Don K4KYV said:
"I hear a lot of guys on the air with AM linears, and most of the time they
claim to be running way too much carrier for the tubes they are using. I
often hear stuff like 200 watts out with a pair of 811A's or 250 watts out
with a pair of 813's in g-g. If so, they are cooking the tubes, or else the
efficiency is running too high, and they are flat-topping all over the
place."
I agree with your comments and yes, there is no doubt that those rigs are
flat-topping. I've been running linear amps on AM for years and I find that
the cleanest, best sounding signal comes from a linear with LOTS OF PLATE
DISSIPATION and HEADROOM. In addition, an audio signal generator and an
oscilloscope are mandatory to get clean results. Not everyone has this
equipment and it is a hobby after all so folks do the best they can.
As a rule of thumb I consider the plate dissipation divided by 3 to be a
general yardstick for AM linears, some tubes giving a little more and some a
little less. In the case of a pair of 3-500Z's I rate them in the 333-350
watt carrier class. So while a pair of 813's can be a wonderful plate
modulated rig, as an AM linear they are not worth all the trouble of
building them. It seems to me to be a waste of time to build an AM linear
with much less than 1000 watts of plate dissipation. Of course if you have
a low power rice box and already own some kind of linear, well go ahead, but
if one is building an AM linear, more dissipation is better.
73,
Ken W2DTC
