Hi folks,
I'd like to get a feel for how precise I should be
when tuning up a linear amplifier. Say I've got an LKZ500 with
vernier drives on the plate and load caps and a 1-100 dial on
each. Say I'm on 20 meters with an antenna that goes from SWR 1.3:1
to 1:1 to 1.3:1 across the phone band. I work out the proper
settings for max power at the low, middle, and high end of the
band and put them on a chart (for my license, abt 14.179, 14.260,
14.395mhz). So now I want to fire up at say 14.220mhz. Can I
look on the chart, put the plate and load caps at an interpolated
position between my "low" and "medium" settings and let fly
with confidence or do I need to touch up the tuning while monitoring
power? Likewise if I am tuning around the band, how do I know when
I've strayed too far and should touch up the tuning? Is there a rule
of thumb for how much change in output power should signal a touchup?
Any guidance (or metrics) would be appreciated.
How hard is it on an amplifier to run into a 2:1 SWR load?
How about a 3:1 load or higher? If the load has a 3:1 SWR and I tune for
max power but then reduce the drive so peak voltages are the same as
with a 1:1 load and full power, does this produce equivalent wear of
the tubes? (Assuming I can do the initial tuning without arcing of course).
I noticed with an AL1200 that after getting everything tuned
up for 1500 watts output with CW, switching the amp to its SSB position and
driving it with an SSB signal without speech processing resulted in grid
current maximums that were similar to when I was tuning. But if I cranked in
15db of RF speech processing (this was with an FT990) the grid current
peaks climbed significantly. Indicated output power stayed close to the
same value although it sometimes exceeded 1500 so I'd reduce drive to stay
within the limits.
Should I use a different tuning procedure to adjust for the use
of aggressive levels of RF speech processing? My solution was to avoid
high processing levels except that a few times I had to have the extra
punch, so I cranked it up but reduced drive too to keep the grid current
within limits. Is there a better way to deal with this?
(This was within a DX contest setting, not what I would be
concerned about for a casual situation of course).
Regards,
Pete
KS4XG
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