Jim Reid wrote:
>
>Jim, KH7M wrote, in part:
>
>>>In fact, with 100 watts into the 8K, and it's 13 dB gain, you will
>>>get out around 2000 watts, so better cut the drive in back until
>>>the Bird in the 8K reads no more than 1500 out with the key down,
>>>SSB peaks.
>
>
>Rich, AG6K responded:
>
>>However, the tank of the Henry 8k is designed for a much lower RL --
>>i.e., the tank is designed for 2.5a of anode current. In order to
>>produce this much anode current, c. 400w of drive is needed.
>>>......
>
>And, presumably under this plate current, the QL, the
>loaded Q of the tank will be something above 10, or so.
>
>
>However, Rich has caused me to revert to Terman, hi.
[...]
>So, I see no harm to the 8K in operating it at low drive,
>and constraining it's power output to legal limits, where
>it evidently provides very linear operation; but at the
>cost of probably very poor efficiency.
Another way of looking at it is that the purpose of the tank circuit is
to create a load line for the tube. It does that by transforming the 50
ohm load impedance "backwards" up to typically a few k-ohms.
As seen on the tube characteristic curves, the effect of the RF drive is
to move the instanteneous operating point back and forth along the load
line, once every RF cycle.
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.
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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