At 15:02 13-02-99 -0800, Vic, K2VCO wrote:
>My thought was to set the no-signal bias high enough to cut off the plate
>current completely. Then the operating point would be a little past
>As long as the bias is not high enough to start sharpening up the keying, why
>should I care about IMD? This would give me more efficiency and simplify the
Your idea is valid and one that I have used myself several times. Most
notable was the pair of 4CX250's that ran 1400 watts out for 2 meter EME
work. Some observations based on my experiences:
1) I prefer to bias to somewhat less then class B for my normal CW amps.
The increase in efficiency is slight and there are benefits to running
some idle current. Since I exclusively use choke input anode supply, I
typically adjust the idle current to match the choke used.(Critical
inductance - look it up)
2) Sometimes the efficiency does matter, such as when you are trying for
many times the rated output of the tubes. In this case, more efficiency
means less anode dissipation.
3) If you are really after efficiency, don't quit at class B. Things really
start to get good at around 110 degrees conduction angle. Depending on a
number of things, you may not even produce noticeable key clicks, but you
do need to check for them. There was an old rule of thumb that said you
could run one class C stage after a properly shaped exciter without
noticeable clicks. Always worked for me.
4) G-G amps as a rule are harder to get right in class C than TGTP
topology. The drive requirement increases a lot and the cathode impedance
swings are bigger as well. Might be a good idea to run higher then normal Q
in the cathode network. I remember the GG pair of TZ40's I had many years
ago. Had 'em biased to cutoff, took almost 100 watts to drive them to full
output. Had to have been around 80% efficiency if the color of the plates
were any indication!
5) If you are just looking for a neat project, try a pair of 4-400's in
push-pull TGTP class C. It would be real hard to make it multi-band, but it
sure would be a thing of beauty in this age of tiny ceramic tubes and
pi-nets. Run around 4000 volts on them, with bias around -225, and maybe
see around 3000 out! If you really want to amaze the locals, I could make
you a deal on a pair of NOS 250TH's.
6) Being able to run class B and C amplifier is just one of the many
advantages of CW. Real hams only transmit one tone at a time!
You could really have some fun with a project like this. Let me know how it
all works out.
Larry - W7IUV
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