I used to run a Kenwood TS-440, driving a SGC SG-500 amplifier. The
SG-500 requires about 55 to 60 watts of drive for full output. It has
protection circuitry, of various kinds. There are probably about a half
dozen conditions that will shut the amplifier down to prevent damage,
and there is an input attenuator, about 3 dB, that automatically
switches in if you overdrive the amplifier. You don't want to overdrive
the amplifier and you don't want that attenuator automatically switching
in on every leading CW element of a transmission. Turning the RF output
down on the TS-440 results in a funny sort of slow rise envelope. I
investigated the possibility of adjusting the ALC (or mis-adjusting it)
so that it would have a proper keying envelope at 55 watts output, and
probably could have done that. Then I would not be able to get the full
100 watts out of the TS-440 when I wanted it, running without the amplifier.
So, I made a power attenuator out of a bunch of 2 watt carbon
composition resistors. I don't recall the exact values. It is a T
network with about 2.5 to 3 dB attenuation, and four or five, two watt
resistors in parallel for each of the three elements of the T. It could
probably dissipate 30 watts continuously in air, and at CW duty cycle
would be fine dissipating 50 watts. Just to be sure, I built it in a one
quart paint can filled with mineral oil. When using the SG-500 and
transmitting and receiving on a single antenna the attenuator was in
line for both transmit and receive. My keying envelope was good, and the
attenuation was not even noticeable in receive.
Perhaps a better setup would be to have the power attenuator in the
input circuitry of the amplifier, after the input/bypass switching
relay. If you could get the right carborundum, or other non-inductive
power resistors, to make the right sized (both in attenuation and power
rating) pad, it might fit inside the amplifier.
If you do a google search on " t pad attenuator calculator " you'll get
plenty of results. The first one on the list worked for me.
DE N6KB
How much drive power does the PA take? The Orion is a bit notorious for
setting power only with the ALC while keeping drive up (no adjustment
for drive in CW mode) which tends to make a sharp corner when the rising
power gets to the power setting, instead of the desired rounded corner.
Slower rise time doesn't force that top corner rounding. You might try
inserting attenuation between the Orion and the PA to allow the Orion to
run full tilt so the top corners on the envelope are rounded.
Scopes are notoriously poor about triggering on modulation envelopes,
they'd rather trigger on the individual sinewaves of the carrier.
Triggering on the audio or the key closures is more reliable.
73, Jerry, K0CQ
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