[AMPS] Low Output on 10 meters

[Pete Smith]

At 09:26 AM 8/26/98 -0500, Jon Ogden wrote:
>
>>INPUT TUNING: What I found to be the hot ticket has been the MFJ 259B
>>antenna analyzer. Remove drive cable from your amp and connect it to the
>>259, key the amp and adjust the slugs. Just watch the meters as you crank
>>the slug into resonance.
>>
>>Others, more astute than myself, will provide the answer to the 10 meter
>>problem...
>
>There is a problem using this analyzer to do input tuning:
>
>The input impedance of a tube varies wildly over the drive cycle and 
>drive level.  So in other words to really match it properly, you need to 
>apply full drive to the amp and then tune it.  The MFJ analyzer really 
>puts out micro watts or so and that is not enough to drive the tube from 
>its cutoff region to active region.  Yes, it may get you "close" but 
>"close" is not necessarily a good match at high power.  Large signal 
>impedances are different from small signal impedances.
>
>I didn't believe this at first when people told me, but I read about it 
>and then I watched it in real life.  I watched as my SWR literally varied 
>significantly as I increased or decreased drive to the PA.

Jon's right about this.  I sometimes use my SB-220 on 12 meters, and for
maximum output I need to use my TS-930's internal tuner to match it.  The
only problem is that as the transferred power changes, the "target" moves.
I often have to abort the tuning process by switching out of "SEND" when
it's close enough.

Why not approximate with the antenna analyzer, and then connect the
transceiver for final tuning?  Use either the internal SWR meter or an
external unit between them.

73,  Pete Smith N4ZR
In wild, wonderful, fairly rare WEST Virginia



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