no longer worth
the hassle of adding CW keying circuits to an outboard amplifier
when you could just throw a little more input power in, and have one
circuit design that worked for all modes.

Class C PAs are still widely used for FM service, and in plate (OK 
collector) modulated AM rigs such as CBs,  and even VHF 
aircraft radios. These are all "no-tune" transistor circuits, and
the PA is biased pure class C. Operating class C  simplifies
AM transmitter design, and lowers power and heat sinking
requirements for any modes were it can be employed.

Note that even though this was a long post, it is MUCH shorter than
many 1 liners which then go on to quote, in full, three earlier posts.