FWIW, one of the things I like best about my KPA-1500 is that you can
use its antenna tuner with the amp on Standby
73, Pete N4ZR
Check out the Reverse Beacon Network
at <http://reversebeacon.net>, now
spotting RTTY activity worldwide.
For spots, please use your favorite
"retail" DX cluster.
On 9/1/2019 4:02 PM, K9MA wrote:
On 8/31/2019 6:02 PM, Roger D Johnson wrote:
Isn't this exactly the way some of the first ham linears were designed
for an input of 2000 watts in SSB and 1000 watts in CW? There
was a switch on the front panel for SSB/CW mode which dropped
the plate voltage for CW mode so that the tubes saw the same
load impedance they did in SSB mode thereby keeping the efficiency
high and not requiring retuning between modes.
I think it was more to make it harder to exceed 1 kW input on CW. It
is almost always possible to load an amplifier (with the usual PI
network) for efficient operation at less than maximum output. It just
has to be loaded lightly and not driven too hard. That conflicts,
however, with the usual procedure of loading for maximum output. Also,
with two plate voltage settings you could usually tune for maximum
output at the low setting at about 1 kW input, and it would be tuned
about right for 2 kW at the high setting. This reduced the stress
during tuning, and probably was the most common strictly legal way to
tune for 2 kW PEP input in those days.
Actually, legally tuning for 2 kW input in those days was tricky.
Collins used a bridge which compared RF input and output voltages and
allowed tuning at low power. I used that circuit in an amplifier I
built about 50 years ago, and it worked pretty well.
A PI network in a tube amplifier allows properly loading the amplifier
into a range of load impedances, but the operator or autotuner is
looking at grid current, plate current, input/output voltage ratios,
etc. to get the tuning right. This sets the tuning for efficient,
linear operation. A solid state amplifier and ATU work very
differently. The amplifier is designed to operate properly into a 50
Ohm resistive load, and the job of the ATU is to make the actual load
as close to that as possible. One result of that is that a rather
small mismatch can have a large effect on efficiency and/or linearity.
73,
Scott K9MA
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