I would first carefully test the Omni into a dummy load, and make sure on
10m, the Omni meter is stable with key down for a long dash. (or several
long dashes)
If you can verify the Omni meter as showing 1:1 SWR with a dummy load on
10m, then; you would be able to use it as an indicator. I would not adjust
the 811 Amplifier coil, unless you KNOW the tubes of the 811 are good, and
not old and possibly marginal. (That would show up on 10m probably sooner
than lower frequencies).
I take it the coil is slug tuned, which surprises me. At 100 watts drive,
many amplifiers have untuned input, or higher power coils than a typical
slug coil.
In any adjustment that is possibly NOT the problem, you have to note the
position of the coil screw adjuster slot, and count turns, as you turn away
from a setting. If you get no lowering of the SWR, you should stop and turn
the other direction. It is unlikely that you would need to turn more than
one turn, unless the amplifier was used in CB service and someone attempted
to adjust it to 27 MHz. The gradual nature of the SWR increase makes me
think of heat affecting a solder joint, or the tubes themselves causing this
as they heat up.
Exhaust ALL other possibilities before changing alignment adjustments on any
rig, receiver or transmitter. These things stay pretty close for many
years. There is an old ham fallacy, that tweaking the coils is the thing to
do.
If the symptom was constant, that might be the case, but any changing
symptom makes me want to try other things first.
-Stuart
K5KVH
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