[Elecraft] K3: 630m amplifiers

[Alan]

In the "good old" tube days, typically the transmitter's final amplifier 
was the keyed stage, or at least one of them.  So the key shaping was 
not degraded by a subsequent amplifier.  It's OK if the PA is a class C 
amplifier as long as the key shaping circuit takes that into account.

One way to get away with using a separate class C amplifier it to use a 
two-stage grid bias circuit.  A fixed voltage biases the tube so that it 
draws a little resting current (like a linear class B amplifier) and the 
rest of the bias is developed from a grid-leak resistor, which only 
kicks in when RF is applied to the input.  So as the RF input ramps up 
at the beginning of a dit, there is no abrupt transition from off to on. 
  It still increases the slope of the rise and fall, so it does increase 
key clicks, but not as bad as with pure fixed bias.  You can compensate 
by slowing down the rise/fall times of the key shaping circuit.

Alan N1AL


On 07/27/2015 04:57 PM, Tom Azlin W7SUA wrote:
> Interesting discussion, but I'm now confused.
>
> In all the old handbooks the discussion of amplifier design says ( page
> 76, 1941) "In amateur transmitters, and r.f. amplifier is invariably
> operated Class C ( see Chaptr 3)."