Pete Soper wrote:
> 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?
In a grounded grid AB2 amplifier, grid current is the best indicator
of
proper 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.
If you observe a noticable increase in grid current, or a reduction
in
output, it is certainly time to consider retuning.
> How hard is it on an amplifier to run into a 2:1 SWR load?
The only compoonent that really "knows the difference" is the loading
capacitor and compnents between the loading cap and the antenna jack.
> 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).
The tubes care less what the SWR is, as long as the tank is tuned.
> 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.
Then you are splattering, and overdriving the tube. You should NEVER
see
rated grid current, or near rated grid current, on SSB with voice
modulation. Typically the VOICE grid current is 1/7th to 1/3 the CW
value.
> 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?
Always tune for maximum output power with full drive on CW, and then
reduce the exciter if you run too much power. Underloading the PA is
the
worse possible thing to do. The voltage in the tank soars, linearity
suffers, and the grid of the tube gets pounded.
If you make a mistake in tuning, it's always better to overcouple
(open
the load cap a little MORE than normal) than undercouple.
You should NEVER, when talking, see anywhere near the rated grid
current. A scope should be a legal requirement, without one grid current
is the next best thing.
73 Tom
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