Radiodan W7RF wrote:
>Ian,
>
>You wrote,
>>
>> Therefore if you tune up at full power and then reduce the drive without
>> touching the output network again, you're simply driving the operating
>> point a smaller distance along the *same* load line. The load line
>> itself doesn't move, so I don't think the loaded Q of the output network
>> changes with drive level either.
>
>I always notice a difference in tuning for for a given maximum at any
>particular level.
>Normally, when I tune my 3K Premier with 10-20 watts drive for maximum power
>at this level, inevitably there will be a slight retune needed at full 100
>watts drive.
>Looking at your last statement, what's different? Is it the small plate
>voltage drop causing a different plate load impedance at the full drive
>level that makes for the change in tuning?
When you tune-up the PA for maximum RF output, you're presenting the
tube with a load line that optimizes the efficiency. When you change the
drive level and re-tune for max, you're RE-optimizing it. Maximum
efficiency at the new drive level will require a different load line, so
you'd expect to need to re-tune a little.
The normal procedure is to tune up at low drive first, and then increase
the drive, because that obviously involves less stress on the PA - the
tuning and loading are already close to the correct settings before you
apply full power.
On the other hand, to get closer to optimum IMD you'd need to tune and
load for maximum output at the highest drive level, then increase the
loading (which usually improves IMD but notably does not give the
maximum possible output or efficiency) and then reduce the drive without
touching anything else.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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